2.2 OpenGL as a Basis for a Rendering Service

A render service needs a rendering model and protocol to communicate rendering requests. OpenGL [10] is a widely adopted programming interface for high-performance, interactive 3D graphics. The OpenGL graphics system has features that make it well suited for use as the underlying rendering model and protocol of a render service:

• OpenGL's X Window System extension known as GLX [4] already provides a standard, inter-operable protocol for issuing OpenGL rendering commands.

• OpenGL is window system independent, making it possible to specify a window system independent render service programming interface.

• OpenGL has existing extensions (with associated GLX protocol) for advanced rendering capabilities like multisampling and 3D textures, as well as image processing support for operations like convolutions and histograms.

• OpenGL is widely available and is an accepted standard with existing, well-tuned high-end implementations. Low-end support for OpenGL makes possible hybrid rendering schemes combining local OpenGL rendering results with render service usage.

• OpenGL cleanly separates rendering state from frame buffer state.

• OpenGL is amendable to hardware acceleration [7].

• OpenGL display lists and texture objects allow caching server-side command sequences and textures, reducing transport overhead during rendering.

The GLR render service uses OpenGL's programming interface so access to the graphics hardware can be shared by various clients requesting high-performance rendering. The programming interface and implementation of GLR are discussed in Section 4. Using GLR, multiple clients on a network (including clients running on the same machine that provides the render service) can share a single, expensive graphics subsystem for their sophisticated rendering needs.

Typically, a GLR client renders a scene using OpenGL commands, then retrieves the rendered image from the frame buffer using OpenGL's glReadPixels. The retrieved image can then be displayed on a low-end workstation's screen, saved to a file, directed to a printer, or otherwise manipulated.