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For just about as long as there has been graphics hardware, there has been programmable graphics hardware. Over the years, building flexibility into graphics hardware designs has been a necessary way of life for hardware developers. Graphics APIs continue to evolve, and because a hardware design can take two years or more from start to finish, the only way to guarantee a hardware product that can support the then current graphics APIs at its release is to build in some degree of programmability from the very beginning.

Until recently, the realm of programming graphics hardware belonged to just a few people, mainly researchers and graphics hardware driver developers. Research into programmable graphics hardware has been taking place for many years, but the point of this research has not been to produce viable hardware and software for application developers and end users. The graphics hardware driver developers have focused on the immediate task of providing support for the important graphics APIs of the time: PHIGS, PEX, Iris GL, OpenGL, Direct3D, and so on. Until recently, none of these APIs exposed the programmability of the underlying hardware, so application developers have been forced into using the fixed functionality provided by traditional graphics APIs.

Hardware companies have not exposed the programmable underpinnings of their products because of the high cost of educating and supporting customers to use low-level, device-specific interfaces and because these interfaces typically change quite radically with each new generation of graphics hardware. Application developers who use such a device-specific interface to a piece of graphics hardware face the daunting task of updating their software for each new generation of hardware that comes along. And forget about supporting the application on hardware from multiple vendors!

As we moved into the 21st century, some of these fundamental tenets about graphics hardware were challenged. Application developers pushed the envelope as never before and demanded a variety of new features in hardware in order to create more and more sophisticated onscreen effects. As a result, new graphics hardware designs became more programmable than ever before. Standard graphics APIs were challenged to keep up with the pace of hardware innovation. For OpenGL, the result was a spate of extensions to the core API as hardware vendors struggled to support a range of interesting new features that their customers were demanding.

The creation of a standard, cross-platform, high-level shading language for commercially available graphics hardware was a watershed event for the graphics industry. A paradigm shift occurred, one that took us from the world of rigid, fixed functionality graphics hardware and graphics APIs to a brave new world where the visual processing unit, or VPU (i.e., graphics hardware), is as important as the central processing unit, or CPU. The VPU is optimized for processing dynamic media such as 3D graphics and video. Highly parallel processing of floating-point data is the primary task for VPUs, and the flexibility of the VPU means that it can also be used to process data other than a stream of traditional graphics commands. Applications can take advantage of the capabilities of both the CPU and the VPU, using the strengths of each to optimally perform the task at hand.

This book describes how graphics hardware programmability is exposed through a high-level language in the leading cross-platform 3D graphics API: OpenGL. This language, the OpenGL Shading Language, lets applications take total control over the most important stages of the graphics processing pipeline. No longer restricted to the graphics rendering algorithms and formulas chosen by hardware designers and frozen in silicon, software developers are beginning to use this programmability to create stunning effects in real time.

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