glReadPixels — read a block of pixels from the frame buffer
void glReadPixels( | GLint x, |
| GLint y, | |
| GLsizei width, | |
| GLsizei height, | |
| GLenum format, | |
| GLenum type, | |
GLvoid * data); |
x, ySpecify 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_ALPHA,
GL_RGB, and
GL_RGBA.
type
Specifies the data type of the pixel data.
Must be one of
GL_UNSIGNED_BYTE,
GL_UNSIGNED_SHORT_5_6_5,
GL_UNSIGNED_SHORT_4_4_4_4, or
GL_UNSIGNED_SHORT_5_5_5_1.
dataReturns the pixel data.
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 data.
The GL_PACK_ALIGNMENT parameter, set with the
glPixelStorei
command, affects the processing of the pixel data before it is placed into
client memory.
glReadPixels returns values from each pixel with lower left corner at
format specifies the format for 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.
The final values are clamped to the range
Finally, the components
are converted to the proper format,
as specified by type.
When type is GL_UNSIGNED_BYTE,
each component is multiplied by
type is GL_UNSIGNED_SHORT_5_6_5,
GL_UNSIGNED_SHORT_4_4_4_4, or GL_UNSIGNED_SHORT_5_5_5_1,
each component is multiplied by
Return values are placed in memory as follows.
If format is
GL_ALPHA,
a single value is returned and the data for the
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 data.
Storage parameter GL_PACK_ALIGNMENT, set by
glPixelStorei,
affects the way that data is written into memory.
See glPixelStorei for a description.
If the currently bound framebuffer is not the default framebuffer object, color
components are read from the color image attached to the
GL_COLOR_ATTACHMENT0 attachment point.
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 and
GL_IMPLEMENTATION_COLOR_READ_TYPE.
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 data.
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 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
and GL_IMPLEMENTATION_COLOR_READ_TYPE.
GL_INVALID_FRAMEBUFFER_OPERATION is generated if
the currently bound framebuffer is not framebuffer complete (i.e. the
return value from glCheckFramebufferStatus
is not GL_FRAMEBUFFER_COMPLETE).
glGet with argument
GL_IMPLEMENTATION_COLOR_READ_FORMAT
or GL_IMPLEMENTATION_COLOR_READ_TYPE
glGet with argument GL_PACK_ALIGNMENT
Copyright © 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.