Tutorial 27 - Codea v1.5.2: Export to Xcode

Figure 1. Tap & Hold the Project Icon.

27.1 Exporting to Xcode is Simple

The team at Two Lives Left continue to innovate. The latest version of Codea includes an export to Xcode function which is so simple to use that you can now submit to the App store without knowing any Objective C at all. Previously, incorporating your Lua code into the Codea runtime was something of an arcane art (see our earlier tutorial on submitting an app to the Apple App Store). Now, it is a matter of a few taps. This latest addition really rounds out Codea as the only development environment that you need.

Figure 2. Check the Export Settings

Loading your Codea apps into Xcode is useful for a couple of reasons:

1. To submit apps to the App Store, this is a necessary interim step;
2. To add additional functionality which isn't currently available in Codea (e.g. iAds, Game Center, creating a universal app - albeit most of this can be done in Codea, audio support, or hooking into some other Objective C methods). We will discuss this in the next tutorial.

Figure 3. Add a Custom Icon

Figure 4. Select an icon from your Photo Library

Figure 5. Tap on the selected icon

Once you are ready to export an app to Xcode, follow the following steps:

1. If you want to use a custom icon for your app, you can either attach this as part of the export process or do it later in Xcode. Assuming you want to include it in the export package, you will need to have it completed and stored in your iPad photo library before you start exporting. Codea will create four icons from the one that you include. Their dimensions are 57 x 57 pixels, 114 x 114 pixels, 72 x 72 pixels and 144 x 144 pixels. This is for the standard and retina versions of the iPhone and iPad icons. I suggest attaching an icon with a resolution of at least 144 x 144 pixels to ensure the best quality. If you are submitting to the app store you will need a version with 1024 x 1024 pixel resolution.
2. In Codea, tap and hold the icon of the project that you wish to export (Figure 1). This will bring up the pop up menu shown. Tap on Export.
3. Check that the Project settings are correct (Figure 2). Taking these one by one: 

• The first item is the app name which is taken from the name of your project (not the "Description" meta tag for your project created using saveProjectInfo). 
• The second item is the developer/author name which is derived from the "Author" meta tag and assigned using saveProjectInfo(). For example, saveProjectInfo("Author", "Reefwing Software"). 
• The third item, "Company Name" is optional. Note that the bundle ID will use a "short form" of the author or company name you include (it will prefer the company name if specified). A short version of a name simply removes any spaces or special characters in the name. So "Two Lives Left" becomes "twolivesleft", and the bundle ID then becomes com.twolivesleft.projectShortName. The bundle ID is important, as it connects your app in Xcode with your app submission in iTunes Connect. I always use a bundle ID of the form au.com.reefwing.LunarLander, so I will need to tweak this in Xcode regardless.
• The next item determines whether your app will display the status bar or not (the black bar right at the top of your iPad which shows your wireless connection strength, network provider, time, battery level, etc). This is an option in your applications pList, so if you change your mind later you can change it in Xcode.
• The final item in Project Settings is Icon. If you are happy with the default Codea icon for your app then you need do nothing. Most people will want to assign a custom icon however, and tapping the arrow to the right of "Icon" will allow you to do this (Figure 3). Tap on the "+" in the "Choose Icon" dialog to bring up your photo library and select the icon (Figure 4). If you created the icon on your Mac then importing it into iPhoto will add the image to your Photo Stream (if enabled) which is then available on your iPad to select.
• The "Included Assets" section allows you to include specific sprite images and shaders in the export bundle. Codea normally does a good job of finding these for you but it will have problems if you are procedurally generating the sprite/shader names in your code. If you find assets are being left out then they can be added here.
• Once you have confirmed all of the Project Settings, tap the Export Button in the top right hand corner of the navigation bar. Codea will chug away (Figure 6) and create your Xcode bundle. When complete (Figure 7), you are given the option to save your exported project bundle somewhere. You need to get this file onto your Mac, so the simplest method is to use Dropbox. If you haven't already signed up for this then I recommend that you do - it is free - and then install the Dropbox app on your iPad. This makes the file movement very simple.

Figure 6. Exporting your Project...

Figure 7. Exported and ready to save.

• Assuming you are using Dropbox, tap the "Open With" button shown in Figure 7, and you will be given the option to select Dropbox (Figure 8). Select this and then save the zipped bundle when prompted (Figure 9). You will then have the bundle saved in your Dropbox repository (Figure 10) which you can unzip and access from your Mac.

Figure 8. Select Export Bundle to Dropbox.

Figure 9. Save the zipped Project Bundle.

Figure 10. Bundle Saved in Dropbox.

27.2 Importing your Bundle into Xcode

1. Open up Finder on your Mac and navigate to the Dropbox folder containing your exported project bundle. Double click on this to unzip the folder (Figure 11).
2. Open up Xcode and select your unzipped project to open (Figure 12). Optionally you may want to save this folder locally first.
3. And that's it! You should now be able to run your imported project on the iPad or iPhone simulator.

Figure 11. Unzip your project bundle on the Mac.

Figure 12. Open the Project with Xcode.

Figure 13. Configure your Project in Xcode.

