glReadPixels
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-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:
- 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 0.0 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].
- 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
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^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 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).
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.
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_VALUE is generated if either width or height
is not 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 called
between a call to glBegin and the
corresponding call to glEnd.
ASSOCIATED GETS
glGet with argument
GL_INDEX_MODE
SEE ALSO
glCopyPixels,
glDrawPixels,
glPixelMap,
glPixelStore,
glPixelTransfer,
glReadBuffer
back to the OpenGL index page
© 1995 Uwe Behrens. All rights reserved.