Converting some images to PICT results in files that will crash a classic Mac system

[briankendall]

ImageMagick version

7.1.1-34

Operating system

MacOS

Operating system, version and so on

macOS 13.6.5

Description

Converting certain images to a PICT file will produce files that will crash the system when attempting to view them on a classic Mac OS system. I've reproduced the crash on Macintosh System 7.1, Mac OS 8, and Mac OS 9.

Converting the PICT file back to a png using ImageMagick does not present any issues. I have not yet identified a modern app that presents a problem viewing the bad PICT file ImageMagick produces.

I've attached both the png file that exhibits this bug and the resulting PICT file that will crash a classic Mac.

Steps to Reproduce

1. Convert the attached png file to a pict with the following:
   magick bad.png bad.pict
2. Copy bad.pict to a system running classic Mac OS and open it in any application that can view PICT files (such as SimpleText)
3. System freezes or displays a system error

Images

bad.pict.zip

[fmw42]

Perhaps Apple changed the PICT format between old classic Mac and newer OSX versions.

[briankendall]

Perhaps Apple changed the PICT format between old classic Mac and newer OSX versions.

I don't believe that's the case. The latest PICT format is "Extended Version 2" which has been around since at least the 1990s, and which is what ImageMagick is producing based off of the version information in the outputted file.

[dlemstra]

Is this not an issue/bug inside the SimpleText application?

[briankendall]

SimpleText uses the same system-level API to display a PICT as every other app in classic Mac OS. I can trigger the crash using any application that can read PICT files.

It's possible that their implementation has a bug that would cause it to crash reading an otherwise valid PICT, but I think that's unlikely. Also, given that Apple's PICT implementation from classic Mac OS is essentially the canonical implementation, I think we need to assume that ImageMagick is producing invalid files.

[briankendall]

I do believe I found the issue. The key is in page 119 of Inside Macintosh: Volume V, where it describes how the pixel data of a PICT should be written out. Here's a screen shot of the relevant section:

The key part is where it says "If rowBytes > 250". In bad.pict, rowBytes is 240, but the byteCount of each scanline is two bytes rather than one. I just manually modified the file so that the byteCount entries for each scanline is only one byte, and it fixed the file, in that classic Mac OS can now properly read it. (See : fixed.pict.zip )

This bug apparently affects ImageMagick's PICT reader as well, as it's unable to open fixed.pict and produces this error:

error/pict.c/ReadPICTImage/1267.

In fact, I think that points me to the error, see:

ImageMagick/coders/pict.c

Line 504 in b77eb77

if (bytes_per_line > 200)

That should be:

if (bytes_per_line > 250)

Same here:

ImageMagick/coders/pict.c

Line 694 in b77eb77

if (bytes_per_line > 200)

And here too:

ImageMagick/coders/pict.c

Line 1087 in b77eb77

if (length > 200)

[briankendall]

Since I went through all the effort of writing that out, why not have a PR! #7840

[dlemstra]

The PR has been merged and thanks for taking the time to fix this for us.

[bobfriesenhahn]

Failing to be able to read the existing PICT files that ImageMagick wrote for perhaps 30 years seems pretty brutal.