27.3 Pre-App Submission Tweaks

If you are going to submit your app to the Apple App Store for approval and listing on iTunes, then there are a few things you still need to do. Have a read of our previous tutorial on this for the full story but in short:

1. Click on the project (top left of the project navigator) to bring up the project configuration (Figure 13).
2. Check that the bundle identifier is the same as that entered in iTunes Connect.
3. Determine whether you want to distribute a universal app (works on iPad and iPhone) or target a specific device. The default is universal.
4. Check the supported orientations.
5. You may want to add your own default load images. The default ones say "made with Codea".
6. When adding your App to iTunes Connect you will need a version of your icon which is 1024 x 1024 pixels in size.

As shown above, the process is now much simpler than it used to be. Good luck with your submissions!

Tutorial 26 - RLE Image Compression for Spritely

1. Introduction

Todays tutorial is brought to you courtesy of Dermot Balson from the lovely city of Perth (the most remote city on Earth according to Bill Bryson). Dermot noted that Spritely (the image editor which comes with Codea) produces some fairly large and unwieldy image files. Given Dermot's background in compression he immediately worked out a way to dramatically reduce these files in size.

Essentially, Dermot creates an array of unique colours, and builds a string for each of them which consist of two-digit cell addresses. 

Let me hand over to Dermot (@Ignatz on the Codea Forum) to explain how it all works.

2. Description of RLE compression code

Run Length Encoding (RLE) is a well known compression technique. Essentially, you run through the cells, and every time the color changes, you store the last color and the number of cells it applied to.

The RLE code can be downloaded below and has several classes.

3. Functionality

The RLE compression code can turn any image (with reason - see below) into code strings, and the Decode function can reassemble the image from these strings.

It can take images from the libraries, or Spritely code, and convert to compressed code.

The ImageCoder:code function takes the image as input and prints the code to recreate it. 

The Test tab shows how this code is used. NB this function also returns the two strings required to decode the image. This is only for testing purposes so the Main tab can show how it works - in practice, users will use the printed code.

The Decoder:Decode function takes two strings as input and returns an image

The Main function carries out several tests:

1. It reads in an image from the standard library, codes and then decodes it back to an image
2. It takes an image created by Spritely code (in the IconImages tab), and encodes/decodes it back to an image
3. The code strings created for the Spritely code are used in the Test tab to demonstrate how to use them to decode an image

All three images above are drawn on screen. The first two images have their originals drawn above them for comparison purposes.

4. Using it in Practice

The ImageCoder tab is needed to encode images.

The Decoder tab needs to be included in any project where you want to decode the RLE images.

5. Compression

The level of compression depends on many factors, but as one indication, the size of the Spritely code in this example is about 28,000 chars, compared to 1,000 chars for the RLE version.

To be fair, this is largely because the Spritely image has a background color, meaning all 1,024 cells have to be coded. An image without a background would be much smaller.

6. Image Limits

The practical limits on image size and complexity are processing speed and storage, because the greater the number of colors and cells, the more space we have to allow for each item. Currently, it can allow for up to 4,096 unique r,g,b,a combinations, and up to 4,096 characters of the same color in a sequence. Increasing storage from 3 to 4 chars per item would increase capacity by a factor of 16, but would blow out the data by 1/3.

There are all sorts of ways to tweak for speed, but this may be good enough. I will tweak if there is demand for it.

7. Download the Code

Dermot has been kind enough to share his RLE coding and decoding classes. It is very well commented, so you should have no problems in following what he has done. You can download the complete project (useful for pasting directly into Codea) or the individual classes using the links below:

1. RLE Complete v1.lua - This includes all of the individual classes and test images.
2. RLE Readme.lua - Replicates most of the text description shown above.
3. RLE Main.lua - The Main tab from the complete project.
4. RLE Coder.lua - The RLE coding class.
5. RLE Decoder.lua - The RLE decoding class.
6. RLE Test.lua - sample string to test decoding.
7. Spritely Flag Image.lua - The flag image from the MineSweeper program, produced using Spritely and used here to demonstrate the compression capability of the RLE algorithm.

Tutorial 25 - A Lua Primer for Codea (Part 1)

25.1 Introduction

Every year around Christmas there is a surge of activity on the Codea Forums and on this tutorial site. A common theme in the comments and emails that we receive, is that this site assumes quite a bit of knowledge and as Codea appears to be attractive to first time programmers there are some gaps which need to be filled.

This tutorial is an attempt to fill in some of those gaps. In particular, we will try and provide a quick primer on the Lua language which Codea uses. To ensure that Codea does not incur the wrath of Apple, there are some features of the Lua language which have been disabled in Codea. We will discuss these briefly but the list is diminishing with every new release of Codea and there isn't anything which will likely cause you any problems.

25.2 Lua

Lua is a light weight scripting language written in C. By light weight we don't mean that it isn't capable of writing complicated code but that the syntax is stream lined and straight forward. This makes it a great first language to learn. 

Lua means "moon" in Portugese, it is the evolution of another language called SOL (Portugese for "sun"). It was created in 1993 by Roberto Ierusalimschy, Luiz Henrique de Figueiredo, and Waldemar Celes, members of the Computer Graphics Technology Group (Tecgraf) at the Pontifical Catholic University of Rio de Janeiro, in Brazil (hence the Portugese naming).

