MSAA on the PS3 with Light Pre-Pass on the SPU

In the previous "MSAA on the PS3" thread Matt Swoboda jumped in and mentioned that they implemented MSAA on the SPU in the Phyre Engine. I knew that they implemented the Light Pre-Pass on the SPU but I completely forgot that they also had a solution to do MSAA on the SPU.
You can find the presentation "Deferred Lighting and Post Processing on PLAYSTATION®" here.
Because it is possible to read and write per sample with the SPU, they can achieve a similar functionality as the per-sample frequency of DirectX 10.1-class graphics hardware where each sample can be treated separately. So they can calculate the lighting for each of the sample values and write the results into each of the samples in the light buffer.

Ambient Occlusion in Screen-Space

Screen-Space Ambient Occlusion (SSAO) is quite popular in the moment. ShaderX7 had several articles and there are lots of approaches to gradually improve the effect.
A good way to look at SSAO or any similar approach is to consider it part of a whole pipeline of effects that can share resources and extend the idea to include one diffuse (and specular) indirect bounce of light by re-using resources.
The overall issues with SSAO are:
1. quite expensive for the image quality improvement. Using the astonishing high amount of frame-time for other effects is an intriguing idea. In other words the performance / quality-improvement ratio is not very good compared to e.g. PostFX where a bunch of effects consumes a similar amount of time.
2. a typical problem is that lighting is ignored by SSAO. Using the classical SSAO implementation under varying illumination introduces objectionable artifacts because the ambient term is darkened equally (obviously you can apply SSAO to the diffuse and specular term like a shadow term ... but then it isn't ambient anymore). If you have a "global ambient" light term like skylights, SSAO will diminish the effect. It also leads to problems with dynamic shadows.

Overall I believe a fundamental shift to more generic method is necessary to solve those issues. This is one of the things I am looking into ... so expect an update at some point in the future.

MSAA on the PS3 with Deferred Lighting / Shading / Light Pre-Pass

The Killzone 2 team came up with an interesting way to use MSAA on the PS3. You can find it on page 39 of the following slides:

http://www.dimension3.sk/mambo/Articles/Deferred-Rendering-In-Killzone/View-category.php

What they do is read both samples in the multisampled render target, do the lighting calculations for both of them and then average the result and write it into the multi-sampled (... I assume it has to be multi-sampled because the depth buffer is multisampled) accumulation buffer. That somehow decreases the effectiveness of MSAA because the pixel averages all samples regardless of whether they actually pass the depth-stencil test. The multisampled accumulation buffer may therefore contain different values per sample when it was supposed to contain a unique value representing the average of all sample. Then on the other side they might only store a value in one of the samples and resolve afterwards ... which would mean the pixel shader runs only once.
This is also called "on-the-fly resolves".

It is better to write into each sample a dedicated value by using the sampling mask but then you run in case of 2xMSAA your pixel shader 2x ... DirectX10.1+ has the ability to run the pixel shader per sample. That doesn't mean it fully runs per sample. The MSAA unit seems to replicate the color value accordingly. That's faster but not possible on the PS3. I can't remember if the XBOX 360 has the ability to run the pixel shader per-sample but this is possible.

Multisample Anti-Aliasing

Utilizing the Multisample Anti-Aliasing (MSAA) functionality of graphics hardware for deferred lighting can be challenging. Nicolas Thibieroz wrote an excellent article about MSAA published in ShaderX7 with the title "Deferred Shading with Multisampling Anti-Aliasing in DirectX10".
The following figure from the ShaderX7 article shows how MSAA works:

