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2.6. Key Benefits

The following key benefits are derived from the choices that were made during the design of the OpenGL Shading Language.

Tight integration with OpenGL The OpenGL Shading Language was designed for use in OpenGL. It is designed in such a way that an existing, working OpenGL application can easily be modified to take advantage of the capabilities of programmable graphics hardware. Built-in access to existing OpenGL state, reuse of API entry points that are already familiar to application developers, and a close coupling with the existing architecture of OpenGL are all key benefits of using the OpenGL Shading Language for shader development.

Runtime compilation Source code stays as source code, in its easiest-to-maintain form, for as long as possible. An application passes source code to any conforming OpenGL implementation that supports the OpenGL Shading Language, and it will be compiled and executed properly. There is no need for a multitude of binaries for a multitude of different platforms.[1]

[1] At the time of this writing, the OpenGL ARB is still considering the need for an API that allows shaders to be specified in a form other than source code. The primary issues are the protection of intellectual property that may be embedded in string-based shader source code and the performance that would be gained by allowing shaders to be at least partially precompiled. When such an API is defined, shader portability may be reduced, but application developers will have the option of getting better code security and better performance.

No reliance on cross-vendor assembly language Both DirectX and OpenGL have widespread support for assembly language interfaces to graphics programmability. High-level shading languages could be (and have been) built on top of these assembly language interfaces, and such high-level languages can be translated into these assembly language interfaces completely outside the environment of OpenGL or DirectX. This does have some advantages, but relying on an assembly language interface as the primary interface to hardware programmability restricts innovation by graphics hardware designers. Hardware designers have many more choices for acceleration of an expressive high-level language than they do for a restrictive assembly language. It is much too early in the development of programmable graphics hardware technology to establish an assembly language standard for graphics programmability. C, on the other hand, was developed long before any CPU assembly languages that are in existence today, and it is still a viable choice for application development.

Unconstrained opportunities for compiler optimization plus optimal performance on a wider range of hardware As we've learned through experience with CPUs, compilers are much better at quickly generating efficient code than humans are. By allowing high-level source code to be compiled within OpenGL, rather than outside of OpenGL, individual hardware vendors have the best possible opportunity to deliver optimal performance on their graphics hardware. In fact, compiler improvements can be made with each OpenGL driver release, and the applications won't need to change any application source code, recompile the application, or even relink it. Furthermore, most of the current crop of assembly language interfaces are string based. This makes them inefficient for use as intermediate languages for compilation because two string translations are required. First, the string-based, high-level source must be translated into string-based assembly language, and then that string-based assembly language must be passed to OpenGL and translated from string-based assembly language to machine code.

A truly open, cross-platform standard No other high-level graphics shading language has been approved as part of an open, multivendor standard. Like OpenGL itself, the OpenGL Shading Language will be implemented by a variety of different vendors for a variety of different environments.

One high-level language for all programmable graphics processing The OpenGL Shading Language is used to write shaders for both the vertex processor and the fragment processor in OpenGL, with very small differences in the language for the two types of shaders. In the future, it is intended that the OpenGL Shading Language will bring programmability to other areas of OpenGL as well. Areas that have already received some discussion include programmability for packing/unpacking arbitrary image formats and support for programmable tessellation of higher-order surfaces in the graphics hardware.

Support for modular programming By defining compilation and linking as two separate steps, shader writers have a lot more flexibility in how they choose to implement complex shading algorithms. Rather than implement a complex algorithm as a single, monolithic shader, developers are free to implement it as a collection of shaders that can be independently compiled and attached to a program object. Shaders can be designed with common interfaces so that they are interchangeable, and a link operation joins them to create a program.

No additional libraries or executables The OpenGL Shading Language and the compiler and linker that support it are defined as part of OpenGL. Applications need not worry about linking against any additional runtime libraries. Compiler improvements are delivered as part of OpenGL driver updates.

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