25.3 Variables and Data Types

Variables are memory locations that hold data. There are three kinds of variables in Lua: global variables, local variables, and table fields. All variables are global unless defined otherwise. Defining an identifier as local is done using the local keyword. Local identifiers are not visible outside the block in which they were declared, but are visible inside sub-blocks. This is called lexical scoping. For Codea a tab is considered a chunk,  which translates as follows:

1. If you define a variable within any of the tabs it will, by default be treated as global.
2. If you define a local variable within a tab (but outside a function), you will only be able to access it within that tab.
3. If you define a local variable within a function, you can only access it from within that function. However, if a function is enclosed in another function, then it has access to all the local variables of that function. In these circumstances, the external local variable is called an "upvalue". We cover this in Interlude 7.

Assigning to a variable that has not been declared locally within the current chunk will recursively search for that name in the parent chunk, up to the top-level. If the name is found, the assignment is made to that variable. If the name is not found, the assignment becomes a global (either creating a new variable, or replacing an existing global). The consequence of this behaviour is that using local variables is much faster than globals.

Global identifiers are stored in the implicit global environment table, which can explicitly be accessed through the name _G (see section 25.5).  

Variable names must start with a letter or underscore and can contain letters, digits or underscores. Defined keywords in Lua can not be used as a variable name (e.g. and, break, do, else and end). As a convention, variables in capitals starting with an underscore (e.g. _G or  _VERSION) are reserved for internal Lua global variables.

Lua supports only a small number of data types. The ones you will use the most are boolean (true or false), numbers, strings and tables. 

By default, Lua's number type is represented by double-precision floating-point numbers. However, Codea's Lua interpreter uses another internal representation for numbers: single-precision float. This gives a precision of 6 to 9 significant decimal digits and a range for positive values of between about 1.4e−45 to about 3.4e+38 (reference from Codea Wiki).

Lua strings can hold any 8-bit character, including embedded zeros. Strings can be enclosed in single or double quotes, pick one style and stick with it, we tend to go with double quotes. Strings in Lua are immutable values, and thus you cannot change a character inside a string. You can also delimit literal strings using matching double square brackets. For example:

aLongString = [[
                   line 1
                   line 2
                   line 3
                   line 4
              ]]

Boolean variables are a relatively recent introduction to Lua. The boolean type has two values, true and false. Conditional tests consider false and nil as false and anything else as true (including 0 and "").

Lua is a dynamically typed language, which means that variables do not have a defined type, they get their type based on the data assigned to them. This means that the following is a valid chunk of code in Lua (albeit probably not best practise as it would make your program hard to follow):

x = 1
print("x as an integer: " .. x)
x = 3.141592654
print("x as a float or real number: " .. x)
x = 0xFE
print("x as a hexadecimal converted to decimal: " .. x)
x = "now a string"
print("x as a string: " .. x)
x = true
print(x)
x = {value = 999}
print("x as a table: " .. x.value)
x = function(n) return n*2 end
print("x as a function: " .. x(2))

A few comments about the preceding code and some general observations on variables: 

• Lua is case sensitive, so x and X are different variables. 
• In the example above, when we store a hexadecimal value in x it is automatically converted and printed as a decimal.
• You can concatenate (i.e. join) two strings or a string and a number using the".." operator. Numbers are automatically converted to strings in this situation. You can't concatenate a boolean.
• This automatic conversion (or to use the technical term - coercion,  works the other way around as well, a string will be treated as a number if used in that context. Any arithmetic operation applied to a string tries to convert this string to a number. Note that comparison operators (== ~= < > <= >=) do not coerce their arguments. Thus a number is not equal to its string representation.
• Each line of code is terminated by a new line. You can optionally use a semi-colon (as used in C, but it isn't recommended). 
• You can do multiple assignments on the one line (e.g. x, y = 1, 2), and you can use this to swap two variables (e.g. x, y = y, x).
• A variable that hasn't been assigned a value will be nil by definition. 
• Unlike C, the value 0 is not a false test condition in Lua, only nil or false is. You can use the fact that nil equates to false to assign default values to a variable (e.g. x = x or 2 will assign the variable x a value of 2 if it hasn't been previously assigned a value).
• Note that you can assign a function to a variable. As an example, we make use of this in Codea to pass call back functions to our Button class to indicate what function to call when a button is tapped. The technical term for this feature is first-class functions. As such, they can be created during runtime, stored in variables, and passed to and returned from other functions.

There are two other data types called userdata and thread. Userdata and tables will be covered in subsequent sections. Thread is way outside the scope of this tutorial so we are just going to ignore them.

25.4 Tables

Tables in Lua are amazingly versatile. Which is just as well since they are the only built in composite data type available. The technical term for tables in Lua is a hashed heterogeneous associative array and they are worthy of a separate tutorial, which is exactly what we have done. You can access our tutorials on tables here:

1.  Understanding Tables;
2.  Converting a string to table and table to string; 
3.  Saving and Loading complicated tables; and
4.  Classes in Lua and Codea.

25.5 The Global Variable Table _G

This section goes beyond the scope of this tutorial, so feel free to skip ahead. It is included here out of interest. We mentioned above that Lua has a number of internal variables. One of these is _G, a global variable which points to the global environment. It includes all of the global variables and functions and even includes a reference to itself! It is sometimes useful to understand what has been defined globally and you can use the following to display the contents of _G: 

for k, v in pairs(_G) do print(k, v) end

25.6 Userdata

Userdata are variables that encapsulate arbitrary C/C++ data within a Lua interface. Many Lua modules extend the capabilities of Lua by binding external libraries, including the creation of new types as userdata. Userdata variables can only be created using the C API (i.e. in our context this means the Codea runtime), this can't be done in Lua.

