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by Patrick Cozzi, who works on the Cesium WebGL engine.

With the new shader editor in Firefox 27 (available now in Aurora), WebGL tools are taking a big step in the right direction. This article reviews the current state of WebGL debugging and profiling tools with a focus on their use for real engines, not simple demos. In particular, our engine creates shaders dynamically; uses WebGL extensions like Vertex Array Objects; dynamically creates, updates, and deletes 100’s of MB of vertex buffers and textures; renders to different framebuffers; and uses web workers. We’re only interested in tools that provide useful results for our real-world needs.

Firefox WebGL Shader Editor

The Firefox WebGL Shader Editor allows us to view all shader programs in a WebGL app, edit them in real-time, and mouse over them to see what parts of the scene were drawn using them.

What I like most about it is it actually works. Scenes in our engine usually have 10-50 procedurally-generated shaders that can be up to ~1,000 lines. The shader editor handles this smoothly and automatically updates when new shaders are created.


The skybox shader is shown in the editor and the geometry is highlighted in red. (Click on any image for its full-screen version.)

I was very impressed to see the shader editor also work on the Epic Citadel demo, which has 249 shaders, some of which are ~2,000 lines.


Live editing is, of course, limited. For example, we can’t add new uniforms and attributes and provide data for them; however, we can add new varying variables to pass data between vertex and fragment shaders.

Given that the editor needs to recompile after our edits, attribute and uniform locations could change, e.g., if uniforms are optimized out, which would break most apps (unless the app is querying these every frame, which is a terrible performance idea). However, the editor seems to handle remapping under-the-hood since removing uniforms doesn’t break other uniforms.

Recompiling after typing stops works well even for our large shaders. However, every editor I see like this, including JavaScript ones we’ve built, tends to remove this feature in favor of an explicit run, as the lag can otherwise be painful.

There are some bugs, such as mousing over some shaders causes artifacts or parts of the scene to go away, which makes editing those shaders impossible.


Even though this is in a pre-beta version of Firefox, I find it plenty usable. Other than spot testing, I use Chrome for development, but this tool really makes me want to use Firefox, at least for shader debugging.

We planned to write a tool like this for our engine, but I’m glad the Mozilla folks did it instead since it benefits the entire WebGL community. An engine-specific tool will still be useful for some. For example, this editor uses the shader source provided to WebGL. If a shader is procedurally-generated, an engine-specific editor can present the individual snippets, nodes in a shade tree, etc.

A few features that would make this editor even better include:

  • Make boldface any code in #ifdef blocks that evaluate to true. This is really useful for ubershaders.
  • Mouse over a pixel and show the shader used. Beyond debugging, this would be a great teaching aid and tool for understanding new apps. I keep pitching the idea of mousing over a pixel and then showing a profile of the fragment shader as a final project to my students, but no one ever bites. Easy, right?
  • An option to see only shaders actually used in a frame, instead of all shaders in the WebGL context, since many shaders can be for culled objects. Taking it a step further, the editor could show only shaders for non-occluded fragments.

For a full tutorial, see Live editing WebGL shaders with Firefox Developer Tools.

WebGL Inspector

The WebGL Inspector was perhaps the first WebGL debugging tool. It hasn’t been updated in a long time, but it is still useful.

WebGL Inspector can capture a frame and step through it, building the scene one draw call at a time; view textures, buffers, state, and shaders; etc.

The trace shows all the WebGL calls for a frame and nicely links to more info for function arguments that are WebGL objects. We can see the contents and filter state of textures, contents of vertex buffers, and shader source and current uniforms.



One of WebGL Inspector’s most useful features is highlighting redundant WebGL calls, which I use often when doing analysis before optimizing.


Like most engines, setting uniforms is a common bottleneck for us and we are guilty of setting some redundant uniforms for now.

WebGL Inspector may take some patience to get good results. For our engine, the scene either isn’t visible or is pushed to the bottom left. Also, given its age, this tool doesn’t know about extensions such as Vertex Array Objects. So, when we run our engine with WebGL Inspector, we don’t get the full set of extensions supported by the browser.

The WebGL Inspector page has a full walkthrough of its features.

Chrome Canvas Inspector

The Canvas Inspector in Chrome DevTools is like a trimmed-down WebGL Inspector built right into Chrome. It is an experimental feature but available in Chrome stable (Chrome 31). In chrome://flags/, “Enable Developer Tools experiments” needs to be checked and then the inspector needs to be explicitly enabled in the DevTools settings.

Although it doesn’t have nearly as many features as WebGL Inspector, Canvas Inspector is integrated into the browser and trivial to use once enabled.


