NAME
glReadPixels - read a block of pixels from the frame buffer
C SPECIFICATION
void glReadPixels( GLint x,
GLint y,
GLsizei width,
GLsizei height,
GLenum format,
GLenum type,
GLvoid *pixels )
PARAMETERS
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.
width, height
Specify the dimensions of the pixel rectangle. width
and height of one correspond to a single pixel.
format
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, and
GL_LUMINANCE_ALPHA.
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.
pixels
Returns the pixel data.
DESCRIPTION
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
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 are:
GL_COLOR_INDEX
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
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, and
GL_PIXEL_MAP_I_TO_A. Each table must be of size
2^n, but n may be different for different tables.
Before an index is used to look up a value in a
table of size 2^n, it must be masked against 2^n-
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 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.
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_LSB_FIRST and
GL_PACK_SWAP_BYTES, affect the way that data is written into
memory. See glPixelStore for a description.
NOTES
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.
ERRORS
GL_INVALID_ENUM is generated if format or type is not an
accepted value.
GL_INVALID_ENUM is generated if type is GL_BITMAP and format
is not GL_COLOR_INDEX or GL_STENCIL_INDEX.
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.
ASSOCIATED GETS
glGet with argument GL_INDEX_MODE
SEE ALSO
glCopyPixels, glDrawPixels, glPixelMap, glPixelStore,
glPixelTransfer,
glReadBuffer