#28 Invisible images

[Neil Strickland]

I have some questions that involve dynamically
generated images (produced by p> plot(...), and handled
by the `aim/ImageCache` class). A significant number
of students have reported that they cannot see the
images: their browser indicates that the expected gif file
cannot be downloaded. I have not been able to
reproduce this behaviour. It may possibly be caused by
different Maple processes messing up each other's
images, but that is just a guess. The code is supposed
to prevent that from happening, anyway.

[Greg Gamble]

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At least some of the time this is caused by a student
clicking a link within the page before it has fully
downloaded. This may be a problem restricted to Internet
Explorer (possibly an IE bug?) It's only come up for me
very occasionally, and
I've worked around it by generating the same page as
administrator and copying the .gif to the right name (after
scanning the source .html for the student's page)
in the imagecache/ directory. Sometimes the students can
help themselves by
backing out and re-entering the quiz. Possibly, if they hit
`Validate' and waited for
a full download the whole page would be regenerated ... but
I haven't tested that idea. I don't think it's the result
of clashing Maple processes.

[Neil Strickland]

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The problem can be fixed in practice by increasing the
timeout in the Clean method of the ImageCache class from
600 to 6000 seconds. This means that the relevant images
must get deleted by the Clean method. That method only
deletes unlocked images, so the code that should lock the
relevant images must be broken.