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5 GIF and Animated GIF Writing

 (require mrlib/gif)


(write-gif bitmap filename)  void?

  bitmap : 
(or/c (is-a?/c bitmap%)
      (-> (is-a?/c bitmap%)))
  filename : path-string
Writes the given bitmap to filename as a GIF image, where bitmap is either an instance of bitmap% or a thunk (to be called just once) that generates such an object. If the bitmap uses more than 256 colors, it is automatically quantized using a simple algorithm; see quantize. If the bitmap has a mask bitmap via get-loaded-mask, it is used to determine transparent pixels in the generated GIF image.


(write-animated-gif bitmaps 
  [#:loop loop? 
  #:one-at-a-time? one-at-a-time? 
  #:last-frame-delay last-frame-delay]) 
  bitmaps : 
  (listof (or/c (is-a?/c bitmap%)
                (-> (is-a?/c bitmap%))))
  delay-csec : (integer-in 0 4294967295)
  filename : path-string
  loop? : any/c = (and delay-csec #t)
  one-at-a-time? : any/c = #f
  last-frame-delay : (or/c (integer-in 0 4294967295) false/c)
   = #f
Writes the bitmaps in bitmaps to filename as an animated GIF. The bitmaps list can contain a mixture of bitmap% objects and thunks (each called just once) that produce bitmap% objects. The delay-csec argument is the amount of time in 1/100s of a second to wait between transitions. If loop? is a true value, then the GIF is marked as a looping animation.

If one-at-a-time? is #f, then the content of all images is collected and quantized at once, to produce a single colortable; a drawback to this approach is that it uses more memory, and it allows less color variation among animation frames. Even when one-at-a-time? is #f, the result of each thunk in bitmaps is converted to a byte-string one at a time.

If one-at-a-time? is true, then the bitmaps are quantized and written to the file one at a time; that is, for each thunk in bitmaps, its result is written and discarded before another thunk is called. A drawback to this approach is that a separate colortable is written for each frame in the animation, which can make the resulting file large.

If last-frame-delay is not false, a delay of last-frame-delay (in 1/100s of a second) is added to the last frame. This extra delay is useful when loop? is true.