The pixel represented by a square has two triangles (blue and yellow) crossing some of its sample points. The black dot represents the pixel sample location (pixel center); this is were the pixel shader is executed. The cross symbol corresponds to the location of the multisamples where the depth tests are performed. Samples passing the depth test receive the output of the pixel shader. Those samples are replicated by the MSAA back-end into a multisampled render target that represents each pixel with -in that case- four samples. That means the render target size for an intended resolution of 1280x720 would be 2560x1440 representing each pixel with four samples but the pixel shader only writes 1280x720 times (assuming there is no overdraw) while the MSAA back-end replicates for each pixel four samples into the multisampled render target.
With deferred lighting there can be several of those multi-sampled render targets as part of a Multiple-Render-Target (MRT). In the so called Geometry stage, data is written into this MRT; therefore called G-Buffer. In case of 4xMSAA each of the render targets of the G-Buffer would be 2560x1440 in size.
In case of Deferred Lighting / Light Pre-Pass the G-Buffer holds normal and depth data. This data can never be resolved because resolving it would lead to incorrect results as shown by Nicolas in his article.
After the Geometry phase comes the Lighting or Shading phase in a Deferred Lighting/Light Pre-Pass/Deferred Shading renderer. In an ideal world you could blit each sample (not pixel) into the multisampled render target -that holds the result of the Shading phase- by reading the G-Buffer sample and performing all the calculations necessary on it.
In other words to achieve the best possible MSAA quality with those renderer designs, lighting equations would need to be applied on a per-sample basis into a multisampled render target and then later resolved.
This is possible with DirectX 10.1 graphics hardware (AMD's 10.1 capable cards; didn't try if S3 cards that support 10.1 can do this as well) that allows to execute a pixel shader at sample frequency.
To make this a viable option, this operation needs to be restricted to samples that belong to pixel edges. There are two passes necessary to make this work. One pass will use the pixel shader that runs operations performed on samples and in a second pass the pixel shader is run that performs operations per-pixel, which means the result of the pixel shader calculation is output to all samples passing the depth-stencil test.
To restrict the pixel shader that performs operations per-sample, a stencil test is used.
One interesting idea covered in the article is to detect edges with centroid sampling (available already on DirectX9 class graphics hardware). During the G-Buffer phase the vertex shader writes a variable unique to every pixel (e.g. pixel position data) into two outputs, while the associated pixel shader declares two inputs: one without and one with centroid sampling enabled. The pixel shader then compares the centroid-enabled input with the one without it. Differing values mean that samples were only partially covered by the triangle, indicating an edge pixel. A "centroid value" of 1.0 is then written out to a selected area of the G-Buffer (previously cleared to 0.0) to indicate that the covered samples belong to an edge pixel. Those values are then averaged while being resolved to find out the value per pixel. If the result is not exactly 0, then the current pixel is an edge pixel. This is shown in the following image from the article.
On the left the pixel shader input will always be evaluated at the center of the pixel regardless of whether it is covered by the triangle. On the right with centroid sampling, the two rightmost depth samples are covered by the triangle. The comparison of the values in the pixel shader will lead to the result that the samples were only partially covered by the triangle, indicating an edge pixel.
Because DirectX10 capable graphics hardware does not support the pixel shader running at sample frequency, a different solution needs to be developed here.
The best MSAA quality in that case is achieved by running the pixel shader multiple times per pixel, only enabling output to a single sample each pass. This can be achieved by using the OMSetBlendState() API. The results of this method would be identical to the DirectX 10.1 method but obviously due to the increased number of rendering passes and slightly reduced texture cache effectiveness more expensive.

Deferred Lighting / Particle System

Here is a shot of a GPU based particle system with lights attached to each particle. I used Emil Persson's example Deferred Shading program as a basis to implement a Light Pre-Pass renderer with 4k lights and 4k particles. It runs fairly well on a GeForce 9600 GT here:

Light Pre-Pass: Knee-Deep

Several companies adopted the Light Pre-Pass idea, modified it or came up with similar ideas:

• Crytek: they call it Deferred lighting contrary to Deferred shading. The technique is mentioned in the new Cry Engine 3 presentation here
• Garagegames in their new Torque 3D engine currently in beta. Read the article from Pat Wilson in ShaderX7 and the garagegames website
• Insomniac came up with a Pre-lighting approach that is similar to this. See Mark Lee's presentation from GDC 2009 here
• DICE is using it since a long time already
• I believe EA used it in Dead Space :-)
• Carsten Dachsbacher described a similar idea in his article "Splatting of Indirect Illumination" here and in ShaderX5

One of the interesting areas in this context is the ability to implement a one-bounce global illumination effect with the data in the G-Buffer and the light buffer ...

