SICP图片语言中below过程实现行为异常的原因排查请求

Hey there! Let's figure out why your below procedure is acting wonky. First, let's look at the code you shared:

#lang sicp
(#%require sicp-pict)
(define (transform-painter painter origin corner1 corner2)
  (lambda (frame)
    (let ((m (frame-coord-map frame)))
      (let ((new-origin (m origin)))
        (painter
         (make-frame new-origin
                     (vector-sub (m corner1) new-origin)
                     (vector-sub (m corner2) new-origin)))))))
(define (below painter1 painter2)
  (let ((split-point (make-vect 0.0 0.5)))
    (...

The Core Issue #

Right off the bat, your below procedure is incomplete—you've defined the split-point but left the critical logic (transforming both painters and combining them) as .... Even if you had started filling that in, there's a good chance you might have mixed up the transformation coordinates for the upper and lower painters, which would cause unexpected behavior.

How to Fix It #

The below procedure needs to do three key things:

1. Transform painter1 to draw in the bottom half of the target frame.
2. Transform painter2 to draw in the top half of the target frame.
3. Combine these two transformed painters into a single painter that draws both.

Here's the complete, working implementation using your existing transform-painter definition:

(define (below painter1 painter2)
  (let ((split-point (make-vect 0.0 0.5)))
    ;; Transform painter1 to occupy the bottom half of the frame
    (let ((paint-bottom (transform-painter painter1
                                           (make-vect 0.0 0.0)   ; new origin: bottom-left corner
                                           (make-vect 1.0 0.0)   ; new corner1: bottom-right corner
                                           split-point))         ; new corner2: middle-left point
          ;; Transform painter2 to occupy the top half of the frame
          (paint-top (transform-painter painter2
                                        split-point             ; new origin: middle-left corner
                                        (make-vect 1.0 0.5)     ; new corner1: middle-right corner
                                        (make-vect 0.0 1.0))))  ; new corner2: top-left corner
      ;; Return a new painter that draws both the bottom and top parts
      (lambda (frame)
        (paint-bottom frame)
        (paint-top frame)))))

Why This Works #

• For paint-bottom: We map painter1 to a frame that starts at the bottom-left of the original frame, stretches to the bottom-right, and goes up to the midpoint (y=0.5). This constrains the painter to the lower half.
• For paint-top: We map painter2 to a frame that starts at the midpoint (y=0.5) on the left, stretches to the midpoint on the right, and goes up to the top-left of the original frame. This constrains the painter to the upper half.
• The final lambda combines both painters by drawing the bottom one first, then the top one over it (the order might not matter for simple shapes, but it's good practice to define it clearly).

Bonus Alternative #

If you want a more concise version, you can reuse the beside procedure by rotating the painters and the result:

(define (below painter1 painter2)
  (rotate270 (beside (rotate90 painter2) (rotate90 painter1))))

This relies on having rotate90 and rotate270 implemented (which are also built with transform-painter), but it's a clever way to leverage existing code.

内容的提问来源于stack exchange，提问作者mishless