glReadPixels — read a block of pixels from the color buffer
void glReadPixels( | GLint x, |
| GLint y, | |
| GLsizei width, | |
| GLsizei height, | |
| GLenum format, | |
| GLenum type, | |
GLvoid * pixels); |
x, y
Specify the window coordinates of the first pixel that is read from the color 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. Must be either
GL_RGBA or the value of
GL_IMPLEMENTATION_COLOR_READ_FORMAT_OES.
type
Specifies the data type of the pixel data. Must be either
GL_UNSIGNED_BYTE or the value of
GL_IMPLEMENTATION_COLOR_READ_TYPE_OES.
pixels
Returns the pixel data.
glReadPixels
returns pixel data from the color buffer, starting with the
pixel whose lower left corner is at location
(x, y),
into client memory starting at location
pixels.
The processing of the pixel data before it is placed into
client memory can be controlled with
glPixelStorei.
glReadPixels
returns values from each pixel with lower left corner at
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 of the returned pixel values; accepted values are:
GL_ALPHAGL_RGBGL_RGBARGBA color components are read from the color buffer. Each color component is converted to floating point such that zero intensity maps to 0.0 and full intensity maps to 1.0.
Unneeded data is then discarded. For example,
GL_ALPHA
discards the red, green, and blue 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].
Finally, the components are converted to the proper, as
specified by type where each
component is multiplied by
n is the number of bits
per component.
Return values are placed in memory as follows. If
format is
GL_ALPHA,
a single value is returned and the data for the
ith pixel in the
jth row is placed in location
GL_RGB returns three values and
GL_RGBA returns four values for each pixel, with all values
corresponding to a single pixel occupying contiguous space in
pixels.
Storage parameter GL_PACK_ALIGNMENT set by
glPixelStorei,
affects the way that data is written into memory. See
glPixelStorei
for a description.
Only two format/type parameter pairs are
accepted. GL_RGBA/GL_UNSIGNED_BYTE is
always accepted, and the other acceptable pair can be discovered by querying
GL_IMPLEMENTATION_COLOR_READ_FORMAT_OES and
GL_IMPLEMENTATION_COLOR_READ_TYPE_OES.
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.
GL_INVALID_ENUM is generated if
format is not
GL_RGBA or the value of
GL_IMPLEMENTATION_COLOR_READ_FORMAT_OES.
GL_INVALID_ENUM is generated if
type is not
GL_UNSIGNED_BYTE or the value of
GL_IMPLEMENTATION_COLOR_READ_TYPE_OES.
GL_INVALID_VALUE is generated if either
width or
height is negative.
GL_INVALID_OPERATION is generated if type is
GL_UNSIGNED_SHORT_5_6_5
and format is not GL_RGB.
GL_INVALID_OPERATION is generated if type is
GL_UNSIGNED_SHORT_4_4_4_4 or
GL_UNSIGNED_SHORT_5_5_5_1
and format is not GL_RGBA.
GL_INVALID_OPERATION is generated if format
and type are neither GL_RGBA and
GL_UNSIGNED_BYTE, respectively, nor the format/type pair
returned by querying GL_IMPLEMENTATION_COLOR_READ_FORMAT_OES
and GL_IMPLEMENTATION_COLOR_READ_TYPE_OES.
glGet
with argument GL_IMPLEMENTATION_COLOR_READ_FORMAT_OES
glGet
with argument GL_IMPLEMENTATION_COLOR_READ_TYPE_OES
glGet with argument GL_PACK_ALIGNMENT
Copyright © 2003-2004 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.