OpenGL man pages
glReadPixels - read a block of pixels from the frame buffer
void glReadPixels( GLint x,
GLvoid *pixels )
x, y Specify the window coordinates of the first pixel that is read from
the frame buffer. This location is the lower left corner of a
rectangular block of pixels.
Specify the dimensions of the pixel rectangle. width and height of
one correspond to a single pixel.
Specifies the format of the pixel data. The following symbolic values
are accepted: GL_COLOR_INDEX, GL_STENCIL_INDEX, GL_DEPTH_COMPONENT,
GL_RED, GL_GREEN, GL_BLUE, GL_ALPHA, GL_RGB, GL_RGBA, GL_LUMINANCE,
type Specifies the data type of the pixel data. Must be one of
GL_UNSIGNED_BYTE, GL_BYTE, GL_BITMAP, GL_UNSIGNED_SHORT, GL_SHORT,
GL_UNSIGNED_INT, GL_INT, or GL_FLOAT.
Returns the pixel data.
glReadPixels returns pixel data from the frame buffer, starting with the
pixel whose lower left corner is at location (x, y), into client memory
starting at location pixels. Several parameters control the processing of
the pixel data before it is placed into client memory. These parameters
are set with three commands: glPixelStore, glPixelTransfer, and glPixelMap.
This reference page describes the effects on glReadPixels of most, but not
all of the parameters specified by these three commands.
glReadPixels returns values from each pixel with lower left-hand corner at
(x + i, y + j) for 0<=i<width and 0<=j<height. This pixel is said to be the
ith pixel in the jth row. Pixels are returned in row order from the lowest
to the highest row, left to right in each row.
format specifies the format for the returned pixel values. Accepted values
for format are as follows:
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.
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.
Depth values are read from the depth buffer. Each component is
converted to floating point such that the minimum depth value
maps to 0.0 and the maximum value maps to 1.0. Each component is
then multiplied by GL_DEPTH_SCALE, added to GL_DEPTH_BIAS, and
finally clamped to the range [0,1].
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
GL_PIXEL_MAP_I_TO_R, GL_PIXEL_MAP_I_TO_G, GL_PIXEL_MAP_I_TO_B,
and GL_PIXEL_MAP_I_TO_A tables. Each of these tables must be of
size 2 , but n may be different for different tables. Before an
index is used to look up a value in a table of size 2 it must
be masked against 2 -1.
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 described above are all
specified by glPixelTransfer. The lookup table contents themselves are
specified by glPixelMap.
The final step involves converting the indices or components 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.
If format is GL_RED, GL_GREEN, GL_BLUE, GL_ALPHA, GL_RGB, GL_RGBA,
GL_LUMINANCE, or GL_LUMINANCE_ALPHA and type is not GL_FLOAT, each
component is multiplied by the multiplier shown in the following table. If
type is GL_FLOAT, then each component is passed as is (or converted to the
client's single-precision floating-point format if it is different from the
one used by the GL).
| type | index mask | component conversion |
|GL_UNSIGNED_BYTE | 2^8-1 | (2^8-1)c |
| GL_BYTE | 2^7-1 | [(2^8-1)c-1]/2 |
| GL_BITMAP | 1 | 1 |
|GL_UNSIGNED_SHORT | 2^16-1 | (2^16-1)c |
| GL_SHORT | 2^15-1 | [(2^16-1)c-1]/2 |
| GL_UNSIGNED_INT | 2^32-1 | (2^32-1)c |
| GL_INT | 2^31-1 | [(2^32-1)c-1]/2 |
| GL_FLOAT | none | c |
Return values are placed in memory as follows. If format is
GL_COLOR_INDEX, GL_STENCIL_INDEX, GL_DEPTH_COMPONENT, GL_RED, GL_GREEN,
GL_BLUE, GL_ALPHA, or GL_LUMINANCE, a single value is returned and the data
for the ith pixel in the jth row is placed in location (j) width + i.
GL_RGB returns three values, GL_RGBA returns four values, and
GL_LUMINANCE_ALPHA returns two values for each pixel, with all values
corresponding to a single pixel occupying contiguous space in pixels.
Storage parameters set by glPixelStore, such as GL_PACK_SWAP_BYTES and
GL_PACK_LSB_FIRST, affect the way that data is written into memory. See
glPixelStore for a description.
Values for pixels that lie outside the window connected to the current GL
context are undefined.
If an error is generated, no change is made to the contents of pixels.
GL_INVALID_ENUM is generated if format or type is not an accepted value.
GL_INVALID_VALUE is generated if either width or height is negative.
GL_INVALID_OPERATION is generated if format is GL_COLOR_INDEX and the color
buffers store RGBA color components.
GL_INVALID_OPERATION is generated if format is GL_STENCIL_INDEX and there
is no stencil buffer.
GL_INVALID_OPERATION is generated if format is GL_DEPTH_COMPONENT and there
is no depth buffer.
GL_INVALID_OPERATION is generated if glReadPixels is executed between the
execution of glBegin and the corresponding execution of glEnd.
glGet with argument GL_INDEX_MODE
glCopyPixels, glDrawPixels, glPixelMap, glPixelStore, glPixelTransfer,
Fri Dec 6 11:18:03 EST 1996
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