Script to disassemble GIFs, to help with debugging broken GIFs.

svn path=/trunk/libnsgif/; revision=5949

1 files changed, 109 insertions, 0 deletions

diff --git a/examples/disassemble_gif.pl b/examples/disassemble_gif.pl
new file mode 100755
index 0000000..2a6cbb1
--- /dev/null
+++ b/examples/disassemble_gif.pl
@@ -0,0 +1,109 @@
+#!/usr/bin/perl
+#
+# Disassemble a GIF file and display the sections and chunks within.
+#
+# Warning: only part of the specification is implemented.
+#
+
+use strict;
+use warnings;
+
+die "Usage: $0 IMAGE\n" unless @ARGV == 1;
+my ($image) = @ARGV;
+
+open IMAGE, '<', $image or die "$0: open $image: $!\n";
+undef $/;
+my $gif = <IMAGE>;
+close IMAGE;
+
+print "$image: ", length $gif, " bytes\n\n";
+
+my @gif = unpack 'C*', $gif;
+my $z = 0;
+
+output_chunk('Header', 6);
+output_chunk('Logical Screen Descriptor', 7);
+
+my $global_colors = $gif[10] & 0x80;
+my $color_table_size = 2 << ($gif[10] & 0x07);
+
+if ($global_colors) {
+	output_chunk('Global Color Table', $color_table_size * 3);
+}
+
+while (1) {
+	while ($gif[$z] == 0x21) {
+		if ($gif[$z + 1] == 0xf9) {
+			output_chunk('Graphic Control Extension', 5 + 2);
+		} elsif ($gif[$z + 1] == 0xfe) {
+			output_chunk('Comment Extension', 2);
+		} elsif ($gif[$z + 1] == 0x01) {
+			output_chunk('Plain Text Extension', 13 + 2);
+		} elsif ($gif[$z + 1] == 0xff) {
+			output_chunk('Application Extension', 12 + 2);
+		} else {
+			output_chunk((sprintf 'Unknown Extension 0x%.2x',
+					$gif[$z + 1]), $gif[$z + 2] + 3);
+		}
+		
+		while ($gif[$z] != 0) {
+			output_chunk('Data Sub-block', $gif[$z] + 1);
+		}
+		output_chunk('Block Terminator', 1);
+	}
+	
+	if ($gif[$z] == 0x3b) {
+		output_chunk('Trailer', 1);
+		last;
+	}
+	
+	if ($gif[$z] != 0x2c) {
+		last;
+	}
+	
+	output_chunk('Image Descriptor', 10);
+	
+	output_chunk('Table Based Image Data', 1);
+
+	while ($gif[$z] != 0) {
+		output_chunk('Data Sub-block', $gif[$z] + 1);
+	}
+	output_chunk('Block Terminator', 1);
+}
+
+if ($z != @gif) {
+	output_chunk('*** Junk on End ***', @gif - $z);
+}
+
+
+#
+# Output a chunk of data as hex and characters.
+#
+sub output_chunk
+{
+	my ($description, $length) = @_;
+
+	print "$description";
+	for (my $i = 0; $i != $length; $i++) {
+		if ($i % 8 == 0) {
+			print "\n";
+			printf "%8i:  ", $z + $i;
+		}
+		if ($z + $i == @gif) {
+			print "EOF\n\n";
+			print "Unexpected end of file\n";
+			exit;
+		}
+		my $c = $gif[$z + $i];
+		printf "%.2x ", $c;
+		if (32 <= $c and $c <= 126) {
+			printf "'%c'", $c;
+		} else {
+			print "   ";
+		}
+		print "   ";
+	}
+	print "\n\n";
+	
+	$z += $length;
+}