Userdata is largely outside the scope of this tutorial but you need to know that Codea extends Lua with 12 user-defined types such as codeaimage, mesh, matrix, vec2, vec3, touch and color. You can read all about these on the Codea User Defined Types page.

Userdata is also useful when we want to expose C/Objective C functions in Lua via the runtime. Have a look at our tutorials on integrating Game Center, building a Universal App or implementing iAds if you are interested.

25.7 Operators

Lua supports the following arithmetic operators:  + (addition), - (subtraction), * (multiplication), / (division), % (modulo),  ^ (exponentiation) and  - (negation).

The relational operators in Lua are: == (equal), ~= (not equal), < (less than), > (greater than), <= (less than or equal to), and >= (greater than or equal to). These operators will return a boolean (true or false).

The logical operators in Lua are and, or, and not. As for control structures, all logical operators consider both false and nil as false and anything else as true.

As mentioned in section 25.3, the string concatenation operator in Lua is denoted by two dots ('..'). If either operand is a number, then it is converted to a string before joining.

The length operator is denoted by #. The length of a string is its number of bytes (i.e. characters). The length of a table is more complicated. For a table used as a simple array the length operator will work as expected and return the number of elements in the array. For more complicated tables, the returned length can be any of the indices that directly precedes a nil value. You can read more about this in our tutorial on tables.

25.8 Classes

In Interlude 11 we spoke about the use of classes in Codea. Codea comes with a built-in global function called class() that is used to emulate the functionality of a class data structure using a table (and metatable). We use classes extensively in our tutorials but we haven't explained the difference between the "." and ":" operators when applied to a class. As this has been the subject of a number of questions, we will cover the proper usage in this section.

In order to demonstrate, we will first construct a simple ship class. By convention the base class name starts with a capital, while instances of the class have names which start in a lower case letter. 

The Codea class() constructor is a function which, when called, sets up a new table and attaches the class metatable to it. The metatable redirects unrecognized events to the class method table (as well as possibly handling events itself). This is covered in some depth on the Codea wiki.

Ship = class()

function Ship:init( hitPoints )
    self.points = hitPoints or 100
end

function Ship:hit( damage )
    self.points = self.points - damage
    print( self.points )
end

In our Main tab, setup() function we may then instantiate our class as follows:

myShip = Ship()

This will create a new object called myShip of class Ship. Note that we called ship without any parameters. This means that hitPoints will be nil and self.points will be assigned 100 (our default value) since nil is interpreted as false by Lua. 

One of the potentially confusing parts of the class definition is where does "self" come from and why do we use it? The self variable is created automatically by calling class() and provides a reference to the object created when we instantiate a class. This means that when we change the points variable using self.points we only change it for that object, not for every object of class Ship. So in our example above self = myShip. 

Try converting the Ship class so that it uses points instead of self.points. If you do this and have created another ship (e.g. myOtherShip = Ship()), then every time you call myShip:hit(20) this will also reduce the hit points of myOtherShip, which is usually not what you want.

If our ship gets hit then we want to record the damage and print out the current hit points using the hit(damage) function of our class. There are two ways you could do this (the right way and the wrong way!). In the Main tab of your program, you could use:

myShip:hit(20)

or you could try to use:

myShip.hit(20)

Which is correct? To work this out we have to understand the difference between the two statements. Using ":" we are actually passing two parameters to hit(), the damage and a hidden reference to the object being hit (i.e. self). So myShip:hit(20) is equivalent to myShip.hit(self, 20). 

The second option - myShip.hit(20) will throw an error in Codea. If you want to access the class instance variables directly from the Main tab, you can do something like:

myShip.points = myShip.points - 20

This will operate as expected, but only if you have defined it as self.points in your class.

25.9 Other Resources

If what is provided here isn't sufficient then have a look at the official Lua Tutorial site, which has a MUCH more detailed treatment on Lua. The Codea reference documentation is also very useful, as is the active forum and wiki (in particular have a look at the Hints and Tips page which contains references to items not covered elsewhere). 

Codea (v1.4) uses version 5.1 of Lua (you can determine the current version of Lua using the statement print(_VERSION)). The definitive treatment of the Lua language can be found at the official on-line Lua 5.1 Reference Manual. 

Interlude 14 - Update on MineSweeper Sales

14.1 Overview

MineSweeper was initially launched at the end of August 2012. To date it has been downloaded by 2,484 people which is an average of 20 per day. As the graph above shows, averages don't paint an accurate picture of the download distribution. The first month (September) accounts for 2,393 or 96% of these downloads. Since the end of September, the average download rate has been 1 per day.

14.2 Version 1.7 Release

The first published update to MineSweeper was version 1.7 which made the App universal, included Game Center support and added iAds. You can see the bump in downloads on the release day in the graph above. This bump is a pale shadow compared to the initial release bump (11 vs 471).

14.3 Geographic Distribution

37% of downloads has been from the US which is not unexpected, but the next largest region is Germany at 9% which is curious.

14.4 What can we deduce?

These are only statistics from one application over a 3 month period and the enhancements from v1.7 have only be around for 3 days so we need to be careful not to identify trends that don't exist.

