GIF89a to GIF87a

[PiMaster]

Does anyone know how to convert from a GIF89a to a GIF87a? Newgrounds requires a gif that has no transparency, so it must require GIF87a because it doesn't support it.

[Duthomhas]

You don't have to change it to a GIF87a. All you need to do is strip the offending GCE block out of the file. (Unless you have found a really ancient GIF decoder that you must use, you can safely assume that all GIF decoders can handle a GIF89a.)

You might find my old gifblock package (for Tcl) useful. With it you can load, modify, and save the blocks of a GIF very easily.
http://wiki.tcl.tk/16127

The complete GIF89a specification can be found here
http://www.w3.org/Graphics/GIF/spec-gif89a.txt

The only difference between the 87a and 89a formats is the introduction of a number of extension blocks, such as the GCE (and, of course, the fifth byte of the file is a '9' instead of a '7').


If you don't have Tcl, you can get something quick and easy to use here
http://code.google.com/p/tclkit/

Just download and save the executable file (say tclkitsh-8.5.9-win32.upx.exe ). You might want to name it something easier to type, like "tclsh.exe". Save the "gifblock.tcl" file I linked for you in the same place as you did the executable. Also save this as "nogce.tcl" in the same place:

source gifblock.tcl
namespace import gifblock::*

# Make sure the user gave the proper arguments
if {$argc != 2} {
  puts "usage:\n  $argv0 ORIGINAL.gif MODIFIED.gif\n"
  puts {Removes all instances of the GCE from the ORIGINAL GIF file}
  puts {and saves the changes as the MODIFIED GIF file.}
  exit
  }

# Load the GIF data
if {[catch {gif.load gifdata [lindex $argv 0]} msg]} {
  puts $msg
  exit 1
  }

puts {Original GIF}
foreach name [gif.blocknames gifdata] { puts "  $name" }

# Remove the GCE blocks
set ns [gif.index gifdata {Graphic Control} 0]
foreach n [lreverse $ns] {
  set gifdata [lreplace $gifdata $n $n]
  }

puts {Modified GIF}
foreach name [gif.blocknames gifdata] { puts "  $name" }

# Save the modified file
if {[catch {gif.save gifdata [lindex $argv 1]} msg]} {
  puts $msg
  exit 2
  }

puts Done

Then, from the command prompt, run the program, substituting your own filenames of course:

C:\tcl> tclsh  nogce.tcl  image-to-modify.gif  modified-image.gif

Assuming all went well you should see output something like

Original GIF
  GIF Header
  Color Table
  Logical Screen Descriptor
  Graphic Control
  Image Descriptor
Modified GIF
  GIF Header
  Color Table
  Logical Screen Descriptor
  Image Descriptor
done

Hope this helps.

[edit] Fixed an error