Both must be nonnegative. Description glCopyPixels copies a screen-aligned rectangle of pixels from the specified frame buffer location to a region relative to the current raster position. Its operation is well defined only if the entire pixel source region is within the exposed portion of the window. Results of copies from outside the window, or from regions of the window that are not exposed, are hardware dependent 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 effects on glCopyPixels of most, but not all, of the parameters specified by these three commands. This pixel is said to be the ith pixel in the jth 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.