combining images where one is negatively scaled creates a blank image

[HeladoDeBrownie]

here is a program that unexpectedly produces a blank image:

#lang racket/base
(require pict)
(require racket/math)

(define original-shape (rotate (filled-rectangle 10 100) (/ pi 3)))

(ht-append original-shape (scale original-shape -1 1))

here are two programs, very similar to that one, that have no unexpected behavior:

#lang racket/base
(require pict)
(require racket/math)

(define original-shape (rotate (filled-rectangle 10 100) (/ pi 3)))

(ht-append original-shape original-shape)

#lang racket/base
(require pict)
(require racket/math)

(define original-shape (rotate (filled-rectangle 10 100) (/ pi 3)))

(ht-append (scale original-shape -1 1) (scale original-shape -1 1))

[rfindler]

I think that maybe the content of the pict in this example is disappearing because DrRacket clips the pict to its bounding box? This example shows something:

#lang racket/base
(require pict)
(require racket/math)

(define original-shape (rotate (filled-rectangle 10 100) (/ pi 3)))

(cc-superimpose
 (blank 200 200)
 (ht-append original-shape (scale original-shape -1 1)))

[maueroats]

There is a test case that contains the following:

(define (reflect p)
  (inset (scale p 1 -1) 0 (pict-height p)))

The inset is there to compensate for the negative bounding box by adding the original height to the top and bottom of the pict.

I don't like the idea that pict-height could be negative... but clearly at some point someone thought it was ok.

Demonstration that pict-height can be negative:

(require pict)
(define shape0 (filled-rectangle 100 10))
(define shape1 (scale shape0 1 -1))
(pict-height shape1)

Making the width/height of a scaled image always be positive is a breaking change, but it's also a pretty big surprise to have negatives in the first place... I'm not really sure what's an appropriate way to fix it.

What does the boss think?

[maueroats]

See Issue #29