glAccum

  • Article

The glAccum function operates on the accumulation buffer.

void glAccum(
  GLenum op,  GLfloat value);

Parameters

  • op
    The accumulation buffer operation. The accepted symbolic constants are:

    Value Description
    GL_ACCUM Obtains R, G, B, and A values from the buffer currently selected for reading (see glReadBuffer). Each component value is divided by 2n 1, where n is the number of bits allocated to each color component in the currently selected buffer. The result is a floating-point value in the range [0,1], which is multiplied by value and added to the corresponding pixel component in the accumulation buffer, thereby updating the accumulation buffer.
    GL_LOAD Similar to GL_ACCUM, except that the current value in the accumulation buffer is not used in the calculation of the new value. That is, the R, G, B, and A values from the currently selected buffer are divided by 2n 1, multiplied by value, and then stored in the corresponding accumulation buffer cell, overwriting the current value.
    GL_ADD Adds value to each R, G, B, and A in the accumulation buffer.
    GL_MULT Multiplies each R, G, B, and A in the accumulation buffer by value and returns the scaled component to its corresponding accumulation buffer location.
    GL_RETURN Transfers accumulation buffer values to the color buffer or buffers currently selected for writing. Each R, G, B, and A component is multiplied by value, then multiplied by 2n 1, clamped to the range [0, 2n 1 ], and stored in the corresponding display buffer cell. The only fragment operations that are applied to this transfer are pixel ownership, scissor, dithering, and color writemasks.

  • value
    A floating-point value used in the accumulation buffer operation. The op parameter determines how value is used.

Remarks

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. You can create effects such as antialiasing (of points, lines, and polygons), motion blur, and depth of field 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 the arguments GL_ACCUM_RED_BITS, GL_ACCUM_GREEN_BITS, GL_ACCUM_BLUE_BITS, and GL_ACCUM_ALPHA_BITS, respectively. Regardless of the number of bits per component, however, the range of values stored by each component is [ 1, 1]. The accumulation buffer pixels are mapped one-to-one with framebuffer pixels.

The glAccum function 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 are applied identically to the red, green, blue, and alpha components of each pixel. The contents of an accumulation buffer pixel component are undefined if the glAccum operation results in a value outside the range [ 1,1].

To clear the accumulation buffer, use the glClearAccum function to specify R, G, B, and A values to set it to, and issue a glClear function with the accumulation buffer enabled.

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

The following functions retrieve information related to the glAccum function:

glGet with argument GL_ACCUM_RED_BITS

glGet with argument GL_ACCUM_GREEN_BITS

glGet with argument GL_ACCUM_BLUE_BITS

glGet with argument GL_ACCUM_ALPHA_BITS

Error Codes

The following are the error codes generated and their conditions.

Error code Condition
GL_INVALID_ENUM op was not an accepted value.
GL_INVALID_OPERATION There was no accumulation buffer or the function glAccum was called between a call to glBegin and the corresponding call to glEnd.

Requirements

**  Windows NT/2000:** Requires Windows NT 3.5 or later.
**  Windows 95/98:** Requires Windows 95 or later. Available as a redistributable for Windows 95.
**  Header:** Declared in Gl.h.
**  Library:** Use Opengl32.lib.

See Also

glBegin, glBlendFunc, glClear, glClearAccum, glCopyPixels, glEnd, glGet, glLogicOp, glPixelStore, glPixelTransfer, glReadBuffer, glReadPixels, glScissor, glStencilOp