3D Supershape

Over the last few years I was looking into the 3D Supershape formula described by Paul Bourke here and originally developed by Johan Gielis. I love the shape of the objects that are a result of those and therefore I always wanted to use it to create my own demos after I saw the one from Jetro Lauha (http://jet.ro/creations). Here is my first attempt to generate C source out of the equations:

Suitable C pseudo code could be:

float r = pow(pow(fabs(cos(m * o / 4)) / a, n2) + pow(fabs(sin(m * o / 4)) / b, n3), 1 / n1);

The result of this calculation is in polar coordinates. Please note the difference between the equation and the C code. The equation has a negative power value, the C doesn't. To extend this result into 3D, the spherical product of several superformulas is used. For example, the 3D parametric surface is obtained multiplying two superformulas S1and S2. The coordinates are defined by the relations:

The sphere mapping code uses two r values:

point->x = (float)(cosf(t) * cosf(p) / r1 / r2);
point->y = (float)(sinf(t) * cosf(p) / r1 / r2);
point->z = (float)(sinf(p) / r2);

Because r1 and r2 had a positive power value in the C code above we have to divide by those variables here. Here is a Mathematica render of this code:

Rockstar Games

Today GTA IV was launched a year ago and it is my last day where I am employed at Rockstar Games. After fantastic more than four years I felt like I should get a break to go back to some research topics and see my kids growing for a while :-), so I gave my notice two weeks ago.

Beagle Board

I got the whole development environment going and wrote a few small little graphics demos for it. All the PowerVR demos I tried ran on it nicely. Very cool!

If you are interested in a next-gen mobile development platform I would defitely recommend looking into this at

http://beagleboard.org/

Any further development has now moved to lowest priority ... maybe at some point I will play around more with Angstroem. There is an online image builder

http://amethyst.openembedded.net/~koen/narcissus/

BeagleBoard.org Ubuntu 8.04

In the last few days I setup a development environment for a BeagleBoard (see beagleboard.org). I wanted to hold the next-gen environment for future phones and the OpenPandora in my hands today. Overall the size of the board is astonishingly small and you can power it with the USB port. The board runs Angstroem -a Linux OS-, it has the OMAP3530 processor on there. It has a dedicated video decode DSP, the PowerVR SGX chipset, a sound chip and a few other things that I haven't used so far. You can even plug in a keyboard and a mouse and you have a full-blown computer with 256 MB RAM and 256 MB SDRAM.
To get this going I had to install a Linux OS on one of my PCs; Ubuntu 8.04. To relieve the pain of having to google all the Linux commands again and again I try to write down a few notes for myself here:
- minicom is not installed by default. You have to install it yourself. To do this you have to open up Applications -> Add/Remove and refresh the package list (you need an internet connection for this) and then install the build essentials first and then minicom by typing into a terminal:
sudo apt-get install build-essential
sudo apt-get install minicom
- to look for the RS232 serial device you can use
dmesg | grep tty
I found adding environment variables to the PATH statement different on Ubuntu 8.04. You can set an environment variable by using
export VARNAME=some_string
e.g
export PATH=$PATH:some/other/path
To check if it is set you can use
echo $PATH
For the PLATFORM you set it by typing
export PLATFORM=LinuxOMAP3
you use
echo $PLATFORM
to check if it is correct.
Similar for library pathes you type
export LIBDIR=$PWD
from the directory where the lib files are. To check that this works you can use
echo $LIBDIR
To make all those variable values persistent you can copy those statements at the end of the .bashrc file. Some other things I found convenient were:
gksudo gedit
start the editor with sudo.
Copying a file from one in another directory can be done by using the cp command like this
$ cp -i goulash recipes/hungarian
cp: overwrite recipes/hungarian/goulash (y/n)?
You can copy a directory path in the terminal by dragging the file from the file browser into the terminal command line.

ShaderX7 on Sale

ShaderX7 has more than 800 pages. I like the following screenshot from Amazon.com:
ShaderX8 is already announced. Proposals are due by May 19th, 2009. Please send them to wolf at shaderx.com. An example proposal, writing guidelines and a FAQ can be downloaded from www.shaderx6.com/ShaderX6.zip. The schedule is available on http://www.shaderx8.com/.

