Exercise 2.50
Define the transformation flip-horiz, which flips painters horizontally, and transformations that rotate painters counterclockwise by 180 degrees and 270 degrees.
这道题挺有意思的，搞明白这道题就明白了 frame 的3个点的位置。如上图所示，为了更好区分，特意用了长方形而不是正方形，第一幅图是原图，O 表示 origin, A 表示 edge1，B 表示 edge2。无论进行何种变换，左下坐标都是 (0, 0)，左上都是(0, 1)， 右下都是(1, 0)， 右上都是(1, 1)，我们只要把变换后的图形中 O, A, B 的新位置作为参数传进去就行。
#lang racket(require (planet "sicp.ss" ("soegaard" "sicp.plt" 2 1)))(define sub-vect vector-sub); Transforming and combining painters
(define (transform-painter painter origin corner1 corner2)(lambda (frame)(let ((m (frame-coord-map frame)))(let ((new-origin (m origin)))(painter (make-framenew-origin(sub-vect (m corner1) new-origin)(sub-vect (m corner2) new-origin)))))))(define (flip-horiz painter)(transform-painter painter(make-vect 1.0 0.0) ; new origin(make-vect 0.0 0.0) ; new end of edge1(make-vect 1.0 1.0))) ; new end of edge2(define (rotate90 painter)(transform-painter painter(make-vect 1.0 0.0) ; new origin(make-vect 1.0 1.0) ; new end of edge1(make-vect 0.0 0.0))) ; new end of edge2(define (rotate180 painter)(transform-painter painter(make-vect 0.0 1.0) ; new origin(make-vect 1.0 1.0) ; new end of edge1(make-vect 0.0 0.0))) ; new end of edge2(define (rotate270 painter)(transform-painter painter(make-vect 0.0 1.0) ; new origin(make-vect 0.0 0.0) ; new end of edge1(make-vect 1.0 1.0))) ; new end of edge2(paint einstein)
(paint (flip-horiz einstein))
(paint (rotate90 einstein))
(paint (rotate180 einstein))
(paint (rotate270 einstein))
效果如下图所示，为了跟水平翻转对比，特意把原图加上了：
