GLREADPIXELS()GLREADPIXELS()NAMEglReadPixels - 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 )
delim $$
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.
DESCRIPTIONglReadPixels 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, glPixel‐
Transfer, 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 $i$th pixel in the $j$th 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 map‐
pings 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 compo‐
nent 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 dis‐
cards 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 keep‐
ing 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 spec‐
ified 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 com‐
ponent 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 differ‐
ent 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 $i$th pixel in the $j$th 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
GLREADPIXELS()
