glAccum

glAccum: man2/glAccum.xml

The accumulation buffer is an extended-range color buffer. Images are not rendered into it. Rather, images rendered into one of the color buffers are added to the contents of the accumulation buffer after rendering. Effects such as antialiasing (of points, lines, and polygons), motion blur, and depth of field can be created by accumulating images generated with different transformation matrices. Each pixel in the accumulation buffer consists of red, green, blue, and alpha values. The number of bits per component in the accumulation buffer depends on the implementation. You can examine this number by calling glGetIntegerv four times, with arguments GL_ACCUM_RED_BITS, GL_ACCUM_GREEN_BITS, GL_ACCUM_BLUE_BITS, and GL_ACCUM_ALPHA_BITS. Regardless of the number of bits per component, the range of values stored by each component is -1 1. The accumulation buffer pixels are mapped one-to-one with frame buffer pixels. glAccum operates on the accumulation buffer. The first argument, op, is a symbolic constant that selects an accumulation buffer operation. The second argument, value, is a floating-point value to be used in that operation. Five operations are specified: GL_ACCUM, GL_LOAD, GL_ADD, GL_MULT, and GL_RETURN. All accumulation buffer operations are limited to the area of the current scissor box and applied identically to the red, green, blue, and alpha components of each pixel. If a glAccum operation results in a value outside the range -1 1, the contents of an accumulation buffer pixel component are undefined. The operations are as follows: To clear the accumulation buffer, call glClearAccum with R, G, B, and A values to set it to, then call glClear with the accumulation buffer enabled.

Only pixels within the current scissor box are updated by a glAccum operation.