Both must be nonnegative. Its operation is well defined only if the entire pixel source region is within the exposed portion of the window. Results from copies from outside the window, or from regions of the window that are not exposed, are hardware dependant and undefined. Both width and height must not be negative.
Several parameters control the processing of the pixel data while it is being copied.
This reference page describes the effect on glCopyPixels of most, but not all, of the parameters specified by these three commands. This pixel is said to be the i th pixel in the j th row. Pixels are copied in row order from the lowest to the highest row, left to right in each row. If the GL is in color index mode, each index that is read from this buffer is converted to a fixed-point format with an unspecified number of bits to the right of the binary point. In either case, zero bits fill otherwise unspecified bit locations in the result.