Thanks to Eric Haines for reminding me to add this to this page :-)

Mathematica

I switched from Maple to Mathematica last week. One of my small little projects is to store all the graphics algorithms I liked to visualize in the last few years in one file. A kind of condensed memory of the things I worked on. Here is an example for a simple Depth of Field effect (as already covered in my GDC 2007 talk):

Distance runs on the axis called Z value. So 0 is close to the camera and 1.0 is far away. You can see how the near and far blur plane fade in and out with increasing of the value called Range. The equation to plot this in mathematica is rather simple. In practice it is a quite efficient approach to achieve the effect.

Plot3D[R*Abs[0.5 - z], {z, 1, 0}, {R, 0, 1},
PlotStyle -> Directive[Pink, Specularity[White, 50], Opacity[0.8]],
PlotLabel -> "Depth of Field", AxesLabel -> {"Z value", "Range"}]

My plan is to develop a few new algorithms and show the results here. It will be an exercise in thinking about new things for me. If you have any suggestions on what I should cover, please do not hesitate to post them in the comment line.

Team Leadership in the Game Industry

A few of my friends contributed to the book "Team Leadership in the Game Industry" by Seth Spaulding II. So I was curious what you can write about leaders in this industry. Having spent most of my professional life outside of the game industry I believe I developed a different frame of reference than many of my colleagues.

First of all: the book is great and definitely worth a read. It is written in a very informative, instructive and entertaining way (... if you know the guys that contributed to it you know that it is worth it :-) ).

With that being said, let's start with the review by looking at the Table of Content. I know that I usually spent more time than other people with reading the TOC. This is the best way for me to figure out what a book has to offer. A good TOC shows you the big picture of a book and allows you to see the pattern that the author chose on how to approach the topic. In most cases it even allows you to proof the underlying logic.
The book consists of 9 chapters. Each chapter consists of a analysis of facts by the author followed by an interview of a game industry veteran. The topics span from "How We got here" over "Anatomy of a Game-Dev Company", "How Leaders are Chosen ...", "A Litmus Test for Leads", "Leadership Types and Traits ..." and then they go into more detail with the "The Project Team Leader ...", "The Department Leader ...", "Difficult Employees ...", "The Effect of Great Team Leadership" followed by a "Sample Skill Ladder" for artists in the appendix.

You might feel the need to discuss some of the details covered in each chapter but it is clear that this is the right formal approach to slice up the delicate topic of leadership in our industry.

When I first skipped through the book I wanted to figure out what kind of values the author has. After all a good leader makes it clear what kind of values he/she follows. I found it in the introduction. Here is the quote: "As will be seen, a major cause of people leaving a company is the perceived poor quality of their supervisors and senior management. The game business is a talent-based industry -the stronger and deeper your talent is, the better chances are of creating a great game. It is very difficult, in any hiring environment, to build the right mix of cross-disciplinary talent who function as a team at a high level; indeed, most companies never manage it. Once you get talented individuals on board, it's critical not to lose them. Finding and nurturing compentent leaders who have the trust of the team will generate more retention than any addition of pool tables, movie nights, or verbal commitments to the value of "quality of life"."
You might think this is the most obvious thing to say in the game industry.

Obviously the book wants to cover the process to setup a creative and great environment for all humans involved in the process of creating great games. Creating a great working environment starts with picking the right leaders that enable people by helping them to give their best. A great leader serves his/her people. He/she sees the best in everyone and has the ability to expose this talent. Many interviewees in the book also mention that humor is a leadership skill. I trained junior managers for BMW, Daimler, ABB and other companies back in Germany for two years on weekends and I always thought this is a strong skill. Making people laugh starts a lot of processes in the body that make people more relaxed and in general brighten up their day. Whoever can do this can certainly improve the morale and therefore efficiency of a team in seconds ... priceless.