1. iAds - Requests and Impressions can given you a feel for usage of your App each day. It is very early days so hard to conclude anything yet. In order to maximise revenue the download curve would suggest that you would be better off including iAds in your initial release rather than a subsequent upgrade.
2. Game Center - will also give you an indication of app usage. In the 3 days that Game Center has been available, 9 people have recorded high scores in the Easy Difficulty leader board.
3. Geographic Distribution - Based on the geographic download distribution, if we were to localise the app then German would be the first language that we should implement.
4. Upgrades - For the 3 days that v1.7 has been released there has been 419 upgrades downloaded. We would expect another 100 or so over time but upgrades give you a feeling for how many people keep your App on their device after the initial download.

Tutorial 24 - Basic 3D Graphics

Figure 1. Ripple Shader.

24.1 Setting the Scene

Version 1.5 of Codea is a huge update. In addition to camera access, image blend modes and a tween library for simple animation, it includes full access to shaders and a shader editor. This feature gives you full access to GLSL (OpenGL Shading Language) vertex and fragment shaders (which can be used to apply the ripple shader effect shown in Figure 1). To understand how to implement and use shaders we need to take a few steps back and provide some graphical foundations.

24.2 OpenGL

OpenGL is a multipurpose open-standard graphics library. Although it is actually a specification, it is usually thought of as an Application Programming Interface (API), which is the manifestation of this specification. The OpenGL API uses C and GLSL is very similar in structure to C but has its own peculiarities. As a C API, OpenGL integrates seamlessly with Objective-C based Cocoa Touch applications. The OpenGL API is defined as a state machine (see Tutorial 5), and almost all of the OpenGL functions set or retrieve some state in OpenGL. The only functions that do not change state are functions that use the currently set state to cause rendering to happen.

OpenGL for Embedded Systems (OpenGL ES) is a simplified version of OpenGL that provides a library which is easier to learn and implement on mobile graphics hardware. Apple provides implementations of OpenGL ES v1.1 and OpenGL ES v2.0. Codea uses v2.0.

OpenGL ES 2.0 is very similar to OpenGL ES 1.1, but removes functions that target the fixed-function vertex and fragment pipeline stages. Instead, it introduces new functions that provide access to a general-purpose shader-based pipeline. Shaders allow you to write custom vertex and fragment functions that execute directly on the graphics hardware (which is very fast). 

24.3 Rendering Graphics

Everything displayed on your iPad screen is a 2 dimensional array of pixels. Each pixel has a particular colour defined by a red, green, blue and alpha (transparency) value in the range 0 to 1. It is the purpose of the graphics pipeline to determine what colour to put in each pixel to provide a representation of your image. Displaying a 2D image is fairly straight forward but what about 3D? The process of converting a 3D world into a 2D image is called rendering.

There are many different rendering systems. The one that we will concern ourselves with is called rasterization, and a rendering system that uses rasterization is called a rasterizer. In rasterizers, all objects that you see are represented by empty shells made up of many triangles. These series of triangles are called "geometry", "model" or "mesh". We will use the term mesh as that is what is used in Codea.

Figure 2. Rasterizing a Triangle.

The process of rasterization has several phases. These phases are ordered into a graphics pipeline (figure 3), where the mathematical model of your image, consisting of a mesh of triangles, enter from the top and a 2D pixel image comes out the bottom. This is a gross simplification but may help in the understanding of the process. The order which triangles from your mesh are submitted to the pipeline can effect the final image. Pixels are square, so they only approximate the triangles (Figure 2), just as the triangles approximate the 3D image. The process of converting your triangles to pixels is called scan conversion, but before we can do this we need to perform some mathematics to check whether the triangle is visible and convert it from 3D to a 2D representation.

Figure 3. OpenGL ES 2.0 Graphics Pipeline.

24.4 Graphics Pipeline Overview

Triangles are described by 3 vertices, each of which define a point in three dimensional space (x, y, z). To represent these in two dimensions we have to project the vertex co-ordinates onto a plane. We maintain the illusion of depth by using tricks like perspective (i.e. things the same size appear smaller the further away they are). We get to influence the graphics pipeline at two points, the vertex shader and the fragment shader, shown in orange in Figure 3.

If you are interested in a much more detailed explanation then we suggest that you read Andrew Stacey's tutorial on Using Matrices in Codea.

Step 1 - Vertex Shader (Clip Space Transformation)

The first phase of rasterization is to transform the vertices of each triangle into "clip space". Everything within the clip space region will be rendered to the output image, and everything that falls outside of this will be discarded. In clip space, the positive x direction is to the right, the positive y direction is up, and the positive z direction is away from the viewer. Clip space can be different for different vertices within a triangle. It is defined as a region of 3D space with the range [-w, w] in each of the x, y, and z directions. 

This is difficult to visualise and use so the vertices are normalised by dividing each co-ordinate (x, y, z) by w. After being normalised, the (x, y, z) co-ordinates will be in the range of -1 to +1. Dividing by w also applies a perspective effect to each of our triangles.

The entire process can be thought of as a mapping from the projection volume to a 2 unit cube with the origin at (0, 0, 0).

Figure 4. Clip Space Transformation & Normalisation.

