Category Archives: WebGL

Double Precision in OpenGL and WebGL

This post talks about double precision numbers in OpenGL and WebGL, and how to emulate them if there is no native hardware support.

In GLSL 4.00.9 (which is OpenGL 4.0) and higher, there is a native double precision floating point type. And if your graphics card is able to run OpenGL 4.0, it most likely has native hardware support for doubles (except for a few ATI/AMD cards). There are some caveats, though:

  1. Not all functions are supported with double precision arguments. For instance, there are no trigonometric and exponential functions. (The available functions may be found here).
  2. You can not pass double precision ‘varying’ parameters from the vertex shader to the fragment shader, and have the GPU automatically interpolate them. Double precision varying variables must be flat.
  3. Double precision performance may be artificially limited by the hardware manufacturers. This is the case for Nvidia’s Fermi architecture, where the scientific computing brand, the Tesla series, can execute double precision arithmetics at half the speed of single precision, while the consumer brand, the GeForce series, only can execute double precision arithmetics at 1/8 the speed of single precision. For Nvidia’s brand new Kepler architecture used in the GeForce 600 series, things change again: here the difference between single and double precision will be a whopping factor 24! Notice, that this will also be the case for some cards in the Kepler Tesla branch, such as the Tesla K10.
  4. In Fragmentarium (and in general, in Qt’s OpenGL wrapper classes) it is not possible to set double precision uniforms. This should be easy to circumvent by using the OpenGL API directly, though.


(Non-related Fragmentarium image)

In order to use double precision, you must either specify a GLSL version 4.00 (or higher) or use the extension:

Older cards, like the GeForce 310M in my laptop, does not support double precision in hardware. Here it is possible to use emulated double precision instead.

I used the functions by Henry Thasler described here in his posts, to emulate a double precision number stored in two single precision floats. The worst part about doing emulated doubles in GLSL, is that GLSL does not support operator overloading. This means the syntax gets ugly for simple arithmetics, e.g. ‘z = add(mul(z1,z2),z3)’ instead of ‘z = z1*z2+z3′.

On Nvidia cards, it is necessary to turn off optimization to use Thasler’s code – this can be done using the following pragmas:


(Non-related Fragmentarium image)

Performance

To test performance, I used a Mandelbrot test scene, rendered at 1000×500 with 1000 iterations in Fragmentarium. The numbers show the performance in frames per second. The zoom factor was determined visually, by noticing when pixelation occurred.

Geforce 570GTX Tesla 2075 Max Zoom
(~300USD) (~2200USD)
Single 140 100 10^5
Double 41 70 10^14
Emulated Double 16 11 10^13

Some observations:

  • Emulated double precision is slightly less accurate then true hardware doubles, but not much in this particular scenario.
  • Emulated doubles are roughly 1/9th the speed of single precision. Amazingly, this suggest that on the Kepler architecture it might make more sense to use emulated double precision than the built-in hardware support!
  • Hardware doubles on the 570GTX performs better than expected (they should perform at roughly 1/8 the speed). This is probably because double precision arithmetics isn’t the only bottleneck in the shader.

Notice that the Tesla card was running on Windows in WDDM mode, not TCC mode (since you cannot use GLSL shaders in TCC mode). Not that I think performance would change.

WebGL and double precision

WebGL does not support double precision in its current incarnation. This might change in the future, but currently the only choice is to emulate them. This, however, is problematic since WebGL seems to strip away pragmas! Henry Thasler’s emulation code doesn’t work under the ANGLE layer either. In fact, the only configuration I could get to work, was on a Intel HD 3000 GPU with ANGLE disabled. I did create a sample application to test this which can be tried out here:
Click to run WebGL app. Left side is single-precision, right side is emulated double precision. Here shown on Firefox without ANGLE on a Intel HD 3000 card.

It is not clear why the WebGL version does not work on Nvidia cards. Floating points may run at lower resolution in WebGL, but I’m using the ‘precision highp’ qualifiers. I also tried querying the resolution using glContext.getShaderPrecisionFormat(…), but had no luck – it is only available on Firefox, and on my GPU’s it just returns precision=0.

The most likely explanation is that Nvidia drivers perform some optimizations which spoils the emulation code. This is also the case for desktop OpenGL, but here the pragma’s solve the problem.

The emulation code uses constructs like:

z = a - (a - b);

which I suspect the well-meaning compiler might translate to ‘z=b’, since the rounding errors normally would be insignificant. Judging from some comments on Thasler’s original posts, it might be possible to prevent this using constructs such as: ‘z = a – float(a-b)’, but I have not pursued this.

