The convert-external-image function returns an image derived from external-image . The image is ready for drawing to the given graphics port. If cache is non- nil images conversions are cached by (color-device a b ) in the external-image where a is the transparent-pixel-index and b the image-background-color of the port. If force-new is non- nil a new image is always created, and put in the cache. By default, cache has the value *cache-external-images-p* .