imagetruecolortopalette()
converts a truecolor image to a palette image. The code for
this function was originally drawn from the Independent JPEG
Group library code, which is excellent. The code has been
modified to preserve as much alpha channel information as
possible in the resulting palette, in addition to preserving
colors as well as possible. This does not work as well as
might be hoped. It is usually best to simply produce a
truecolor output image instead, which guarantees the highest
output quality.
dither indicates if
the image should be dithered - if it is
TRUE then dithering will be used which will
result in a more speckled image but with better color
approximation.
ncolors sets the
maximum number of colors that should be retained in the
palette.
注: This function was added in PHP 4.0.6 and requires GD 2.0.1 or later