LOSTCIRCUITS

Historically, geometry processing had to be carried out by the CPU, however, with the introduction of hardware-accelerated transform and lighting (T&L), the GPU has taken over these tasks. This happened about 2 years ago with the introduction of the GeForce256. The latest developments in hardware and software, however take it a few steps further by introducing programmable vertex shaders that can be used to modify the incoming vertices.

What are vertex shaders?

A vertex shader is software that modifies vertex data within the geometry -processing pipeline. In the case of ATI's SMARTSHADER, each vertex can consist of up to 16 distinct pieces of data, which are read by the vertex shader as individual streams. These pieces of data can contain positional coordinates, texture coordinates, lighting values or weighting values for matrix skinning. A vertex shader program can have a maximum length of 128 instructions, and make use of up to 96 constant values and 12 temporary data registers. The actual instructions are very similar to those found in CPU assembly language and allow for easy manipulation of vertex data.

Schematic overview of the SMARTSHADER topology with its 16 Stream units (picture courtesy of ATI).

The implementation of vertex shaders allows a huge variety of new graphics effects. Since the vertex shaders modify the position of a vertex and, therefore, the geometry, the effects involve mostly modifications to the shape or position of object. Vertex shaders can also be optimized in many cases to provide superior performance in fixed function T&L operations.

Examples

Procedural Deformation

Procedural deformation is another word to assign a mathematical function (vertex shader) to incoming vertices. Objects that change shape according to these functions (soap bubbles, waves and flags) can be modeled within the GPU without requiring additional bandwidth.

Shadow Volumes

Shadows are a very important part of any scene, since create depth and atmosphere. Vertex shaders can generate convincing shadows that can be fully animated and extended to multiple light sources. The shader is used to create transparent volumes that extend behind objects away from any light sources, creating shadows where the volumes contact other surfaces. The closer the light source is to the shadow-casting object, the darker the shadow is.

Shadow volumes are defined by size and proximity of the light source to the shadow-casting object (picture courtesy of ATI).

Other vertex shader effects include, but are not limited to:

• Fur Rendering - grow realistic fur from any object that can be made to curl or blow in the wind if desired.
• Particle Systems - animate large numbers of small particles by assigning and modifying physical properties such as mass, velocity, acceleration, etc. Useful for modeling fire, sparks, explosions, smoke, rain, snow, and more.
• Lens Effects - distort an image as if it were being viewed through a lens.
• Matrix Palette Skinning - allow skeletally animated characters with many bones to move freely and bend naturally.
• Advanced Keyframe Interpolation - perform animation and morphing effects using non-linear blending between two or more keyframes. Useful for modeling complex facial expressions and speech.

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