imagecopyresampled() copies a
rectangular portion of one image to another image, smoothly
interpolating pixel values so that, in particular, reducing
the size of an image still retains a great deal of clarity.
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().
注: imagecopyresampled() requires GD 2.0.l or greater.
See also imagecopyresized().