Fragmentarium and Double Precision

Except that there are no double-precision sliders (uniforms), it is straight-forward to use double precision code in Fragmentarium. The only thing to remember is that you cannot pass doubles from the vertex shader to the fragment shader, which is the standard way of passing camera information to the shader in Fragmentarium.

I’ve also included a small port of Thaslers GLSL code in the distribution (see “Include/EmulatedDouble.frag”). It is quite easy to use (for an example, try the included “Theory/Mandelbrot – Emulated Doubles.frag”).

WebGL for Shader Graphics

Web applications are becoming popular, not at least because of Google’s massive effort to push everything through the browser (with Chrome OS being the most extreme example, where everything is running through a browser interface).

Before WebGL, the only way to create efficient graphics was through plug-ins, such as Adobe’s Flash, Microsoft’s Silverlight, Unity, or Google’s O3D and Native Client. But WebGL is a vendor independent technology, directly integrated with the browser’s JavaScript language and DOM model.

Unfortunately, WebGL browser support is limited. WebGL is not available in Internet Explorer on Windows, and is not enabled by default in Safari on Mac OS X. This means that roughly 50% of all internet users won’t have access to WebGL content. WebGL is not supported on iOS devices either (even though it is accessible for iAds, and can be enabled on jail-broken devices).

What is worse, is that Microsoft do not even plan to support WebGL, since they consider it a security threat. Their concerns are reasonable, but their solution is not: it would be much better if they simply showed a dialog box message, warning the user that executing WebGL provides a security risk, and giving a choice to continue or not – the same way they warn about plugins and downloaded executables.

Some very impressive stuff has been done using WebGL, though: for instance ro.me, Path Tracing (Evan Wallace) , Cars (Altered Qualia), Terrain Editor (Rob Chadwick), Traveling Wavefronts (Felix Woitzel), Hartverdrahtet.

Using WebGL for Fractals

There are already some great tools available for experimenting with WebGL: ShaderToy, GLSLSandbox, WebGL Playground. Their main weakness is that it is difficult to store state information (for instance, if you want a movable camera), since this cannot be done in the shader itself, without using weird hacks. So, I decided to start out from scratch to get a feeling for WebGL.

WebGL (specification) is a JavaScript API based on OpenGL ES 2.0, a subset of the desktop OpenGL version designed for embedded devices such as cell phones.

Being a ‘modern’ OpenGL implementation, there is no support for fixed pipeline rendering: there is no matrix stack, no default shaders, no immediate mode rendering (you cannot use glBegin(…) – instead you must use vertex buffers). WebGL also misses some of more advanced features of the desktop OpenGL version, such as 3D textures, multiple render targets, and double precision support. And float texture support is an optional extension.

The first example I made was this Mandelbrot viewer: It demonstrates how to initialise WebGL and compile shaders, render a full-canvas quad, and process keyboard and mouse events and pass them through uniforms to the fragment shader.
Click the image to try out the WebGL demo.

A few programming comments. First JavaScript: I’m not very fond of JavaScript’s type system. The loose typing means that you risk finding bugs later, at run-time, instead of when compiling. It also means that it can be hard to read third-party code (which kind of parameters are you supposed to provide to a function like ‘update(ev, ui)’?). As for numerical types, JavaScript only has the Number type: an IEEE 754 double precision type – no integers!. Some browsers also silently ignore errors during run-time, which makes it even harder to find bugs. On the positive side is the quick iteration time, and the Firebug Firefox plugin, which is an extremely powerful tool for debugging web and JavaScript code.

As for the HTML, I still find it difficult to do table-less layout using floating div’s and css. I’m missing the flexible layout managers that many desktop UI kits provide, which makes it easy to align components and control how they scale when resized (but I may be biased towards desktop UI’s). Also, as HTML was not designed with UI widgets in mind, you have to use a third-party library to display a simple slider: I chose jQuery UI, which was easy to setup and use.

Finally the WebGL: The WebGL GLSL shader code is very similar to the desktop GLSL dialect. The biggest difference is the way loops are handled. Only ‘for’ loops are available, and with a very restricted syntax. It seems the iteration count must be determinable at compilation time (probably because some implementations unroll all loops), which means you no longer can use uniforms to control the loops (you can, however, ‘break’ out of loops dynamically based on run-time variables). This means, that in order to pass the iteration count and number of samples to the Mandelbrot shader, I have to do direct text substitutions in the shader code and recompile.

But my biggest frustation was caused by the ANGLE translation layer. Even for this very simple example, I had several issues with ANGLE – see the notes below.

