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fix alpha-only mode of `get-argb-pixels'

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Closes PR 11927
This commit is contained in:
Matthew Flatt 2011-05-17 15:49:21 -07:00
parent b65054134a
commit 905eb11651
3 changed files with 31 additions and 2 deletions
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2
collects/racket/draw/private/bitmap.rkt
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@ -718,7 +718,7 @@
(for ([j (in-range y (min (+ y h) height))]) (for ([j (in-range y (min (+ y h) height))])
(let ([row (* j row-width)]) (let ([row (* j row-width)])
(for ([i (in-range x (min (+ x w) width))]) (for ([i (in-range x (min (+ x w) width))])
(let ([p (* 4 (+ i (* j w)))] (let ([p (* 4 (+ (- i x) (* (- j y) w)))]
[q (+ row (* i 4))]) [q (+ row (* i 4))])
(bytes-set! bstr p (bytes-ref data (+ q A))))))))) (bytes-set! bstr p (bytes-ref data (+ q A)))))))))

3
collects/scribblings/draw/bitmap-dc-class.scrbl
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@ -76,7 +76,8 @@ The pixel RGB values are copied into @scheme[pixels]. The first byte
If @scheme[alpha?] is false, if the bitmap does not have an alpha If @scheme[alpha?] is false, if the bitmap does not have an alpha
channel, then the alpha value for each pixel is set to 255. If channel, then the alpha value for each pixel is set to 255. If
@scheme[alpha?] is true, then @italic{only} the alpha value is set @scheme[alpha?] is true, then @italic{only} the alpha value is set
for each pixel, based on each pixel's inverted value. Thus, when a for each pixel; if the bitmap has no alpha channel, then the alpha
value is based on each pixel's inverted RGB average. Thus, when a
bitmap has a separate mask bitmap, the same @scheme[pixels] byte bitmap has a separate mask bitmap, the same @scheme[pixels] byte
string is in general filled from two bitmaps: one (the main image) string is in general filled from two bitmaps: one (the main image)
for the pixel values and one (the mask) for the alpha values. for the pixel values and one (the mask) for the alpha values.

28
collects/tests/gracket/dc.rktl
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@ -365,4 +365,32 @@
;; ---------------------------------------- ;; ----------------------------------------
(let ()
(define (get-column-alpha bm x y)
(define bs (make-bytes 4))
(send bm get-argb-pixels x y 1 1 bs #t)
bs)
(define abm (make-object bitmap% 2 2 #f #t))
(define nbm (make-object bitmap% 2 2 #f #f))
(define (avg bstr) (- 255
(quotient (+ (bytes-ref bstr 0)
(bytes-ref bstr 1)
(bytes-ref bstr 2))
3)))
(send abm set-argb-pixels 0 0 2 2 #"0123456789abcdef")
(send nbm set-argb-pixels 0 0 2 2 #"0123456789abcdef")
(test (bytes (char->integer #\0) 0 0 0) 'a0+0 (get-column-alpha abm 0 0))
(test (bytes (char->integer #\4) 0 0 0) 'a1+0 (get-column-alpha abm 1 0))
(test (bytes (char->integer #\8) 0 0 0) 'a0+1 (get-column-alpha abm 0 1))
(test (bytes (char->integer #\c) 0 0 0) 'a1+1 (get-column-alpha abm 1 1))
(test (bytes (avg #"123") 0 0 0) 'n0+0 (get-column-alpha nbm 0 0))
(test (bytes (avg #"567") 0 0 0) 'n1+0 (get-column-alpha nbm 1 0))
(test (bytes (avg #"9ab") 0 0 0) 'n0+1 (get-column-alpha nbm 0 1))
(test (bytes (avg #"def") 0 0 0) 'n1+1 (get-column-alpha nbm 1 1)))
;; ----------------------------------------
(report-errs) (report-errs)