In terms of the graphical pipeline (figure 3), this transformation is coded in the Vertex Shader. Open up the Shader Lab in Codea and tap on the vertex shader tab. In the main() function, the line:

gl_Position = modelViewProject * position;

performs the clip space transformation for you. 

The inputs to the vertex shader consist of:

• Attributes - per vertex data supplied via vertex arrays (e.g. position, color and texCoord). They are signified by the attribute tag in GLSL;
• Uniforms - constant data used by the vertex shader (e.g. modelViewProjection). Labelled as uniform in GLSL; and
• Samplers - a specific type of uniforms that represent textures used by the vertex shader. These are optional.

The outputs of the vertex shader are called (somewhat redundantly) varying variables.

Step 2 - Primitive Assembly

A primitive is a geometric object which can be drawn by OpenGL ES (e.g. a point, line or triangle). In this stage, the shaded vertices are assembled into individual primitives.

Normalisation and Clipping will happen automatically in the Primitive Assembly stage between the vertex shader and fragment shader. Primitive Assembly will also convert from normalized device coordinates to window coordinates. As the name suggests, window coordinates are relative to the window that OpenGL is running within. Window coordinates have the bottom-left position as the x, y (0, 0) origin. The bounds for z are [0, 1], with 0 being the closest and 1 being the farthest away. Vertex positions outside of this range are not visible. The region of 3D space that is visible on the screen is referred to as the view frustum.

Step 3 - Rasterization

Rasterization converts the graphic primitives from the previous stage to two dimensional fragments. These 2D fragments represent pixels that can be drawn to the screen (Figure 2).

In this stage, the varying values are calculated for each fragment and passed as inputs to the fragment shader. In addition, the colour, depth, stencil and screen co-ordinates are generated and will be passed to the per-fragment operations (e.g. stencil, blend and dither).

Step 4 - Fragment Shader

The fragment shader is executed for each fragment produced by the rasterization stage and takes the following inputs:

• Varying variables - outputs from the vertex shader that are generated for each fragment in the rasteriser using interpolation (e.g. vColor in the Ripple Shader Lab example).;
• Uniforms - constant data used by the fragment shader (e.g. time and freq in the Ripple Shader Lab example).; and
• Samplers - a specific type of uniforms that represent textures used by the fragment shader (e.g. texture in the Ripple Shader Lab example).

The output of the fragment shader will either be a colour value called gl_FragColor or it may be discarded (see Step 5).

Step 5 - Per Fragment Operations

The final step before writing to the frame buffer is to perform (where enabled) the following per fragment operations.

1. Pixel ownership test - checks if the pixel is currently owned by the OpenGL context. If it isn't (e.g. the pixel is obscured by another view) then it isn't displayed.
2. Scissor Test - if enabled, is used to restrict drawing to a certain part of the screen. If the fragment is outside the scissor region it is discarded.
3. Stencil & Depth test - if enabled, OpenGL's stencil buffer can be used to mask an area.The stencil test conditionally discards a fragment based on the value in the stencil buffer. Similarly, if enabled the depth buffer test discards the incoming fragment if a depth comparison fails.
4. Blending - combines the newly generated fragment colour value with the corresponding colour values in the frame buffer at that screen location.
5. Dithering - simulates greater color depth to minimise artifacts that can occur from using limited precision. It is hardware-dependent and all OpenGL allows you to do is to turn it on or off.

24.5 A Simple Shader Example

Version 1.5 of Codea comes with a sample ripple shader (see Figure 1). The following fragment shader code will tint a texture with the tint colour by the tint amount.

// A basic fragment shader with tint.

// This represents the current texture on the mesh

// uniform lowp sampler2D texture;

// The interpolated vertex color for this fragment

// varying lowp vec4 vColor;

// The interpolated texture coordinate for this fragment

// varying highp vec2 vTexCoord;

void main()

{

    // Sample the texture at the interpolated coordinate

    

    lowp vec4 texColor = texture2D( texture, vTexCoord );

    

    // Tint colour - red is currently hard coded.

    // Tint amount - select a number between 0.0 and 1.0

    // Alternatively you could pass the tint color and amount

    // into your shader by defining above:

    //

    // uniform lowp vec4 tintColor;

    // uniform lowp float tintAmount;

    

    lowp vec4 tintColor = vec4(1.0,0.0,0.0,1.0);

    lowp float tintAmount = 0.3;

    tintColor.a = texColor.a;

    // Set the output color to the texture color

    // modified by the tint amount and colour.

    

    gl_FragColor = tintColor * tintAmount + texColor * (1.0 - tintAmount);

}

24.6 Appendix - GLSL Precision Qualifiers

You will notice the lowp, mediump and highp precision specifiers in the shader lab example. It is much faster to use lowp in calculations than highp.The required minimum ranges and precisions for the various precision qualifiers are:

Apple provides the following guidelines for using precision in iOS applications:

• When in doubt, default to high precision.
• Colours in the 0.0 to 1.0 range can usually be represented using low precision variables.
• Position data should usually be stored as high precision.
• Normals and vectors used in lighting calculations can usually be stored as medium precision.
• After reducing precision, retest your application to ensure that the results are what you expect.

Tutorial 23 - Implementing iAds with Codea

23.1 Introduction

A common business model in the App store is to give away your app for free and generate revenue by serving ads to your application. This model has benefits but can annoy your users if done in excess. Everyone likes stuff that is free, so setting a zero cost removes most of the barriers to downloading your app and should maximise download numbers.

