Move NetSurf's gifread.c to libnsgif

svn path=/branches/dynis/libnsgif/; revision=4344

1 files changed, 1004 insertions, 0 deletions

diff --git a/libnsgif.c b/libnsgif.c
new file mode 100644
index 0000000..30fd19e
--- /dev/null
+++ b/libnsgif.c
@@ -0,0 +1,1004 @@
+/*
+ * Copyright 2004 Richard Wilson <not_ginger_matt@users.sourceforge.net>
+ *
+ * This file is part of NetSurf, http://www.netsurf-browser.org/
+ *
+ * NetSurf is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ *
+ * NetSurf is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <stdbool.h>
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include "gifread.h"
+#include "image/bitmap.h"
+#include "utils/log.h"
+
+/*	READING GIF FILES
+	=================
+
+	The functions provided by this file allow for efficient progressive GIF
+	decoding. Whilst the initialisation does not ensure that there is
+	sufficient image data to complete the entire frame, it does ensure that
+	the information provided is valid. Any subsequent attempts to decode an
+	initialised GIF are guaranteed to succeed, and any bytes of the image
+	not present are assumed to be totally transparent.
+
+	To begin decoding a GIF, the 'gif' structure must be initialised with
+	the 'gif_data' and 'buffer_size' set to their initial values. The
+	'buffer_position' should initially be 0, and will be internally updated
+	as the decoding commences. The caller should then repeatedly call
+	gif_initialise() with the structure until the function returns 1, or
+	no more data is avaliable.
+
+	Once the initialisation has begun, the decoder completes the variables
+	'frame_count' and 'frame_count_partial'. The former being the total
+	number of frames that have been successfully initialised, and the
+	latter being the number of frames that a partial amount of data is
+	available for. This assists the caller in managing the animation whilst
+	decoding is continuing.
+
+	To decode a frame, the caller must use gif_decode_frame() which updates
+	the current 'frame_image' to reflect the desired frame. The required
+	'background_action' is also updated to reflect how the frame should be
+	plotted. The caller must not assume that the current 'frame_image' will
+	be valid between calls if initialisation is still occuring, and should
+	either always request that the frame is decoded (no processing will
+	occur if the 'decoded_frame' has not been invalidated by initialisation)
+	or perform the check itself.
+
+	It should be noted that gif_finalise() should always be called, even if
+	no frames were initialised.
+
+	[rjw] - Fri 2nd April 2004
+*/
+
+
+
+/*	Internal GIF routines
+*/
+static int gif_initialise_sprite(struct gif_animation *gif, unsigned int width, unsigned int height);
+static int gif_initialise_frame(struct gif_animation *gif);
+static unsigned int gif_interlaced_line(int height, int y);
+
+
+
+/*	Internal LZW routines
+*/
+static void gif_init_LZW(struct gif_animation *gif);
+static bool gif_next_LZW(struct gif_animation *gif);
+static int gif_next_code(struct gif_animation *gif, int code_size);
+
+/*	General LZW values. They are shared for all GIFs being decoded, and
+	thus we can't handle progressive decoding efficiently without having
+	the data for each image which would use an extra 10Kb or so per GIF.
+*/
+static unsigned char buf[4];
+static unsigned char *direct;
+static int maskTbl[16] = {0x0000, 0x0001, 0x0003, 0x0007, 0x000f, 0x001f, 0x003f, 0x007f,
+			  0x00ff, 0x01ff, 0x03ff, 0x07ff, 0x0fff, 0x1fff, 0x3fff, 0x7fff};
+static int table[2][(1 << GIF_MAX_LZW)];
+static unsigned char stack[(1 << GIF_MAX_LZW) * 2];
+static unsigned char *stack_pointer;
+static int code_size, set_code_size;
+static int max_code, max_code_size;
+static int clear_code, end_code;
+static int curbit, lastbit, last_byte;
+static int firstcode, oldcode;
+static bool zero_data_block = false;
+static bool get_done;
+
+/*	Whether to clear the decoded image rather than plot
+*/
+static bool clear_image = false;
+
+
+/*	Initialises any workspace held by the animation and attempts to decode
+	any information that hasn't already been decoded.
