October 2011 Posts

Terrainicles [WebGL & HaXe]

I have been playing with this thing, tweaking it, making changes for weeks. Theres so many different things I want to add. Different options, scenarios, optimisations etc. I decided however just to follow the ‘release early and often’ mantra and get this thing out now.

Before I go any further check out what im talking about here:

http://mikecann.co.uk/projects/WebGLTerrainicles/
(You will need a WebGL compatible browser, that means no IE) 

Its a continuation of my earlier work on GPU particles using WebGL and HaXE. Im trying to emulate some work I did years ago in XNA, on the LieroXNA project.

It uses the same techniques for updating and rendering particles entirely on the GPU as my previous post. What that means is that is possible have millions of particles interacting updating and rendering simultaneously as all the operations are performed on the GPU.

What I have added this time is another texture for the particles to collide with as they move. I was originally working with the same dirt and grass texture as my XNA project but I thought as it was Halloween I would get into the spirit a little ;)

There are several options on the right hand side that can be used to tweak the properties of the simulation. I spent alot of time playing around with these, there are some really cool effects you can achieve with just simple modifications.

There are so many things I could add to improve this. You can see some of them in a video I made years ago:

There we have some cool stuff like Bloom, forces and multiple rendering layers going on. It would be nice to get those in this sample too.

For now however I think im going to have a break from this sample. I have spent quite a few weeks to get to this point so far, and I think I need a break for a little bit so I can work on other things. I may come back to it soon tho If people are interested or if (probably more likely) I think of some ‘cool new thing’ that will ‘only take 5 mins’.

I have uploaded the source for this sample to Github for people to lookat/fork if they wish:

https://github.com/mikecann/WebGLTerrainicles

Enjoy!

 

 

 

Why Developing for WebGL Sucks!

For some time now I have been working with WebGL and have developed a sort of love/hate relationship with it. I love the ability to instantly target millions of people with GPU accelerated code without any plugins or barriers (excluding the targets that dont support it). However as a developer, writing code that takes advantage of WebGL kinda sucks.

Procedural Based

First off is the way you have to structure your GL calls. For example take a look at the following generic bit of webGL harvested from the net:

[codesyntax lang="javascript" lines="normal"]

texture = gl.createTexture();
   gl.activeTexture(gl.TEXTURE0);
   gl.bindTexture(gl.TEXTURE_2D, texture);
   gl.pixelStorei(gl.UNPACK_ALIGNMENT, 1);
   gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGB, 64, 64, 0,
     gl.RGB, gl.FLOAT, new Float32Array(pix));
   gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.NEAREST);
   gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.NEAREST);

   texture1 = gl.createTexture();
   gl.activeTexture(gl.TEXTURE1);
   gl.bindTexture(gl.TEXTURE_2D, texture1);
   gl.pixelStorei(gl.UNPACK_ALIGNMENT, 1);
   gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGB, 64, 64, 0,
     gl.RGB, gl.FLOAT, new Float32Array(pix1));
   gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.NEAREST);
   gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.NEAREST);

   FBO = gl.createFramebuffer();
   gl.bindFramebuffer(gl.FRAMEBUFFER, FBO);
   gl.framebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0,
       gl.TEXTURE_2D, texture, 0);
   FBO1 = gl.createFramebuffer();
   gl.bindFramebuffer(gl.FRAMEBUFFER, FBO1);
   gl.framebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0,
       gl.TEXTURE_2D, texture1, 0);
   if( gl.checkFramebufferStatus(gl.FRAMEBUFFER) != gl.FRAMEBUFFER_COMPLETE)
     alert(err + "FLOAT as the color attachment to an FBO");

[/codesyntax]

All it does is create a couple of textures, setting their starting values and creates two frame buffers for rendering to. Its just it looks complicated and difficult to understand.

GL works on a procedural basis, so you tell GL that you are about to work on something by calling a function like “bindTexture()” then on the next line you perform an operation on it such as “pixelStorei()”. Now this may have made perfect sense back when we were writing everything in C which is procedural anyway however this is Javascript (or haXe in my case) which is an Object based language, code like this is difficult to understand and follow.

The procedural nature of WebGL means you have to be much more careful about unsetting things you have previously set. For example if you bind a texture to perform some operation, you must then remember to unbind it else you could inadvertently cause operations to be applied to it on subsequent calls elsewhere in your codebase. It this ‘hidden state’ that has caused me alot of headaches when developing my samples.

The principal behind WebGL was to provide a very low-level library which other developers can build upon to build more complex and abstracted libraries. And there are numerous libraries out there. I personally have tried several of them including the very popular three.js. Three.js is great for doing common things like loading models and putting quads on the screen. I however encountered a problem with render targets which I struggled with for days before I discovered that you had to set “needsUpdate” to true on your texture before using it. In the end I decided to drop three.js beacuse of another issue I encountered and instead attempt to reduce my complications by working with webGL directly.

Flash11′s Stage3D has the same philosophy as webGL, to provide a low level API for other developers to build libraries upon. The thing is Flash11′s low-level API makes more sense and is more readable. For example the following to me is much more readable than its webGL equivalent:

[codesyntax lang="actionscript3" lines="normal"]

texture = c.createTexture(logo.width, logo.height, Context3DTextureFormat.BGRA, false);
texture.uploadFromBitmapData(logo);

[/codesyntax]

The Stage3D API also uses language like “upload” to let you know when you are transferring data to the GPU, for a new comer to GL you have no clue when things are going to the GPU. Its small things like this that reduce the “WTF?” factor when tackling the tricky world of hardware-accelerated 3D programming.