Since iOS 4.0 the iAds framework has been available for developers. Using this you can add banner or full screen advertisements to your application. If you are planning to include ads on a screen then you need to ensure that you have left space within your interface to display them. Full screen ads (referred to as interstitial ads by Apple) are only available on the iPad and are only suitable in certain situations, so we wont cover those in this tutorial.

You receive revenue when users view (called impressions) or click advertisements on your app. Some apps use a bit of social engineering to maximise their click through rate. For example the Foxtel (cable TV) app in Australia has ads which appear at the top of the channel selection screen. As per the Apple guidelines these ads are only displayed once the banner has been loaded from the ad server. When this occurs, the ad slides in from the top and moves the content down by the height of the banner. However what happens 50% of the time (for us at least) is that you accidentally tap the ad when you are trying to select a channel.

It is difficult to estimate exactly what revenue you might receive from including iAds in your app. A graph of the impressions that we got for the last year is shown below. There was about 98k in total with 90% coming from one app (Personality). According to Apple the effective revenue per thousand ads (eCPM) served over this period was $0.82. In practice, we get on average $0.25 per day. So while this may not be a get rich quick scheme, it is not bad passive income, particularly given the app I'm deriving revenue on is not optimised for ads. 

We suspect you could do much better with a free game app with high reusability (people tend to use Personality only 2-3 times at best). Apple says that "hundreds of developers are making more than $15,000 per quarter". Given that there are 275,000 iOS developers in the US alone, this is perhaps not that impressive - albeit not all of those developers have iAd enabled apps.

23.2 Banner Views

From a Codea perspective, implementing iAds is easy. If you are happy to display ads on every screen then you just need to leave the appropriate amount of space on your screen to display the ad. In most cases there will be screens that you don't want to display ads on (e.g. the main game screen), so we have extended Codea Lua with a function, showBannerAd(true) to turn ads on and off. For our example, we will only display ads on the Menu and High Score screens.

For the iPad, banner sizes are set at:

• Portrait: 66 (height) x 768 (width) points - see Interlude 13 if you dont understand the difference between pixels and points.
• Landscape: 66 (height) x 1024 (width) points.

For completeness, the iPhone banner sizes are 320 x 50 points for a portrait advertisement and 480 x 32 points for a landscape advertisement.

The banner ads are intended to typically appear either at the top or bottom of the display and are provided in two formats so that ads may be displayed when the device is either in portrait or landscape orientation.

23.3 Setup iAds in iTunes Connect

As with Achievements and in-App purchasing, there are some things that you need to enable in iTunes Connect before your app will be able to receive ads from Apple's servers. Log in to your developer account, select iTunes Connect and then click on Manage your Apps. Click on the appropriate App icon and then click on the Setup iAds link (right hand side towards the top).

Answer the question regarding whether the primary target audience for your app is users under 17 years of age and then click Enable iAds and Save.

That's it! You can disable iAds just as simply but changes to your iAd settings will not take effect until your next app version is approved.

23.4 Creating Your Banner Views in the Codea runtime

You should only create a banner view when you intend to display it to the user. Otherwise, it may cycle through ads and deplete the list of available advertising for your application. As a corollary to this, avoid keeping a banner view around when it is invisible to the user. To demonstrate the implementation of iAds in the runtime we will use our old faithful MineSweeper application.

Based on our careful UI planning (or perhaps just through good luck) we happen to have enough space at the top of the MineSweeper Menu screen to fit in a banner ad, so that is where we will put it. As MineSweeper is now a universal application we will need to handle the iPhone sized banners as well. We have more room at the bottom of the iPhone Menu screen when it is in the portrait orientation, so when we detect that device and orientation we will place the banner at the bottom of the view. There is no room for a banner on an iPhone in landscape orientation, so we will disable ads when we detect this combination.

Banner views use a delegate to communicate with your application. Your application will thus need to implement a banner view delegate to handle common events. In particular we need to handle:

1. when the user taps the banner.
2. when the banner view loads an advertisement.
3. when the banner view encounters an error.

Usually you would place this code in the relevant view controller but for simplicity we will handle this in the CodifyAppDelegate (the alternative is to extend the BasicRendererViewController class in the runtime).

If your application supports orientation changes, your view controller must also change the banner view’s size when the orientation of the device changes.

Step 1 - Add the iAd Framework to your App.

Fire up Xcode and load the runtime version of your Codea application. Click on the CodeaTemplate file at the top of the project navigator then in the summary tab, scroll down to the linked libraries area. Click on the "+" button below your existing frameworks to add a new framework. Find iAds and click on "Add".

Step 2 - Update the CodifyAppDelegate Files.

In CodifyAppDelegate.h you need to import the framework you just added using:

#import <iAd/iAd.h>

then modify the class interface definition so that it looks like this:

@interface CodifyAppDelegate : UIResponder<UIApplicationDelegate, ADBannerViewDelegate>

This indicates that the ADBannerViewDelegate protocol will be implemented. We will provide a link to download the updated CodifyAppDelegate files below. We have added two other things to the CodifyAppDelegate files:

1. A boolean instance variable called displayingAds which is controlled from your Lua code via the modified aGameCenter_Codea class; and
2. A call back method called iAdDisplayStatusChanged which is used to update the banner view if the MineSweeper game state or orientation changes. This also comes via the modified aGameCenter_Codea class.

