[cl-gd-devel] [question] change of colors when tiling an image

[colimal at mails.selgrad.org]

Hello all :)

I'm using cl-gd to generate tiles of a larger image. I'm generating a set of
tiles of each, e.g. 256x256 pixels, from an input image that is much larger,
e.g. 4096x2048.

When using "debug-images" (pngs) with few colors and large areas of the same
color this works well. However, when I want to tile a png file with higher
frequencies and lots of colors the resulting tile images seem to be resampled
somehow, with a huge loss of quality.

Here is part of my code:

  (with-image-from-file (src input-name)
   (let ((width (image-width src))
         (height (image-height src)))

        ;; store a single tile
        (defun generate-tile (x y)
          "Generates a single tile. The index of which is given in x and y"
          (with-image (dest size size)
            (copy-image src dest (* x size) (* y size) 0 0 size size)
            (write-image-to-file (concatenate 'string output-base "-" (write-to-string x) "-" (write-to-string y) "." ext)
                                 :image dest :if-exists :supersede :compression-level 0)))
               
        ;; master's actual code
        (let ((count-x (ceiling (/ width size)))
              (count-y (ceiling (/ height size))))
             (format t "Slicing up ~s (~d x ~d) into (~d x ~d) tiles of size (~d x ~d).~%" input-name width height count-x count-y size size)
             (dotimes (x count-x)
                      (dotimes (y count-y)
                               (generate-tile x y))))))

As you can see, I don't compress the image (doesn't change, if I leave the
compression key out), and only copy a given section to the output image.
The version of cl-gd I'm using is 0.5.7, the version of is libgd 2.0.0.

I would appreciate if someone would hint me to what I'm missing :)

Thanks in advance,
   Kai.