OpenGL Programming Guide

Chapter 10The Framebuffer

The Accumulation Buffer

Advanced

The accumulation buffer can be used for such things as scene antialiasing, motion blur, simulating photographic depth of field, and calculating the soft shadows that result from multiple light sources. Other techniques are possible, especially in combination with some of the other buffers. (For more information on the uses for the accumulation buffer, see The Accumulation Buffer: Hardware Support for High-Quality Rendering by Paul Haeberli and Kurt Akeley (SIGGRAPH 1990 Proceedings, p. 309-318).

OpenGL graphics operations don't write directly into the accumulation buffer. Typically, a series of images is generated in one of the standard color buffers, and these are accumulated, one at a time, into the accumulation buffer. When the accumulation is finished, the result is copied back into a color buffer for viewing. To reduce rounding errors, the accumulation buffer may have higher precision (more bits per color) than the standard color buffers. Rendering a scene several times obviously takes longer than rendering it once, but the result is higher quality. You can decide what trade-off between quality and rendering time is appropriate for your application.

You can use the accumulation buffer the same way a photographer can use film for multiple exposures. A photographer typically creates a multiple exposure by taking several pictures of the same scene without advancing the film. If anything in the scene moves, that object appears blurred. Not surprisingly, a computer can do more with an image than a photographer can do with a camera. For example, a computer has exquisite control over the viewpoint, but a photographer can't shake a camera a predictable and controlled amount.

See "Clearing Buffers" for information about how to clear the accumulation buffer; use glAccum() to control it. void glAccum(GLenum op, GLfloat value);

Controls the accumulation buffer. The op parameter selects the operation, and value is a number to be used in that operation. The possible operations are GL_ACCUM, GL_LOAD, GL_RETURN, GL_ADD, and GL_MULT:

• GL_ACCUM reads each pixel from the buffer currently selected for reading with glReadBuffer() (see Chapter 8 ), multiplies the R, G, B, and alpha values by value, and adds the result to the accumulation buffer.
  
  

• GL_LOAD does the same thing, except that the values replace those in the accumulation buffer rather than being added to them.
  
  

• GL_RETURN takes values from the accumulation buffer, multiplies them by value, and places the result in the color buffer(s) enabled for writing.
  
  

• GL_ADD and GL_MULT simply add or multiply the value of each pixel in the accumulation buffer by value, and then return it to the accumulation buffer. For GL_MULT, value is clamped to be in the range [-1.0,1.0]. For GL_ADD, no clamping occurs.

Scene Antialiasing
Motion Blur
Depth of Field
Soft Shadows
Jittering

OpenGL Programming Guide