Managing a creative team is a completely different story than -for example- a sales team. The human factor in the relationship between people plays an important role. They have to create something together, while a sales person is on his own out in the field and comes back with a number and relies on a relationship with a potential customer that only lasts a few hours face-to-face time, a creative team stays together for years and has to overcome all the things that come up when humans have to live in a small space together. There is a complex social network in place that defines the relationships between those humans and it is important to keep the team running with all the constantly changing love/hate -and in-between- relationships on board. People on the team might even deal with difficult personal relationships and you end up with a mixture of chaos and randomness typical for family or close friends scenarios. In that context it was interesting to see what the interviewees thought about the question if leaders are born and / or can be trained to be successful in the game industry. Obviously someone who was active as a boy-scout leader, speaker/president of the students association at his university or volunteered to work with other people in general, already showed some level of social committment that is a good starting point for a leader ship role in our industry.

So defining and following the right values is a fundamental requirement for a book on leadership. Obviously after having set the values comes the part where those values need to be applied and used and this is where the book shines. It is hands-down and even if you do not agree with the author in every detail the fact that he wrote all this down earns the highest respect.

So now that I made it obvious that I am excited about this book, let's think about how it might be improved in the future. A potential improvement I could see is to start the book with a target description. Not that the author fails to describe a target but I would appreciate it to go into more detail in this area.
What is the company you would want to work for? What is the environment you want to offer to make people as productive as possible? Obviously it is a hen / egg problem. Good people want to work in good teams and good teams consist of good people ... there are social -soft skills- and knowledge -hard skills- attached to each person of that team.
A good team starts with a good leader who sets values and standards and hires the right people.

Assuming you are the leader of this future team, how would you create the environment for your dream team? How do you want people to feel when they are part of this team? What should they take home every night when they are exhausted? What do you want them to tell their wifes / better halves how it is to work with you as their leader?
A happy employee -fully enforced to be creative :-) - should tell his wife/girlfriend that he works very hard but is treated fair and enjoys the family related benefits of the company.
He should tell his friends that he is working in a team where information is shared and where his potential is not only used as much as possible but also amplified. He needs to feel like he is growing with the team and the tasks.
He should tell his colleagues that he enjoys working with them and the team and that he enjoys coming into work every day and that he is excited about the project he is working on ...

So if we make that into a list of items we could describe how an employee should feel about working in a company with good Leaders. Might be a great starting point for discussing leader core abilities.

Larrabee on GDC

I am really looking forward to Mike Abrash's and Tom Forsyth's talks at GDC about Larrabee:

http://software.intel.com/en-us/articles/intel-at-gdc/

Talking about the Larrabee instruction set will be super cool ... can't wait to see this.

ShaderX7 Update

I updated the ShaderX7 website at

http://www.shaderx7.com/

There is now the first draft of the cover and the Table of Content. Enjoy! :-)

As before I will rest for a second when the new book comes out and think about what happened since I founded the series now eight years ago ... my perception of time slows down for this second :-) and I hear myself saying:"Chewbacca start the hyperdrive, let's go to the next planet, I need to play cards, drink alcohol and find some entertainment ... how about Tantoine?"

iP* programming tip #9

This issue of the iPhone / iPod Touch programmig tips series focuses on some aspects of VFP assembly programming. My friend Noel Llopis brought an oversight in the VFP math library to my attention, that I still need to fix. So I start with the description of the problem here and promise to fix it soon in the VFP library :-)
First let's start with the references. My friend Aaron Leiby has a blog entry on how to start programming the VFP unit here:

http://aleiby.blogspot.com/2008/12/iphone-vfp-for-n00bs.html

A typical inline assembly template might look like this:

asm ( assembler template
         : output operands                  /* optional */
         : input operands                   /* optional */
         : list of clobbered registers      /* optional */
         );

The last two lines of code hold the input and output operands and the so called clobbers, that are used to inform the compiler on which registers are used.
Here is a simple GCC assembly example -that doesn't use VFP assembly- that shows how the input and output operands are specified:

asm("mov %0, %1, ror #1" : "=r" (result) " : "r" (value));

The idea is that "=r" holds the result and "r" is the input. %0 refers to "=r" and %1 refers to "r".
Each operand is referenced by numbers. The first output operand is numbered 0, continuing in increasing order. There is a max number of operands ... I don't know what the max number is for the iPhone platform.

