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Refactored the rotation code to support an arbitrary linear transformation

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This commit is contained in:
Robby Findler 2010-06-20 14:13:16 -05:00
parent 83a95970b6
commit 2e67f8bb9f
1 changed files with 87 additions and 2 deletions
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87
collects/mrlib/private/image-core-bitmap.rkt
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@ -6,7 +6,8 @@
(provide rotate-bytes ;; : bytes int[width] int[height] radians[radians] -> bytes
flip-bytes ;; : bytes int[width] int[height] -> bytes
bitmap->bytes
bytes->bitmap)
bytes->bitmap
linear-transform)
;; rotate-bitmap : (-> bytes? natural-number/c natural-number/c real? bytes?)
;; avoid a dependency on scheme/contract, which pulls in too much
@ -85,6 +86,7 @@ instead of this scaling code, we use the dc<%>'s scaling code.
[new-y (- h y 1)])
(bmbytes-ref/safe bmbytes w h new-x new-y)))))
#;
(define (rotate-bytes bmbytes w h theta)
(let* {[theta-rotation (exp (* i theta))]
[theta-unrotation (make-rectangular (real-part theta-rotation)
@ -114,6 +116,89 @@ instead of this scaling code, we use the dc<%>'s scaling code.
(- (imag-part pre-image))))))
new-w
new-h)))
;; linear transform: bytes width height <matrix coodinates> -> (values bytes width height)
;; The matrix is read like this:
;; +- -+
;; | a b |
;; | c d |
;; +- -+
;; The ai, bi, ci, and di are the coordinates of the inverse matrix
(define (linear-transform bmbytes w h a b c d)
(let-values ([(ai bi ci di)
(let ([k (/ (- (* a d) (* b c)))])
(values (* k d) (* k (- b))
(* k (- c)) (* k a)))])
;; mapp : <matrix> complex -> complex
;; applies the matrix represented by abcd(as in the picture above) to p
(define (mapp a b c d p)
(let ([x (real-part p)]
[y (imag-part p)])
(make-rectangular (+ (* a x) (* b y))
(+ (* c x) (* d y)))))
(let* {[f-rotation (λ (p) (mapp a b c d p))]
[f-unrotation (λ (p) (mapp ai bi ci di p))]
[ne (f-rotation w)]
[sw (f-rotation (* i (- h)))]
[se (f-rotation (make-rectangular w (- h)))]
[nw 0]
[pts (list ne sw se nw)]
[longitudes (map real-part pts)]
[latitudes (map imag-part pts)]
[east (apply max longitudes)]
[west (apply min longitudes)]
[nrth (apply max latitudes)]
[sth (apply min latitudes)]
[new-w (round/e (- east west))]
[new-h (round/e (- nrth sth))]}
(values (build-bmbytes new-w
new-h
(λ (x y)
(let* {[pre-image (f-unrotation (make-rectangular (+ west x 1/2) (- nrth y 1/2)))]}
(interpolate bmbytes w h
(real-part pre-image)
(- (imag-part pre-image))))))
new-w
new-h))))
(define (rotate-bytes bmbytes w h theta)
(let* ([theta-rotation (exp (* i theta))]
[x (real-part theta-rotation)]
[y (imag-part theta-rotation)])
(linear-transform
bmbytes w h
x (- y) y x)))
#;
(define (rotate-bytes bmbytes w h theta)
(let* {[theta-rotation (exp (* i theta))]
[theta-unrotation (make-rectangular (real-part theta-rotation)
(- (imag-part theta-rotation)))]
[f-rotation (λ (p) (* theta-rotation p))]
[f-unrotation (λ (p) (* theta-unrotation p))]
[ne (f-rotation w)]
[sw (f-rotation (* i (- h)))]
[se (f-rotation (make-rectangular w (- h)))]
[nw 0]
[pts (list ne sw se nw)]
[longitudes (map real-part pts)]
[latitudes (map imag-part pts)]
[east (apply max longitudes)]
[west (apply min longitudes)]
[nrth (apply max latitudes)]
[sth (apply min latitudes)]
[new-w (round/e (- east west))]
[new-h (round/e (- nrth sth))]}
(values (build-bmbytes new-w
new-h
(λ (x y)
(let* {[pre-image (f-unrotation (make-rectangular (+ west x 1/2) (- nrth y 1/2)))]}
(interpolate bmbytes w h
(real-part pre-image)
(- (imag-part pre-image))))))
new-w
new-h)))
;; Why the offsets of 1/2 in `rotate-bytes` and `interpolate`?
;; We consider a pixel's RGB as a point-sample taken from the 'true' image,
;; where the RGB is the sample at the *center* of the square covered by the pixel.