Draw calls are organized into groups that contain the WebGL state calls and the affected draw call. We can step one draw group or one WebGL call at a time (all WebGL tracing tools can do this). The scene is supposed to be shown one draw call at a time, but we currently need to turn off Vertex Array Objects for it to work with our engine. Canvas Inspector can also capture consecutive frames pretty well.

The inspector is nicely integrated into the DevTools so, for example, there are links from a WebGL call to the line in the JavaScript file that invoked it. We can also view the state of resources like textures and buffers, but not their contents or history.

Tools like WebGL Inspector and Canvas Inspector are also useful for code reviews. When we add a new rendering feature, I like to profile and step through the code as part of the review, not just read it. We have found culling bugs when stepping through draw calls and then asking why there are so many that aren’t contributing to any pixels.

For a full Canvas Inspector tutorial, see Canvas Inspection using Chrome DevTools.

Google Web Tracing Framework

The Google Web Tracing Framework (WTF) is a full tracing framework, including support for WebGL similar to WebGL Inspector and Canvas Inspector. It is under active development on github; they addressed an issue I submitted in less than a day! Even without manually instrumenting our code, we can get useful and reliable results.

Here we’re stepping through a frame one draw call at a time:


For WebGL, WTF has similar trace capability as the above inspectors, combined with all its general JavaScript tracing features. The WebGL trace integrates nicely with the tracks view.


Above, we see the tracks for frame #53. The four purple blocks are texture uploads using texSubImage2D to load new imagery tiles we received from a web worker. Each call is followed by several WebGL state calls and a drawElements call to reproject the tile on the GPU (see World-Scale Terrain Rendering from the Rendering Massive Virtual Worlds SIGGRAPH 2013 course). The right side of the frame shows all the state and draw calls for the actual scene.

Depending on how many frames the GPU is behind, a better practice would be to do all the texSubImage2D calls, followed by all the reprojection draw calls, or even move the reprojection draw calls to the end of the frame with the scene draw calls. The idea here is to ensure that the texture upload is complete by the time the reprojection draw call is executed. This trades the latency of completing any one for the throughput of computing many. I have not tried it in this case so I can’t say for certain if the driver lagging behind isn’t already enough time to cover the upload.


The tracks view gets really interesting when we examine slow frames highlighted in yellow. Above, the frame takes 27ms! It looks similar to the previous frame with four texture uploads followed by drawing the scene, but it’s easy to see the garbage collector kicked in, taking up almost 12ms.


Above is our first frame, which takes an astounding 237ms because it compiles several shaders. The calls to compileShader are very fast because they don’t block, but the immediate call to linkProgram needs to block, taking ~7ms for the one shown above. A call to getShaderParameter or getShaderInfoLog would also need to block to compile the shader. It is a best practice to wait as long as possible to use a shader object after calling compileShader to take advantage of asynchronous driver implementations. However, testing on my MacBook Pro with an NVIDIA GeForce 650M did not show this. Putting a long delay before linkProgram did not decrease its latency.

For more details, see the WTF Getting Started page. You may want to clear a few hours.

More Tools

The WebGL Report is handy for seeing a system’s WebGL capabilities, including extensions, organized by pipeline stage. It’s not quite up-to-date with all the system-dependent values for the most recent extensions, but it’s close. Remember, to access draft extensions in Chrome, we need to explicitly enable them in the browser now. For enabling draft extensions in Firefox you need to go to “about:config” and set the “webgl.enable-draft-extensions” preference to true.


The simple Chrome Task Manager (in the Window menu) is useful for quick and dirty memory usage. Make sure to consider both your app’s process and the GPU process.


Although I have not used it, webgl-debug.js wraps WebGL calls to include calls to getError. This is OK for now, but we really need KHR_debug in WebGL to get the debugging API desktop OpenGL has had for a few years. See ARB_debug_output: A Helping Hand for Desperate Developers in OpenGL Insights.

There are also WebGL extensions that provide debugging info to privileged clients (run Chrome with –enable-privileged-webgl-extensions). WEBGL_debug_renderer_info provides VENDOR and RENDERER strings. WEBGL_debug_shaders provides a shader’s source after it was translated to the host platform’s native language. This is most useful on Windows where ANGLE converts GLSL to HLSL. Also see The ANGLE Project: Implementing OpenGL ES 2.0 on Direct3D in OpenGL Insights.

The Future

The features expected in WebGL 2.0, such as multiple render targets and uniform buffers, will bring us closer to the feature-set OpenGL developers have enjoyed for years. However, API features alone are not enough; we need an ecosystem of tools to create an attractive platform.

