[GM-help] identify doesn't detect truncated gif file (GIMP does)

[Alan J. <ala...@gm...>]

Hi

Ubuntu 12.04 seems to have switched me to GraphicsMagick.  I had a nice 
script to scan for corrupt images, but it's stopped working now it's 
calling gm's "identify" instead of the ImageMagick version.

Like ImageMagick, the manpage claims that "identify" will "report if an 
image is incomplete or corrupt".  Like ImageMagick, the manpage is... 
well, I'd call it a documentation bug :).

$ ls -l icn_enlarge.gif
-r-------- 1 alan alan 200 Jul  9 21:41 icn_enlarge.gif
$ dd bs=1 count=199 if=icn_enlarge.gif of=i.gif
199+0 records in
199+0 records out
199 bytes (199 B) copied, 0.00120877 s, 165 kB/s
$ gm identify i.gif
i.gif GIF 17x17+0+0 PseudoClass 32c 8-bit 199 0.000u 0:01

GIMP loads the image, but briefly shows an error in the status bar 
"EOF/read error on image data."

When I posted a similar problem on the ImageMagick forums, 
<http://www.imagemagick.org/discourse-server/viewtopic.php?f=3&t=20045, 
the answer was that I needed to use "-regardwarnings" to get errors 
reflected in the exit code, and "-verbose" to trick ImageMagick to 
decode all the image data.  (Hence my disillusionment with the 
broad-brush statement in the manpage).  But GraphicsMagick doesn't 
support -regardwarnings, so my script doesn't work any more.

For GraphicsMagick, the manpage suggests that "+ping" might help, but it 
still doesn't recognise the gif file as being truncated.  Is this a 
bug?  Is there some other option I need, in order to request the desired 
behaviour?

Alan