Polygons In Your Pocket: Introducing OpenGL ES

If you're a regular reader of OpenGL Pipeline, you probably know that you can use OpenGL on Macs, PCs (under Windows or Linux), and many other platforms ranging from workstations to supercomputers. But, did you know that you can also use it on PDAs and cell phones? Yes, really!

Okay, not really, at least not yet; but you can use its smaller sibling, OpenGL ES. OpenGL ES is OpenGL for Embedded Systems, including cell phones in particular, but also PDAs, automotive entertainment centers, portable media players, set-top boxes, and -- who knows -- maybe, someday, wrist watches and Coke® machines.

OpenGL ES is defined by the Khronos Group, a consortium of cell phone manufacturers, silicon vendors, content providers, and graphics companies. Work on the standard began in 2002, with support and encouragement from SGI and the OpenGL ARB. The Khronos Group's mission is to enable high-quality graphics and media on both mobile and desktop platforms. In addition to OpenGL ES, Khronos has defined standards for high-quality vector graphics, audio, streaming media, and graphics asset interchange. In 2006, the OpenGL ARB itself became a Khronos working group, and many ARB members are active in OpenGL ES discussions.

Why OpenGL ES?

When the Khronos group began looking for a mobile 3D API, the advantages of OpenGL were obvious: it is powerful, flexible, non-proprietary, and portable to many different OSes and environments. However, just as human evolution left us with tailbones and appendices, OpenGL's evolution has left it with some features whose usefulness for mobile applications is, shall we say, non-obvious: color index mode, stippling, and GL_POLYGON_SMOOTH antialiasing, to name a few. In addition, OpenGL often provides several ways to accomplish the same basic goal, and it has some features that have highly specialized uses and/or are expensive to implement. The Khronos OpenGL ES Working Group saw this as an opportunity: by eliminating legacy features, redundant ways of doing things, and features not appropriate for mobile platforms, they could produce an API that provides most of the power of desktop OpenGL in a much smaller package.

Design Guidelines

The Khronos OpenGL ES Working Group based its decisions about which OpenGL features to keep on a few simple guidelines:

If in doubt, leave it out

Rather than starting with desktop OpenGL and deciding what features to remove, start with a blank piece of paper and decide what features to include, and include only those features you really need.

Eliminate redundancy

If OpenGL provides multiple ways of doing something, include at most one of them. If in doubt, choose the most efficient.

"When was the last time you used this?"

If in doubt about whether to include a particular feature, look for examples of recent applications that use it. If you can't find one, you probably don't need it.

The Principle of Least Astonishment

Try not to surprise the experienced OpenGL programmer; when importing features from OpenGL, don't change anything you don't have to. OpenGL ES 1.0 is defined relative to desktop OpenGL 1.3 – the specification is just a list of what is different, and why. Similarly, OpenGL ES 1.1 is defined as a set of differences from OpenGL 1.5.

OpenGL ES is almost a pure subset of desktop OpenGL. However, it has a few features that were added to accommodate the limitations of mobile devices. Handhelds have limited memory, so OpenGL ES allows you to specify vertex coordinates using bytes as well as the usual short, int, and float. Many handhelds have little or no support for floating point arithmetic, so OpenGL ES adds support for fixed point. For really light-weight platforms, OpenGL ES 1.0 and 1.1 define a "light" profile that doesn't use floating point at all.

To Market

OpenGL ES has rapidly replaced proprietary 3D APIs on mobile phones, and is making rapid headway on other platforms. The original version 1.0 is supported in Qualcomm's BREW® environment for cell phones, and (with many extensions) on the PLAYSTATION®3. The current version (ES 1.1) is supported on a wide variety of mobile platforms.

Learning More

In future issues of OpenGL Pipeline, I'll go into more detail about the various OpenGL ES versions and how they differ from their OpenGL counterparts. But for the real scoop, you'll need to look at the specs. As I said earlier, the current OpenGL ES specifications refer to a parent desktop OpenGL spec, listing the differences between the ES version and the parent. This is great if you know the desktop spec well, but it's confusing for the casual reader. For those who prefer a self-contained document, the working group is about to release a stand-alone version of the ES 1.1 specification, and (hurray!) man pages. You can find the current specifications and (soon) other documentation at http://www.khronos.org/opengles/1_X/.

Take It for a Test Drive

OpenGL ES development environments aren't quite as easy to find as OpenGL environments, but if you want to experiment with it, you have several no-cost options. The current list is at http://www.khronos.org/developers/resources/opengles. Many of these toolkits and SDKs target multiple mobile programming environments, and many also offer the option of running on Windows, Linux, or both. There's also an excellent open source implementation, Vincent.

Watch This Space

Since releasing OpenGL ES 1.1 in 2004, the Working Group has been busier than ever. Later this year we'll release OpenGL ES 2.0, which (you guessed it) is based on OpenGL 2.0, and brings modern shader-based graphics to handheld devices. You can expect to see ES 2.0 toolkits later this year, and ES2-capable devices in 2008 and 2009. Also under development or discussion are the aforementioned stand-alone specifications and man pages, educational materials, an effects framework, and (eventually) a mobile version of Longs Peak.

I hope you've enjoyed this quick overview of OpenGL ES. We'd love to have your feedback! Look for us at conferences like SIGGRAPH and GDC, or visit the relevant discussion boards at http://www.khronos.org.

Tom Olson, Texas Instruments, Inc.
OpenGL ES Working Group Chair