Cross-domain textures

This one cropped up around July time this year and took me ages to work out what was going on. For some inexplicable reason (or so it seemed) my code one day stopped working. When I looked for demo code online it all worked fine, however when I downloaded it and run it locally it also didnt work. I was getting errors like the following:

Uncaught Error: SECURITY_ERR: DOM Exception 18
Uncaught Error: SECURITY_ERR: DOM Exception 18
Uncaught Error: SECURITY_ERR: DOM Exception 18
Uncaught Error: SECURITY_ERR: DOM Exception 18
Uncaught Error: SECURITY_ERR: DOM Exception 18 

I was so baffled that I posted about it on the HaXe mailing list asking for help, thinking it was something I was doing wrong with HaXe. It turns out (after much wall-head-butting) this was a change that they brought into Chrome 13 and Firefox 5 to combat a security problem when using shaders that use textures from a different domain to the one running the code:

http://blog.chromium.org/2011/07/using-cross-domain-images-in-webgl-and.html

Now I have no problem with cross-domain issues, im used to this from Flash where we have the same sort of setPixel() restrictions on cross-domain BitmapData’s. The thing is, it appears that this restriction applies when running code locally too. So If you are developing something on your local machine and trying to access a texture from disk it throws the same security errors because the browser thinks you are reaching across domains to access the image.

At the time the only way to get around this was to create your own webserver that you run on localhost to server up the files. So to do that I had to download python so I could run a simple localhost commandline webserver from my bin directory. What an effort! There may be easier ways these days to solve it but at the time it really frustrated me and formed yet another barrier to developing webGL.

No Error Messages

This is by far the most annoying thing about developing for WebGL. So many times I have been trying to write something that I know SHOULD work but for some reason it doesn’t. I dont get any error messages, nothing. It makes writing something from scratch neigh on impossible.

In my last post “GPU State Preserving Particle Systems with WebGL & HaXe” I started with an idea. I attempted to code it ‘bottom-up’. That is start with nothing and then add more code until I reached what I wanted. Unfortunately having no error messages in WebGL makes this very difficult indeed. I would spend some time writing something really simple, like trying to get a textured quad to render on the screen only to find I get nothing. I double check my camera matrices my vertex and texture buffers, my shader and still nothing. Eventually I found that I hadn’t bound something first before trying to operate on it *sigh*

In the end I found the best way to get anywhere is to go from the other direction, a ‘top-down’ method. Start with something kind of simmilar to what you want then cut bits out one line at a time until you get what you want. Its extremely time consuming and frustrating, but its less frustrating than going from the other way.

There are tools out there that help with debugging what is going wrong. Namely the WebGL Inspector (see above) is intended to provide gDEBugger / PIX like debugging information about what is going on inside webGL. Its a clever bit of tech, it lets you inspect buffers and traces each gl call, however it suffers from the same underlying problem of having no errors. You setup a buffer incorrectly and what you get is “INVALID_VALUE”. No indication as to which of the values is invalid or what part of the call you messed up on :(

Googling Doesn’t Help

If you do happen to get an error message (unlikely) or you word your problem in a sufficiently succinct and googaleble way you will then run into the next big problem with WebGL; theres very few people using it. Now I know I am likely to be flamed for that comment, but it just seems that way to me. Whenever I tried to google my problem, or google for what I was trying to achieve (because working bottom-up doesnt work) there would be a very sparse smattering of relevant posts. Usually the results are forum posts and are OpenGL not WebGL related and are from 5-10 years ago.

But..

Now having just ranted on for several hundred words about why it sucks im going to finish it off by saying that im going to continue to develop for WebGL using haXe regardless. Why? Well I just like making pretty things that run fast and GPGPU programming appeals to me for some unknown (likely sadistic) reason.

GPU State Preserving Particle Systems with WebGL & HaXe

Well this is the post I didnt think was going to happen. I have been struggling for weeks with this little bit of tech, ill explain more about why it has been so difficult in another post. For now however, ill just talk about this sample.

So the idea was to build upon what I had been working with previously with my stateless particles systems with WebGL and HaXe. The intention from the start was to replicate some of my very early work (from 2007) on state preserving particle systems in WebGL.

Before I go any further, you can check it out in action here:
http://mikecann.co.uk/projects/HaxeWebGLParticles/ 

First a quick reminder. The difference between a stateless and state-preserving particle simulation is that in the latter we store and update the positions, velocities and other properties of each particle per frame, allowing us to interact and control the simulation. This differs from the stateless particle simulation (detailed in my previous post), where the position for each particle is calculated each frame based on a fixed algorithm.

A fairly reccent addition to WebGL made this possible, namely texture lookups in the vertex shader (aka Vertex Texture Fetch). I wont go over the exact details how this makes state preserving particle systems possible as I have already documented it in my earlier work. A brief explanation is that it allows you to use the fragment shader to perform the updates on particle state stored in textures then use the vertex shader to map those states to a point-sprite vertex buffer.

Basically what this means is that the entire particle simulation can be contained and updated on the GPU, which means no read-back. This allows us to achieve simulations of millions of particles without too much difficulty (depending on GPU ofcourse).

I have uploaded the source for people to perouse at their leisure:
https://github.com/mikecann/HaxeWebGLParticles

As usual it was written using the JS target of HaXe so it should be fairly easy to understand whats going on if you have written any Ecma-script-like-language. Im going to detail this in my next post, but the code isnt the best I have ever written as its a result of a mish-mash of various samples and examples I have found on the web. If anyone has any comments on things that are wrong or could be done better I would be very happy to hear about them.