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gif colour changes

by matthewsnape (Acolyte)
on Jun 15, 2005 at 22:40 UTC ( #467101=perlquestion: print w/ replies, xml ) Need Help??
matthewsnape has asked for the wisdom of the Perl Monks concerning the following question:

I have a CGI script that changes GIF images. It replaces each colour throughout the image with a different one. I have been using the Image::Magick Opaque function to do this. However, it is to slow and I would like to speed it up. What is the best way of changeing all instances of a colour throughout an image? Can I alter GIF palettes in perl (without GD) without touching the individual pixels?

Comment on gif colour changes
Re: gif colour changes
by cmeyer (Pilgrim) on Jun 15, 2005 at 23:54 UTC

    Yes, you can.


    (Read the fine documentation: GD. Look at: $image->colorDeallocate(colorIndex) and  $image->colorAllocateAlpha(reg,green,blue,alpha) and related friends.

    -Colin.

    WHITEPAGES.COM | INC

Re: gif colour changes
by BrowserUk (Pope) on Jun 16, 2005 at 01:12 UTC

    The GIF file format is widely described on the internet. One of the better descriptions is here.

    This piece of code tweaks the global colour table (simply reversing the indexes):

    #! perl -slw use strict; open GIF, '<:raw', $ARGV[ 0 ] or die $!; my $gif = do{ local $/; <GIF> }; close GIF; ## Extract the colour table my @GCT = unpack '(A3)256', substr $gif, 13, 256 * 3; ## SWap the colours end to end @GCT = reverse @GCT; ## put the colour table back substr( $gif, 13, 256*3 ) = pack '(A3)256', @GCT; open OUT, '>:raw', 'modified.gif' or die $!; print OUT $gif; close OUT;

    To do things properly, you would need to check the size of the colour table entry (see byte 10-bits 4..7 of the header) and account for colour tables that use less that 24-bits per entry. Also, you should check bit-0 of the same byte to determine if global or local colour tables are in use, but the code may get you started.


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