From: Tim Hunter <TimHunter@...>
Date: 2009-08-28T09:45:24+09:00
Subject: Re: RMagick reacts very oddly to an incorrect file

Pito Salas wrote:
> Tim Hunter wrote:
>> Pito Salas wrote:
>>>             Magick::Image.read(f, &block).each { |n| @images << n }
>>>
>>> will have a comment... But if anyone, especially  TimH can shed some
>>> light I would appreciate it!
>>>
>>> I want to put some defensive code into my stuff but I am not sure what I
>>> am checking for. Is it just a very large file or what?
>>>
>>> Thanks!!
>>>
>>> Pito
>> My guess is that you ran out of memory and OS X started swapping to
>> disk. This would explain your disk whirling and your computer getting
>> unresponsive. Killing the process freed the memory and things went back
>> to normal.
> 
> But why would a 40M file run out of memory on a machine with 2G of 
> memory, and why didn't it just throw an error? By the way if I use 
> ImageMagick itself, off the command line with the identify command it 
> has no problem with the file.

Without the file I can only guess. It didn't throw an error because it 
hadn't encountered an error yet. RMagick doesn't report an error until 
ImageMagick reports an error, and if OS X is still swapping stuff out to 
disk trying to allocate memory ImageMagick hasn't failed yet.

Why would a 40M file use up a lot of memory? Thanks to compression the 
file size on disk doesn't have a lot to do with how big the image is in 
memory. What quantum depth is ImageMagick configured to use? You can 
find out by running this command:

convert -version

It'll say either Q8 or Q16. Most people are using Q16. If you are, then 
ImageMagick uses 2 bytes for each of the R, G, B, and A channels, or 8 
bytes per pixel, plus about 1 byte per pixel overhead. So figure 9*W*H 
bytes per image in the file, where W is the width in pixels and H is the 
height. You said this file had about 50 images in it. You didn't say how 
big the images are, but you can do the arithmetic.

Why would identify work? Identify doesn't need to keep the entire image 
in memory.

If you want to put in some defensive code, check out the 
Magick.limit_resource method: 
http://studio.imagemagick.org/RMagick/doc/magick.html#limit_resource.

But all this is just my best guess. If you want me to analyze your 
specific image open a support request on RubyForge and attach the file. 
Also tell me the version of ImageMagick and the version of RMagick 
you're using.

-- 
MagickWand for Ruby - http://magickwand.rubyforge.org/