+	If an error occurs, all previously decoded frames are retained.
+
+	@return GIF_FRAME_DATA_ERROR for GIF frame data error
+		GIF_INSUFFICIENT_FRAME_DATA for insufficient data to process
+		          any more frames
+		GIF_INSUFFICIENT_MEMORY for memory error
+		GIF_DATA_ERROR for GIF error
+		GIF_INSUFFICIENT_DATA for insufficient data to do anything
+		0 for successful decoding
+		1 for successful decoding (all frames completely read)
+*/
+int gif_initialise(struct gif_animation *gif) {
+	unsigned char *gif_data;
+	unsigned int index;
+	int return_value;
+
+	/*	Check for sufficient data to be a GIF
+	*/
+	if (gif->buffer_size < 13) return GIF_INSUFFICIENT_DATA;
+
+	/*	Get our current processing position
+	*/
+	gif_data = gif->gif_data + gif->buffer_position;
+
+	/*	See if we should initialise the GIF
+	*/
+	if (gif->buffer_position == 0) {
+
+		/*	We want everything to be NULL before we start so we've no chance
+			of freeing bad pointers (paranoia)
+		*/
+		gif->frame_image = NULL;
+		gif->frames = NULL;
+		gif->local_colour_table = NULL;
+		gif->global_colour_table = NULL;
+
+		/*	The caller may have been lazy and not reset any values
+		*/
+		gif->frame_count = 0;
+		gif->frame_count_partial = 0;
+		gif->decoded_frame = -1;
+
+		/*	Check we are a GIF
+		*/
+		if (strncmp((const char *) gif_data, "GIF", 3) != 0)
+			return GIF_DATA_ERROR;
+		gif_data += 3;
+
+		/*	Check we are a GIF type 87a or 89a
+		*/
+/*		if ((strncmp(gif_data, "87a", 3) != 0) &&
+				(strncmp(gif_data, "89a", 3) != 0))
+			LOG(("Unknown GIF format - proceeding anyway"));
+*/		gif_data += 3;
+
+		/*	Get our GIF data.
+		*/
+		gif->width = gif_data[0] | (gif_data[1] << 8);
+		gif->height = gif_data[2] | (gif_data[3] << 8);
+		gif->global_colours = (gif_data[4] & 0x80);
+		gif->colour_table_size = (2 << (gif_data[4] & 0x07));
+		gif->background_colour = gif_data[5];
+		gif->aspect_ratio = gif_data[6];
+		gif->dirty_frame = -1;
+		gif->loop_count = 1;
+		gif_data += 7;
+
+		/*	Some broken GIFs report the size as the screen size they were created in. As
+			such, we detect for the common cases and set the sizes as 0 if they are found
+			which results in the GIF being the maximum size of the frames.
+		*/
+		if (((gif->width == 640) && (gif->height == 480)) ||
+				((gif->width == 640) && (gif->height == 512)) ||
+				((gif->width == 800) && (gif->height == 600)) ||
+				((gif->width == 1024) && (gif->height == 768)) ||
+				((gif->width == 1280) && (gif->height == 1024)) ||
+				((gif->width == 1600) && (gif->height == 1200)) ||
+				((gif->width == 0) || (gif->height == 0)) ||
+				((gif->width > 2048) || (gif->height > 2048))) {
+			gif->width = 1;
+			gif->height = 1;
+		}
+
+		/*	Allocate some data irrespective of whether we've got any colour tables. We
+			always get the maximum size in case a GIF is lying to us. It's far better
+			to give the wrong colours than to trample over some memory somewhere.
