HP b2600 Reference Manual page 381

Color indices are read from the color buffer selected by glReadBuffer. Each index is
converted to fixed point, shifted left or right depending on the value and sign of
GL_INDEX_SHIFT, and added to GL_INDEX_OFFSET. If GL_MAP_COLOR is
GL_TRUE, indices are replaced by their mappings in the table
GL_PIXEL_MAP_I_TO_I. GL_STENCIL_INDEX
Stencil values are read from the stencil buffer.
Each index is converted to fixed point, shifted left or right depending on the value and
sign of GL_INDEX_SHIFT, and added to GL_INDEX_OFFSET. If GL_MAP_STENCIL is
GL_TRUE, indices are replaced by their mappings in the table
GL_PIXEL_MAP_S_TO_S.
GL_DEPTH_COMPONENT
Depth values are read from the depth buffer. Each component is converted to floating
point such that the minimum depth value maps to 0 and the maximum value maps to 1.
Each component is then multiplied by GL_DEPTH_SCALE, added to GL_DEPTH_BIAS,
and finally clamped to the range [0,1].
GL_RED, GL_GREEN, GL_BLUE, GL_ALPHA, GL_RGB, GL_RGBA,
GL_LUMINANCE, and GL_LUMINANCE_ALPHA
Processing differs depending on whether color buffers store color indices or RGBA color
components. If color indices are stored, they are read from the color buffer selected by
glReadBuffer. Each index is converted to fixed point, shifted left or right depending on
the value and sign of GL_INDEX_SHIFT, and added to GL_INDEX_OFFSET. Indices
are then replaced by the red, green, blue, and alpha values obtained by indexing the
tables GL_PIXEL_MAP_I_TO_R, GL_PIXEL_MAP_I_TO_G, GL_PIXEL_MAP_I_TO_B,
Each table must be of size 2
Before an index is used to look up a value in a table of size 2
n
2
If RGBA color components are stored in the color buffers, they are read from the color
buffer selected by glReadBuffer. Each color component is converted to floating point such
that zero intensity maps to 0.0 and full intensity maps to 1.0. Each component is then
multiplied by GL_c_SCALE and added to GL_c_BIAS, where c is RED, GREEN, BLUE,
or ALPHA. Finally, if GL_MAP_COLOR is GL_TRUE, each component is clamped to the
range [0, 1], scaled to the size of its corresponding table, and is then replaced by its
mapping in the table GL_PIXEL_MAP_c_TO_c, where c is R, G, B, or A.
Unneeded data is then discarded. For example, GL_RED discards the green, blue, and
alpha components, while GL_RGB discards only the alpha component.
GL_LUMINANCE computes a single-component value as the sum of the red, green, and
blue components, and GL_LUMINANCE_ALPHA does the same, while keeping alpha as
a second value. The final values are clamped to the range [0, 1].
The shift, scale, bias, and lookup factors just described are all specified by
glPixelTransfer. The lookup table contents themselves are specified by glPixelMap.
Finally, the indices or components are converted to the proper format, as specified by
type. If format is GL_COLOR_INDEX or GL_STENCIL_INDEX and type is not
GL_FLOAT, each index is masked with the mask value given in the following table. If
type is GL_FLOAT, then each integer index is converted to single-precision
floating-point format.
Chapter 16
- 1.
n
, but n may be different for different tables.
glReadPixels
n
, it must be masked against
R
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