GLREADPIXELS() MachTen Programmer’s Manual 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 )

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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 pix-
els.

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 follow-
ing 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 $i$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 con-
verted 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 compo-
nents. If color indices are stored, they 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. 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 com-
ponents, 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 multi-
plier 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).

center box tab(:); c | ci | ci c | c | c . 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 con-
tents 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 for-
mat 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 com-
ponents.

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 exe-
cuted between the execution of glBegin and the correspond-
ing execution of glEnd.

ASSOCIATED GETS
glGet with argument GL_INDEX_MODE

SEE ALSO
glCopyPixels, glDrawPixels, glPixelMap, glPixelStore,
glPixelTransfer,
glReadBuffer

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