Building WebGL tools, such as the Firefox Shader Editor and Chrome Canvas Inspector, directly into the browser developer tools is the right direction. It makes the barrier to entry low, especially for projects with limited time or developers. It helps more developers use the tools and encourages using them more often, for the same reason that unit tests that run in the blink of an eye are then used frequently.

The current segmentation of Google’s tools may appear confusing but I think it shows the evolution. WebGL Inspector was first out of the gate and proved very useful. Because of this, the next generation version is being built into Chrome Canvas Inspector for easy access and into the WTF for apps that need careful, precise profiling. For me, WTF is the tool of choice.

We still lack a tool for setting breakpoints and watch variables in shaders. We don’t have what NVIDIA Nsight is to CUDA, or what AMD CodeXL is to OpenCL. I doubt that browser vendors alone can build these tools. Instead, I’d like to see hardware vendors provide back-end support for a common front-end debugger built into the browser.

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Due to a long plane flight, this week I started reading up on WebGL in earnest, going beyond the various tutorials on the web. The two books recommended to me that I’ve started on (and skimmed through in their entirety – I need to sit down with each in front of a computer):

WebGL: Up and Running, by Tony Parisi, O’Reilly Press, August 2012. More info about the book here, including a sample. I recall Tony from way back in the days of VRML, he had one of the first viewers for that file format, so he’s been working in this area of 3D graphics for the web for a long time. This looks to be a good book if your goal is indeed to get something on the screen fast. It’s readable, and I appreciate his use of URLs to WebGL demos and resources and (properly credited) Wikimedia images. The code samples for the book are here on github.

It uses the popular three.js library to insulate you from the OpenGL ES roots of WebGL and hide its raw API nature. The author of three.js, Mr.doob (I don’t make these names up), notes that his work is “a lightweight 3D library with a very low level of complexity — in other words, for dummies”. Which is a fine thing, and I think I’ll be using it myself to teach introductory computer graphics. However, if you want to gaze at the occult pulsing mutation spawned of JavaScript + OpenGL ES + DOM + JSON + HTML 5, I mean, unleash the full power of WebGL itself, you’ll want to read the next book instead.

WebGL Beginner’s Guide, by Diego Cantor and Brandon Jones, Packt Publishing, June 2012 (code here). This is a pretty impressive book overall. The authors deal with WebGL directly, adding only the glMatrix library on github to make matrix manipulation easier (this is about as minimal as you can get). This book walks through much of what WebGL does (essentially, the same as OpenGL, of course), giving lots of code examples and worthwhile illustrations. Some example programs are quite nice, with useful user interfaces allowing you to twiddle values and see the effect. The book deals with more advanced topics towards the end, such as how to render offscreen and sample the results (their example is for performing picking). There are a few minor problems with layout on my iPad (a few illustrations don’t fit and there’s an awkward scrolling interface that doesn’t quite work), and some occasional lapses in grammar, but overall the book is fine. Occasionally the authors will get distracted by side topics, like a full derivation of why you use the inverse transpose of the matrix for transforming normals, or explaining a shader that does simple ray tracing. In general, however, the book works through the key areas of the WebGL API and warns you of potential problems along the way. WebGL itself exposes you directly to vertex and fragment shaders, so if you are planning to do some serious work in this area, this book is perhaps a better choice that Parisi’s (that said, three.js itself gives you easy access to shaders).

Personally, these two WebGL books were cheap enough on Kindle that I bought them both. The Beginner’s Guide in particular is a much better deal on Kindle – a third the physical book’s price. They’re actually better on Kindle, in that they’re in color; the physical books are printed with grayscale images. There are other books on WebGL, but these were the two recommended to me and they are both reasonable choices, depending on your goals. I would not recommend either if your primary goal is learning the fundamentals of computer graphics (though there’s a bit of that type of material in each). For learning WebGL itself and the basics of what it offers, both books are fine, with the “WebGL Beginner’s Guide” being aimed at the more “to the metal” programmer.

JavaScript: The Good Parts, by Douglas Crockford, O’Reilly Press, May 2008. If you use WebGL, you’ll have to know at least a bit about JavaScript. Patrick Cozzi mentioned this as a reasonable guide, along with other writings by the author. I’ve only started it, but it’s pretty amusing. The book’s approach is to indeed teach only the good parts about the language and not let you know how to use the bad parts. The author recommends only one of the dozens of books on JavaScript, JavaScript: The Definitive Guide. He feels the rest are, to quote, “quite awful.”