Step 3 - Update the other Codea runtime Files.

As we saw in Tutorial 19 when integrating Game Center, there are changes required in LuaState.m, OSCommands.h and OSCommands.m, to allow Lua to communicate with our Objective C code. This mostly comprises registering our new showBannerAd(true or false) function.

In LuaState.m add the following to the  - (void) create method:

    //iAds Function

    LuaRegFunc(showBannerAd);

In OSCommands.h add the following below the similar Game Center definitions:

    //iAds

    int showBannerAd(struct lua_State *L);

Finally, in OSCommands.m add the following method below the Game Center definitions:

int showBannerAd(struct lua_State *L){

    [CodeaGameCenter showBannerAd:lua_toboolean(L,1)];

    return 0;

}

23.5 Codea Lua Code Changes

As mentioned in section 23.2, if you are happy to display adds on every screen then there is no changes required to your Lua code (assuming you left space for the ads). If like us you want to select when ads are displayed, then you need to use the new showBannerAd() function. If you pass true in this function then ads will be displayed and if you pass false, they wont.

As an example, the following shows our updated orientationChanged() function. 

function orientationChanged(newOrientation)

    print("Orientation Change Detected.")

    

    if setupHasRun and gameState == stateRun or gameState == stateWon or 

        gameState == stateLost then

            updateGridLocation()

    end 

    

    if setupHasRun and (gameState == stateMenu or gameState == stateScore) then

        showBannerAd(true)

    else

        showBannerAd(false)

    end

    

end

We also use showBannerAd()  when the gameState changes due to button taps.

The other class we need to modify is the SpashScreen. In the fadeAnimation call back, we turn on ads once the splash screen has faded away.

function fadeAnimationDone()

    

    -- Call back function for Fader complete

    

    gameState = stateMenu

    showBannerAd(true)

    

end

23.6 Download the Updated Runtime & Codea Files

The aGameCenter_Codea and CodifyAppDelegate files are available on GitHub. In addition, the following links allow downloading individual files from DropBox:

1. aGameCenter_Codea.h
2. aGameCenter_Codea.m
3. CodifyAppDelegate.h
4. CodifyAppDelegate.m
5. LuaState.m
6. OSCommands.h
7. OSCommands.m
8. MineSweeper - Main.lua v1.7
9. MineSweeper - SplashScreen.lua v1.7

Interlude 13 - Pixels vs Points in iOS

In the previous tutorial we had to deal with a number of different resolutions in order to be able to develop a universal app. The following is a quick refresher on how to do this.

From iOS 4, dimensions are measured in “points” instead of pixels. All coordinate values and distances are specified using points, which are floating-point values. The measurable size of a point varies from device to device and is largely irrelevant. The main thing to understand about points is that they provide a fixed frame of reference for drawing.

The iPhone screen is 320 x 480 points on both iPhone 4 and older models. The iPhone 4 has a screen resolution of 640 x 960 pixels (i.e. a scale factor of 2.0) while older models have a resolution of 320 x 480 pixels (i.e. a scale factor of 1.0).

The iPad screen is 768 x 1024 points. This scales to the following pixel resolutions:

• iPad 1 - 768 x 1024 pixels (i.e. a scale factor of 1.0)
• iPad 2 - 768 x 1024 pixels
• iPad 3 - 1536 x 2048 pixels (i.e. a scale factor of 2.0)
• iPad 4 - 1536 x 2048 pixels
• iPad Mini - 768 x 1024 pixels

Retina is a marketing term developed by Apple. It refers to devices and monitors that have such a high pixel density (normally greater than 300 pixels per inch) that most people can't discern individual pixels at a normal viewing distance. We wont use this term in our explanation because it is inexact and differs by device (e.g. iPhone 4/5 is 326 ppi, iPad mini is 163 ppi, iPad 1 and 2 is 132 ppi, iPad 3 and 4 is 264 ppi - note that the iPad 3 and 4 don't strictly meet the Retina "standard"). 

Querying the scale factor is the best way to determine the relationship between pixels and points on the current device. In iOS 4 and later, UIScreen has a property called scale which is a floating point number that defines the pixel/point relationship. In Objective C you can get this number using:

[[UIScreen mainScreen] scale]

On an iPhone 4 and 5 a point is a four pixel square, so if you draw a one-point line, it shows up as two pixels wide. The theory is that you specify your measurements in points for all devices, and iOS automatically draws everything to the right proportion on the screen. The result is that if you draw the same content on two similar devices, with only one of them having a high-resolution screen, the content appears to be about the same size on both devices. This is why you go to the trouble of creating your @2x images.

The iPhone 5 has a physical resolution of 1136 x 640 pixels which equates to 568 x 320 points. Your WIDTH and HEIGHT constants return points not pixels. So the best test for an iPhone 5 within Codea is to look for the 568 dimension.

Within your Objective C code you can determine the device using something like the following:

if (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad)
        NSLog(@"iPad device detected.");
else if (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPhone)
{
        CGSize result = [[UIScreen mainScreen] bounds].size;
        if (result.height == 480)
        {
            NSLog(@"iPhone/iPod Touch detected (< v5).");
        }
        if (result.height == 568)
        {
            NSLog(@"iPhone/iPod Touch detected (> v5).");
        }
}