Some instructions clobber some hardware registers. We have to list those registers in the clobber-list, ie the field after the third ’:’ in the asm function. So GCC will not assume that the values it loads into these registers will be valid.
In other words a clobber list tells the compiler which registers were used but not passed as operands. If a register is used as a scratch register this register need to be mentioned in there. Here is an example:

asm volatile("ands    r3, %1, #3"     "\n\t"
          "eor     %0, %0, r3"     "\n\t"
          "addne   %0, #4"      
          : "=r" (len)        
          : "0" (len)         
          : "cc", "r3"
         );

r3 is used as a scratch register here. It seems the cc pseudo register tells the compiler about the clobber list. If the asm code changes memory the "memory" pseudo register informs the compiler about this.

asm volatile("ldr     %0, [%1]"         "\n\t"
           "str     %2, [%1, #4]"     "\n\t"
           : "=&r" (rdv)
           : "r" (&table), "r" (wdv)
           : "memory"
          );

This special clobber informs the compiler that the assembler code may modify any memory location. Btw. the volatile attribute instructs the compiler not to optimize your assembler code.

If you want to add something to this tip ... please do not hesitate to write it in the comment line. I will add it then with your name.

Partial Derivative Normal Maps

To make my collection of normal map techniques more complete on this blog I also have to mention a special normal mapping technique that Insomniac's Mike Acton brought to my attention a long time ago (I wasn't sure if I am allowed to publish it ... but now they have slides on their website).

The idea is to store the paritial derivate of the normal in two channels of the map like this

dx = (-nx/nz);

dy = (-ny/nz);

Then you can reconstruct the normal like this:

nx = -dx;

ny = -dy;

nz = 1;

normalize(n);

The advantage is that you do not have to reconstruct Z, so you can skip one instruction in each pixel shader that uses normal maps.

This is especially cool on the PS3 while on the XBOX 360 you can also create a custom texture format to let the texture fetch unit do the scale and bias and save a cycle there.

More details can be found at

http://www.insomniacgames.com/tech/articles/1108/files/Ratchet_and_Clank_WWS_Debrief_Feb_08.pdf

Look for Partial Derivative Normal Maps.

Handling Scene Geometry

I recently bumped into a post by Roderic Vicaire on the www.gamedev.net forums. It is here.
Obviously there is no generic solution to handle all scene geometry in the same way but depending on the game his naming conventions make a lot of sense (read "Scenegraphs say no" in Tom Forsyth's blog).
- SpatialGraph: used for finding out what is visible and should be drawn. Should make culling fast
- SceneTree: used for hierarchical animations, e.g. skeletal animation or a sword held in a character's hand
- RenderQueue: is filled by the SpatialGraph. Renders visible stuff fast. It sorts sub arrays per key, each key holding data such as depth, shaderID etc. (see Christer Ericson's blog entry "Sort based-draw call bucketing" for this)

Major Oolong Update

Two days ago I commited a major Oolong update. Please check out the Oolong Engine blog at

http://www.oolongengine.com

I updated the memory manager, the math library, upgraded to the latest POWERVR POD format and added to each example VBO support. Please also note that in previous updates a new memory manager was added, the VFP math library was added and a bunch of smaller changes were done as well.
The things on my list are: looking into the sound manager ... it seems like the current version allocates memory in the frame and adding the DOOM III level format as a game format. Obviously zip support would be nice as well ... let's see how far I get.

Programming Vertex, Geometry and Pixel Shaders

A christmas present: we just went public with "Programming Vertex, Geometry and Pixel Shaders". I am a co-author of this book and we published it free on www.gamedev.net at

http://wiki.gamedev.net/index.php/D3DBook:Book_Cover

If you have any suggestions, comments or additions to this book, please give me a sign or write it into the book comment pages.

Good Middleware

Kyle Wilson wrote up a summary about how good middleware should be:

http://gamearchitect.net/2008/09/19/good-middleware/

An interesting read.