+		*/
+		gif->global_colour_table = (unsigned int *)calloc(GIF_MAX_COLOURS, sizeof(int));
+		gif->local_colour_table = (unsigned int *)calloc(GIF_MAX_COLOURS, sizeof(int));
+		if ((gif->global_colour_table == NULL) || (gif->local_colour_table == NULL)) {
+			gif_finalise(gif);
+			return GIF_INSUFFICIENT_MEMORY;
+		}
+
+		/*	Set the first colour to a value that will never occur in reality so we
+			know if we've processed it
+		*/
+		gif->global_colour_table[0] = 0xaa000000;
+
+		/*	Initialise enough workspace for 4 frame initially
+		*/
+		if ((gif->frames = (gif_frame *)malloc(sizeof(gif_frame))) == NULL) {
+			gif_finalise(gif);
+			return GIF_INSUFFICIENT_MEMORY;
+		}
+		gif->frame_holders = 1;
+
+		/*	Initialise the sprite header
+		*/
+		if ((gif->frame_image = bitmap_create(gif->width, gif->height, BITMAP_NEW)) == NULL) {
+			gif_finalise(gif);
+			return GIF_INSUFFICIENT_MEMORY;
+		}
+
+		/*	Remember we've done this now
+		*/
+		gif->buffer_position = gif_data - gif->gif_data;
+	}
+
+	/*	Do the colour map if we haven't already. As the top byte is always 0xff or 0x00
+		depending on the transparency we know if it's been filled in.
+	*/
+	if (gif->global_colour_table[0] == 0xaa000000) {
+		/*	Check for a global colour map signified by bit 7
+		*/
+		if (gif->global_colours) {
+			if (gif->buffer_size < (gif->colour_table_size * 3 + 12)) {
+				return GIF_INSUFFICIENT_DATA;
+			}
+			for (index = 0; index < gif->colour_table_size; index++) {
+				char colour[] = {gif_data[0], gif_data[1], gif_data[2], 0xff};
+				gif->global_colour_table[index] = *((int *) colour);
+				gif_data += 3;
+			}
+			gif->buffer_position = (gif_data - gif->gif_data);
+		} else {
+			/*	Create a default colour table with the first two colours as black and white
+			*/
+			gif->global_colour_table[0] = 0xff000000;
+			gif->global_colour_table[1] = 0xffffffff;
+		}
+	}
+
+	/*	Repeatedly try to decode frames
+	*/
+	while ((return_value = gif_initialise_frame(gif)) == 0);
+
+	/*	If there was a memory error tell the caller
+	*/
+	if ((return_value == GIF_INSUFFICIENT_MEMORY) ||
+			(return_value == GIF_DATA_ERROR))
+		return return_value;
+
+	/*	If we didn't have some frames then a GIF_INSUFFICIENT_DATA becomes a
+		GIF_INSUFFICIENT_FRAME_DATA
+	*/
+	if ((return_value == GIF_INSUFFICIENT_DATA) && (gif->frame_count_partial > 0))
+		return GIF_INSUFFICIENT_FRAME_DATA;
+
+	/*	Return how many we got
+	*/
+	return return_value;
+}
+
+
+
+/**	Updates the sprite memory size
+
+	@return -3 for a memory error
+		0 for success
+*/
+static int gif_initialise_sprite(struct gif_animation *gif, unsigned int width, unsigned int height) {
+	unsigned int max_width;
+	unsigned int max_height;
+	struct bitmap *buffer;
+
+	/*	Check if we've changed
+	*/
+	if ((width <= gif->width) && (height <= gif->height))
+		return 0;
+
+	/*	Get our maximum values
+	*/
+	max_width = (width > gif->width) ? width : gif->width;
+	max_height = (height > gif->height) ? height : gif->height;
+
+	/*	Allocate some more memory
+	*/
+	if ((buffer = bitmap_create(max_width, max_height, BITMAP_NEW)) == NULL)
+		return GIF_INSUFFICIENT_MEMORY;
+	bitmap_destroy(gif->frame_image);
+	gif->frame_image = buffer;
+	gif->width = max_width;
+	gif->height = max_height;
+
+	/*	Invalidate our currently decoded image
+	*/
+	gif->decoded_frame = -1;
+	return 0;
+}
+
+
+/*	Attempts to initialise the next frame
+
+	@return -4 for insufficient data to process the entire frame
+		-3 for a memory error
+		-2 for a data error
+		-1 for insufficient data to process anything
+		0 for success
+		1 for success (GIF terminator found)
+*/
+int gif_initialise_frame(struct gif_animation *gif) {
+	int frame;
+	gif_frame *temp_buf;
+
+	unsigned char *gif_data, *gif_end;
+	int gif_bytes;
+	unsigned int flags = 0;
+	unsigned int background_action;
+	unsigned int width, height, offset_x, offset_y;
+	unsigned int extension_size, colour_table_size;
+	unsigned int block_size;
+	bool more_images = true;
+	bool first_image = true;
+
+	/*	Get the frame to decode and our data position
+	*/
+	frame = gif->frame_count;
+
+	/*	Get our buffer position etc.
