imagecopyresized() copies a
rectangular portion of one image to another image. Dst_im is the destination image, src_im is the source image
identifier. If the source and destination coordinates and
width and heights differ, appropriate stretching or shrinking
of the image fragment will be performed. The coordinates
refer to the upper left corner. This function can be used to
copy regions within the same image (if
dst_im is the same as
src_im) but if the regions overlap the results will
be unpredictable.
注: There is a problem due to palette image limitations (255+1 colors). Resampling or filtering an image commonly needs more colors than 255, a kind of approximation is used to calculate the new resampled pixel and its color. With a palette image we try to allocate a new color, if that failed, we choose the closest (in theory) computed color. This is not always the closest visual color. That may produce a weird result, like blank (or visually blank) images. To skip this problem, please use a truecolor image as a destination image, such as one created by imagecreatetruecolor().
See also imagecopyresampled().