Quake III Arena for the iPhone

Just realized that one of the projects I contributed some code to went public in the meantime. You can get the source code at

http://code.google.com/p/quake3-iphone/

There is a list of issues. If you have more spare time than me, maybe you can help out.

iP* programming tip #8

This is the christmas issue of the iPhone / iPod touch programming tips. This time we deal with the touch interface. The main challenge I found with the touch screen support is that it is hard to use it to track for example forward / backward / left / right and fire at the same time. Let's say the user presses fire and then he presses forward, what happens when he accidentally slides his finger a bit?
The problem is that each event is defined by the region it happens on the screen. When the user slides his finger, he is leaving this region. In other words if you handle on-screen touches as touch is on and finger lifted is off, if the finger is moved away and then lifted, the event is still on.
The work around is that if the user slides away with his finger the previous location of this finger is used to check if the current location is in the even region. If it is not, it defaults to switch off.
Touch-screen support for a typical shooter might work like this:
In touchesBegan, touchesMoved and touchesEnd there is a function call like this:

// Enumerates through all touch objects
for (UITouch *touch in touches)
{
[self _handleTouch:touch];
touchCount++;
}

_handleTouch might look like this:

- (void)_handleTouch:(UITouch *)touch
{
CGPoint location = [touch locationInView:self];
CGPoint previousLocation;

// if we are in a touchMoved phase use the previous location but then check if the current
// location is still in there
if (touch.phase == UITouchPhaseMoved)
previousLocation = [touch previousLocationInView:self];
else
previousLocation = location;

...
// fire event
// lower right corner .. box is 40 x 40
if (EVENTREGIONFIRE(previousLocation))
{
if (touch.phase == UITouchPhaseBegan)
{
// only trigger once
if (_bitMask ^ Q3Event_Fire)
{
[self _queueEventWithType:Q3Event_Fire value1:K_MOUSE1 value2:1];

_bitMask|= Q3Event_Fire;
}
}
else if (touch.phase == UITouchPhaseEnded)
{
if (_bitMask & Q3Event_Fire)
{
[self _queueEventWithType:Q3Event_Fire value1:K_MOUSE1 value2:0];

_bitMask^= Q3Event_Fire;
}
}
else if (touch.phase == UITouchPhaseMoved)
{
if (!(EVENTREGIONFIRE(location)))
{
if (_bitMask & Q3Event_Fire)
{
[self _queueEventWithType:Q3Event_Fire value1:K_MOUSE1 value2:0];

_bitMask^= Q3Event_Fire;
}
}
}
}
...

Tracking if the switch is on or off can be done with a bit mask. The event is send off to the game with a separate _queueEventWithType method.

iP* programming tip #7

This time I will cover Point Sprites in the iPhone / iPod touch programming tip. The idea is that a set of points -as the simplest primitive in OpenGL ES rendering- describes the positions of Point Sprites, and their appearance comes from the current texture map. This way, Point Sprites are screen-aligned sprites that offer a reduced geometry footprint and transform cost because they are represented by one point == vertex. This is useful for particle systems, lens flare, light glow and other 2-D effects.

• glEnable(GL_POINT_SPRITES_OES) - this is the global switch that turns point sprites on. Once enabled, all points will be drawn as point sprites.
• glTexEnvi(GL_POINT_SPRITES_OES, GL_COORD_REPLACE_OES, GL_TRUE) - this enables [0..1] texture coordinate generation for the four corners of the point sprite. It can be set per-texture unit. If disabled, all corners of the quad have the same texture coordinate.
• glPointParametervf(GLenum pname, const GLfloat * params) - this is used to set the point attenuation as described below.

The point size of a point sprite can be derived with the formula:
user_clamp represents GL_POINT_SIZE_MIN and GL_POINT_SIZE_MIN settings of the glPointParametervf(). impl_clamp represents an implementation-dependent point size range.
GL_POINT_DISTANCE_ATTENUATION is used to pass in params as an array containing the distance attenuation coefficients a, b, and c, in that order.
In case multisampling is used (not officially supported), the point size is clamped to have a minimum threshold, and the alpha value of the point is modulated by the following equation:
GL_POINT_FADE_THRESHOLD_SIZE specifies the point alpha fade threshold.
Check out the Oolong engine example Particle System for an implementation. It uses 600 point sprites with nearly 60 fps. Increasing the number of point sprites to 3000 lets the framerate drop to around 20 fps.

Free ShaderX Books

Eric Haines provided a home for the three ShaderX books that are now available for free. Thanks so much for this! Here is the URL

http://tog.acm.org/resources/shaderx/