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fix gif decoding of interlaced images

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This commit is contained in:
Matthew Flatt 2010-12-09 20:08:16 -07:00
parent 24a4fe52c7
commit e3c4a0ae98
1 changed files with 39 additions and 17 deletions
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56
collects/racket/draw/gif.rkt
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@ -185,20 +185,42 @@
[w (img-desc-width (image-desc i))]
[h (img-desc-height (image-desc i))]
[t (let ([v (image-transparent i)])
(and v (* v 3)))])
(values
w
h
(list->vector
(for/list ([j (in-range h)])
(let ([bstr (make-bytes (* 4 w) 255)])
(let ([yp (* w j)])
(for ([i (in-range w)])
(let ([pos (* 3 (bytes-ref data (+ yp i)))])
(when (eq? pos t)
;; transparent
(bytes-set! bstr (+ 3 (* i 4)) 0))
(bytes-set! bstr (* i 4) (bytes-ref ct pos))
(bytes-set! bstr (+ 1 (* i 4)) (bytes-ref ct (+ 1 pos)))
(bytes-set! bstr (+ 2 (* i 4)) (bytes-ref ct (+ 2 pos))))))
bstr)))))))
(and v (* v 3)))]
[vec
;; build vector of row byte strings
(list->vector
(for/list ([j (in-range h)])
(let ([bstr (make-bytes (* 4 w) 255)])
(let ([yp (* w j)])
(for ([i (in-range w)])
(let ([pos (* 3 (bytes-ref data (+ yp i)))])
(when (eq? pos t)
;; transparent
(bytes-set! bstr (+ 3 (* i 4)) 0))
(bytes-set! bstr (* i 4) (bytes-ref ct pos))
(bytes-set! bstr (+ 1 (* i 4)) (bytes-ref ct (+ 1 pos)))
(bytes-set! bstr (+ 2 (* i 4)) (bytes-ref ct (+ 2 pos))))))
bstr)))]
[vec2
(if (img-desc-interlace? (image-desc i))
;; reorder rows for interlace decoding
(let ([vec2 (make-vector h #f)])
(for ([i (in-range h)])
(let ([j (let ([count (quotient (- h 1) 8)])
(if (i . <= . count)
(* i 8)
(let ([i (- i (add1 count))]
[count (quotient (- h 5) 8)])
(if (i . <= . count)
(+ (* i 8) 4)
(let ([i (- i (add1 count))]
[count (quotient (- h 3) 4)])
(if (i . <= . count)
(+ (* i 4) 2)
(let ([i (- i (add1 count))])
(+ (* i 2) 1))))))))])
(vector-set! vec2 j (vector-ref vec i))))
vec2)
;; rows are already in order
vec)])
(values w h vec2))))