extra IDATs

John Bowler a écrit :
> ....
>   

i don't understand why you are being so hostile, i'm just
trying to 
help. here, i even made an image that will illustrate the
problem. it's 
toucan.png from the png book with an extra, empty IDAT added
in between 
the last IDAT and IEND. here's the debug output from libpng
used in a 
basic app that just reads everything (just as in the png
tutorial / the 
png book, you can have the source if you like):

.....
        in png_do_background
        in png_combine_row
        in png_read_finish_row
        in png_read_end
                        reading 4 bytes
                        reading 4 bytes
                        reading 4 bytes
Reading IDAT chunk.
libpng error: Too many IDAT's found
        in png_destroy_read_struct
        in png_read_destroy
.....

just as i said. imagemagick is also unable to display it.
but seamonkey 
works for some reason, maybe they are using a different
version.

http://l
ittlesvr.ca/misc/andorbug-toucan.png

cheers



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From: Andrew Smith
>i don't understand why you are being so hostile

Sorry, that wasn't my intention.  I was trying to analyse
whether there is a problem in the absence of an example,
however:

>i even made an image that will illustrate the problem.
it's 
>toucan.png from the png book with an extra, empty IDAT
added in between 
>the last IDAT and IEND. 
>
>http://l
ittlesvr.ca/misc/andorbug-toucan.png

Which disproves my assertion...  That's a valid PNG yet at
least
some versions of libpng fail to display it (I used IE6, I
don't
know for sure that this uses libpng and it is probably still
an
old version).  Windows itself and MS Office, which don't
use
libpng, display it perfectly 

xv 3.10a displays it, but it encounters the error (which it
ignores, I guess that's ok for an image editor).  xv is
using
libpng-1.2.8

I guess a minimal fix would be to remove the PNG_AFTER_IDAT
part of the test, but that would allow zero length IDAT
after
a non-IDAT chunk.

John Bowler <jbowleracm.org>



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