Feel free to use the example as a starting point for further experiments – it is quite simple to modify the 2D shader code.

Notes about ANGLE

A problem with WebGL is poor graphics driver support for OpenGL. Chrome and Firefox have chosen a very radical approach to solve this: on Windows, they convert all WebGL GLSL shader code into DirectX 9 HLSL code through a converter called ANGLE. Their rationale for doing this, is that OpenGL 2.0 drivers are not available on all computers. However, several shaders won’t run due to the ANGLE translation, and the compilation time can be extremely slow. Wrt drivers, older machines with integrated graphics might be affected, but anything with a less than five year old Nvidia, AMD, or Intel HD graphics card should work with OpenGL 2.0.

In my experiments above, I ran into a bug that in some cases make loops with more than 255 iterations fail (I’ve submitted a bug report).

When debugging ANGLE problems, a good first step is to disable ANGLE and test the shaders. In Chrome, this can be done by starting the executable with the comand line argument –use-gl=desktop. You can check your ANGLE version with the URL chrome://gpu-internals/. In Firefox use the about:config URL, and webgl.force-enabled=true and webgl.prefer-native-gl=true to disable ANGLE.

It is also possible to get the translated HLSL code using the WEBGL_debug_shaders extension. However, this extension is only available for privileged code, which means Chrome must be started with the command line parameter –enable-privileged-webgl-extensions. After that the HLSL source can be obtained by calling:

I still haven’t found an workaround for this earlier Mandelbulb experiment (using GLSLSandbox), which fails with another ANGLE bug:
Click the image to try out the WebGL demo (fails on ANGLE systems).

But, I’ll try implementing it from scratch to see if I can find the bug.

Assorted Links

Pixel Bender 3D

Adobe has announced Pixel Bender 3D:


If I understand it correctly, it is a new API for Flash, and not as such a direct extension of the Pixel Bender Toolkit. So what does it do?

As far as I can tell, it is simply a way to write vertex and fragment shader for Flash. While this is certainly nice, I think Adobe is playing catchup with HTML5 here – many browsers already support custom shaders through WebGL (in their development builds, at least). Or compare it to a modern 3D browser plugin such as Unity, with deferred lightning, depth-of-field, and occlusion culling…

And do we really need another shader language dialect?

Flash raytracer

Kris Temmerman (Neuro Productions) has created a raytracer in Flash, complete with ambient occlusion and depth-of-field:

Kris has also produced several other impressive works in Flash:

Chaos Constructions

Quite and Orange won the 4K demo at Chaos Constructions 2010 with the very impressive ‘CDAK’ demo:

(Link at Pouet, including executable).

Ex-Silico Fractals

This YouTube video shows how to produce fractals without a computer. I’ve seen video feedback before, but this is a clever setup using multiple projectors to create iterated function systems.


Vimeo Motion Graphics Award

‘Triangle’ by Onur Senturk won the Vimeo Motion Graphics Award. The specular black material looks good. Wonder if I could create something similar in Structure Synth’s internal raytracer?

Shader Toy

For some time I’ve been wanting to play around with pixel (fragment) shaders, but I couldn’t find a proper playground.

Then I stumbled upon Shader Toy, by Inigo Quilez (whom I’ve mentioned several times on this blog). A couple of things make Shader Toy stand out:

It runs inside your browser. It uses the emerging WebGL standard, which is JavaScript bindings for OpenGL (ES) 2.0. OpenGL can be used directly inside a Canvas HTML element, including support for custom shaders. As Shader Toy demonstrates, this makes it possible to do some very impressive stuff, such as real-time GPU-accelerated raytracing inside an element on a web page.

The examples are great. While Shader Toy itself is mostly a thin wrapper around the WebGL functionality, the great thing about it is the example shaders: 2D fractals and Demo Scene effects, but also complex examples like the Slisesix 4K demo, and examples of raytracing, and complex fractals, like the Quaternion Julia set, and the Mandelbulb.

The only problem with WebGL is, that it is not supported by the current generation of browsers.

The good news is that the nightly builds of Firefox, Safari (WebKit), and Chromium (Google Chrome) all support it, and are quite easy to install: this is a good place for more information. If you use the Chromium builds, you don’t have to worry about messing up your existing browser configuration – the nightly builds are standalone versions and can be run without installation.

There are lots of complex shader tools out there: for instance, NVIDIAs FX Composer, AMDs Rendermonkey, TyphoonLabs OpenGL Shader Designer, and Lumina, but Shader Toy makes it very easy to get started with shaders. And it provides a rare insight into how those amazing 4K demos were made.