DrScheme version 370 contains a bug that manifests itself on Linux systems and other systems running the X11 windowing system, but not on the normal versions of DrScheme for Mac OS X or Microsoft Windows. This bug was fixed in DrScheme version 371. (It is bug number 8821.) The purpose of this web page is to explain the impact of this bug on the functional graphics package and especially to give instructions on how to work around it enough to be able to use the Concrete Abstractions textbook, if for some reason you are stuck using DrScheme 370 rather than a newer version.
The upshot of the bug is that on affected systems, the following
procedures produce incorrect results when given black-and-white images:
quarter-turn-right,
invert, mirror-image, resize-image, and threshold.
For the quilting exercises in chapters 1-3, you can use the version of quilting.scm that is linked here. This provides definitions of the basic blocks that look black-and-white (just like those in the book) but are actually, from a technical perspective, color images; it just happens that the only colors used are black and white. As such, you will not encounter the bug when turning and inverting these images for the quilting exercises. The only exercise that can't be done is exercise 1.11, regarding the overlaying of filled triangles. Rather than limit yourself to the images we provide in this file, you can also use DrScheme's "Insert Image..." feature.
For exercise 2.23, which concerns making images of numbers, you can use the version of digits.scm that is linked here. Again, the images in this file are "color" images that happen to only have black and white in them. As such, they can be turned without trouble, which is a necessary part of putting them side by side.
For the fractal curves in section 4.3 (and exercise 4.20), the bug
shouldn't be a problem because the lines are only
combined using overlay, which works correctly.