14.015 How do I use glDrawPixels() and glReadPixels()?
glDrawPixels() and glReadPixels() write and read rectangular areas to and from the framebuffer, respectively. Also, you can access stencil and depth buffer information with the format parameter. Single pixels can be written or read by specifying width and height parameters of 1.
glDrawPixels() draws pixel data with the current raster position at the lower left corner. Problems using glDrawPixels() typically occur because the raster position is set incorrectly. When the raster position is set with the glRasterPos*() function, it is transformed as if it were a 3D vertex. Then the glDrawPixels() data is written to the resulting device coordinate raster position. (This allows you to tie pixel arrays and bitmap data to positions in 3D space).
When the raster position is outside the view volume, it's clipped and the glDrawPixels() call isn't rendered. This occurs even when part of the glDrawPixels() data would be visible. Here's info on how to render when the raster position is clipped.
glReadPixels() doesn't use the raster position. Instead, it obtains its (X,Y) device coordinate address from its first two parameters. Like glDrawPixels(), the area read has x and y for the lower left corner. Problems can occur when reading pixels if:
The area being read is from a window that is overlapped or partially offscreen. glReadPixels() will return undefined data for the obscured area. (More info.)
Memory wasn't allocated for the return data (the 7th parameter is a NULL pointer) causing a segmentation fault, core dump, or program termination. If you think you've allocated enough memory, but you still run into this problem, try doubling the amount of memory you've allocated. If this causes your read to succeed, chances are you've miscalculated the amount of memory needed.
For both glDrawPixels() and glReadPixels(), keep in mind:
The width and height parameters are in pixels.
If the drawn or read pixel data seems correct, but is slightly off, make sure you've set alignment correctly. Argument values are controlled with the glPixelStore*() functions. The PACK and UNPACK values control sending and receiving pixel data, from and to OpenGL, respectively.