Update: I finished this book and thought it got better as it went along. It starts out a bit language-wonkish, too much about grammar. Skip that chapter. The most useful parts were the appendices, where he explains the various parts of JavaScript clearly and succinctly, calls out the dangerous and evil parts of the language, and explains how to steer clear of them. The author came up with JSLint, which checks your code for badness, but to be honest it’s way too a strict schoolmarm for me, hitting me on the knuckles whenever I put a left brace “{” on the wrong line. I recommend JSHint instead.

Enough on WebGL and JavaScript, my current interests. I’ll mention just two more books that I’ve only just glanced at and so can jump to conclusions.

Real-Time Graphics Rendering Engine (Advanced Topics in Science and Technology in China), by Hujun Bao and Wei Hua, Springer Publishing. Shame on you, Springer, for shame. A 300 page book for $169 is bad enough, but this one has a slap-dash translation that could probably have been done better by Google Translate. I’m judging only from the “Look Inside” text I can access on the Amazon listing, but I see things like a section labelled “Rending Modle” on page 5. This might be a fine book in its native Chinese, the bits of material I tried to read seemed sound enough, but the translation is, well, judge for yourself. Here’s the end of chapter summary, a section I picked at random:

“Real-time rendering is always formed as a pipeline composed of consequential data processing and graphics calculations to generate final rasterized images. In order to achieve amazing rendering effects as well as real-time performance, GPU techniques, especially shader techniques, are introduced to utilize programmable pipelines and the parallel computing structure. Virtual geometric 3D models are transformed to be located in a virtual scene by geometrical transformation matrices. Material and/or other visual properties are then attached to those geometrical models to compute visually pleasant rendering results. As the performance of real-time rendering applications is highly emphasized, tailored shading techniques and BRDF models are employed to achieve real-time rendering results with balanced visual effects.”

I feel terrible for these authors whose work has been so shabbily thrown between two covers.

However, there is certainly a worse book out there, and I have to give the full-sized cover from its Amazon listing, since I expect it to disappear any day now and I want to keep it as a memento.

Suck it, bizzatches, I got me my own book on Amazon. Yes, the title is my name, it’s ostensibly a biography; I’m not the author. I ran across it yesterday morning, and it almost made me laugh. Almost, because I hate these “books”. It’s clearly by the same people who came up with all those other automated offerings, VDM Publishing.

I’ve posted about this so-called publisher before, here and here. VDM now has a new imprint, “Cel Publishing” (one of seventy-nine!), I expect because the Alphascript and Betascript Publisher books are fast disappearing from Amazon’s catalog as the complaints and returns roll in. The short version is that this firm has a program that grabs a random Wikipedia page and follow links from it. Once there’s enough material for a book-sized book, it spits out the contents. Some human – in this case allegedly one “Iustinus Tim Avery” (anagram server’s first hit is “Autism University”) – slaps a cover on it and it’s put in the catalog, to be published on demand.

The most creative part of the process used to be the cover choice. In the past they were sometimes pretty amusing, but it looks like Cel Publishing has removed that fun part of the job for the “editor” and instead just uses random abstract patterns. I personally think of myself as more of a splash of chartreuse paint, but so be it. Also, the book is clearly underpriced at $38 for 60 pages. What did make me laugh is that there’s now a review of the book (thanks, Matt!).

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I asked Patrick Cozzi if he knew of any WebGL editors working in the browser. There turn out to be quite a few, each with their own focus:

Enjoy! And let us know of any others you find.

Note: we’ve added a resource page for WebGL-related information.

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I’ll be gone this weekend, so my dream of catching up on resources by posting every day is slowed a bit. Here’s today’s seven:

  • The free Process Explorer has a lot more functionality than its name implies. One very cool feature is that it actually shows GPU usage. Run it, right-click a process that’s running and select Properties, then go to the GPU Graph tab to watch memory use and GPU load.
  • If you are seriously involved in implementing bump maps, parallax occlusion maps, etc., Morten Mikkelsen’s blog has a lot of chewy information, along with demos and source. He’s doing a lot of interesting work on autogenerating and blending mappings.
  • The game itself is no great shakes, but Google’s Cube has some lovely 3D rendering going on via javascript.
  • Another “3D in the browser” experiment (with WebGL) is sketchPatch. It’s not as simple as advertised, but I like the idea of an interpreted language you just type and see in the same window.
  • There are lots of reasons Unreal Engine 3 is the most popular commercial 3D engine for games. Here’s some nice eye candy from their tutorial on image reflection, which is also just plain educational.
  • Some cool results here using cone tracing for global illumination effects. Seeing these effects for dynamic objects at interactive rates is great stuff, especially since they’re having to update octrees on the fly.
  • I love the colored Japanese woodcuts of classic videogames that Jed Henry has been making:

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