glReadPixels(3G)glReadPixels(3G)NAMEglReadPixels - read a block of pixels from the frame buffer
SYNOPSIS
void glReadPixels(
GLint x,
GLint y,
GLsizei width,
GLsizei height,
GLenum format,
GLenum type,
GLvoid *pixels );
PARAMETERS
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 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_BGR, GL_RGBA, GL_BGRA, GL_LUMINANCE, and GL_LUMI‐
NANCE_ALPHA. 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, GL_FLOAT, GL_UNSIGNED_BYTE_3_3_2,
GL_UNSIGNED_BYTE_2_3_3_REV, GL_UNSIGNED_SHORT_5_6_5,
GL_UNSIGNED_SHORT_5_6_5_REV, GL_UNSIGNED_SHORT_4_4_4_4,
GL_UNSIGNED_SHORT_4_4_4_4_REV, GL_UNSIGNED_SHORT_5_5_5_1,
GL_UNSIGNED_SHORT_1_5_5_5_REV, GL_UNSIGNED_INT_8_8_8_8,
GL_UNSIGNED_INT_8_8_8_8_REV, GL_UNSIGNED_INT_10_10_10_2, or
GL_UNSIGNED_INT_2_10_10_10_REV. 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 pro‐
cessing of the pixel data before it is placed into client memory. These
parameters are set with three commands: glPixelStore(), glPixelTrans‐
fer(), and glPixelMap(). This reference page describes the effects on
glReadPixels() of most, but not all of the parameters specified by
these three commands.
When the GL_ARB_imaging extension is supported, the pixel data may be
processed by additional operations including color table lookup, color
matrix tranformations, convolutions, histograms and minimum and maximun
pixel value computations.
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 for the returned pixel values; accepted values
are: Color indices are read from the color buffer selected by glRead‐
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_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 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]. 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 sup 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
sup 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 com‐
ponent 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-compo‐
nent 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 spec‐
ified by glPixelTransfer(). The lookup table contents themselves are
specified by glPixelMap().
Finally, the indices or components are converted to the proper , 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 .
If format is GL_RED, GL_GREEN, GL_BLUE, GL_ALPHA, GL_RGB, GL_BGR,
GL_RGBA, GL_BGRA, 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 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 and GL_BGR return three values, GL_RGBA and
GL_BGRA return four values, and GL_LUMINANCE_ALPHA returns two values
for each pixel, with all values corresponding to a single pixel occupy‐
ing contiguous space in pixels. Storage parameters set by glPixel‐
Store(), such as GL_PACK_LSB_FIRST and GL_PACK_SWAP_BYTES, affect the
way that data is written into memory. See glPixelStore() for a descrip‐
tion.
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().
GL_INVALID_OPERATION is generated if type is one of
GL_UNSIGNED_BYTE_3_3_2, GL_UNSIGNED_BYTE_2_3_3_REV,
GL_UNSIGNED_SHORT_5_6_5, or GL_UNSIGNED_SHORT_5_6_5_REV and format is
not GL_RGB.
GL_INVALID_OPERATION is generated if type is one of
GL_UNSIGNED_SHORT_4_4_4_4, GL_UNSIGNED_SHORT_4_4_4_4_REV,
GL_UNSIGNED_SHORT_5_5_5_1, GL_UNSIGNED_SHORT_1_5_5_5_REV,
GL_UNSIGNED_INT_8_8_8_8, GL_UNSIGNED_INT_8_8_8_8_REV,
GL_UNSIGNED_INT_10_10_10_2, or GL_UNSIGNED_INT_2_10_10_10_REV and for‐
mat is neither GL_RGBA nor GL_BGRA.
The formats GL_BGR, and GL_BGRA and types GL_UNSIGNED_BYTE_3_3_2,
GL_UNSIGNED_BYTE_2_3_3_REV, GL_UNSIGNED_SHORT_5_6_5,
GL_UNSIGNED_SHORT_5_6_5_REV, GL_UNSIGNED_SHORT_4_4_4_4,
GL_UNSIGNED_SHORT_4_4_4_4_REV, GL_UNSIGNED_SHORT_5_5_5_1,
GL_UNSIGNED_SHORT_1_5_5_5_REV, GL_UNSIGNED_INT_8_8_8_8,
GL_UNSIGNED_INT_8_8_8_8_REV, GL_UNSIGNED_INT_10_10_10_2, and
GL_UNSIGNED_INT_2_10_10_10_REV are available only if the GL version is
1.2 or greater.
ASSOCIATED GETSglGet() with argument GL_INDEX_MODE
SEE ALSOglCopyPixels(3), glPixelMap(3), glDrawPixels(3), glPixelStore(3),
glPixelTransfer(3), glReadBuffer(3)glReadPixels(3G)