+	*/
+	gif_data = (unsigned char *)(gif->gif_data + gif->buffer_position);
+	gif_end = (unsigned char *)(gif->gif_data + gif->buffer_size);
+	gif_bytes = (gif_end - gif_data);
+
+	/*	Check we have enough data for at least the header, or if we've finished
+	*/
+	if ((gif_bytes > 0) && (gif_data[0] == 0x3b)) return 1;
+	if (gif_bytes < 11) return -1;
+
+	/*	We could theoretically get some junk data that gives us millions of frames, so
+		we ensure that we don't have a silly number
+	*/
+	if (frame > 4096) return GIF_DATA_ERROR;
+
+	/*	Get some memory to store our pointers in etc.
+	*/
+	if ((int)gif->frame_holders <= frame) {
+		/*	Allocate more memory
+		*/
+		if ((temp_buf = (gif_frame *)realloc(gif->frames,
+					(frame + 1) * sizeof(gif_frame))) == NULL)
+			return GIF_INSUFFICIENT_MEMORY;
+		gif->frames = temp_buf;
+		gif->frame_holders = frame + 1;
+	}
+
+	/*	Store our frame pointer. We would do it when allocating except we
+		start off with one frame allocated so we can always use realloc.
+	*/
+	gif->frames[frame].frame_pointer = gif->buffer_position;
+	gif->frames[frame].virgin = true;
+	gif->frames[frame].frame_delay = 100;
+	gif->frames[frame].redraw_required = false;
+
+	/*	Invalidate any previous decoding we have of this frame
+	*/
+	if (gif->decoded_frame == frame)
+		gif->decoded_frame = -1;
+
+	/*	We pretend to initialise the frames, but really we just skip over all
+		the data contained within. This is all basically a cut down version of
+		gif_decode_frame that doesn't have any of the LZW bits in it.
+	*/
+	while (more_images) {
+
+		/*	Ensure we have some data
+		*/
+		if ((gif_end - gif_data) < 10)
+			return GIF_INSUFFICIENT_FRAME_DATA;
+
+		/*	Decode the extensions
+		*/
+		background_action = 0;
+		while (gif_data[0] == 0x21) {
+			/*	Get the extension size
+			*/
+			extension_size = gif_data[2];
+
+			/*	Check we've enough data for the extension then header
+			*/
+			if ((gif_end - gif_data) < (int)(extension_size + 13))
+				return GIF_INSUFFICIENT_FRAME_DATA;
+
+			/*	Graphic control extension - store the frame delay.
+			*/
+			if (gif_data[1] == 0xf9) {
+				gif->frames[frame].frame_delay = gif_data[4] | (gif_data[5] << 8);
+				background_action = ((gif_data[3] & 0x1c) >> 2);
+				more_images = false;
+
+			/*	Application extension - handle NETSCAPE2.0 looping
+			*/
+			} else if ((gif_data[1] == 0xff) &&
+					(gif_data[2] == 0x0b) &&
+					(strncmp((const char *) gif_data + 3, 
+						"NETSCAPE2.0", 11) == 0) &&
+					(gif_data[14] == 0x03) &&
+					(gif_data[15] == 0x01)) {
+				gif->loop_count = gif_data[16] | (gif_data[17] << 8);
+			}
+
+			/*	Move to the first sub-block
+			*/
+			gif_data += 2;
+
+			/*	Skip all the sub-blocks
+			*/
+			while (gif_data[0] != 0x00) {
+				gif_data += gif_data[0] + 1;
+				if ((gif_end - gif_data) < 10) return GIF_INSUFFICIENT_FRAME_DATA;
+			}
+			gif_data++;
+		}
+
+		/*	We must have at least one image descriptor
+		*/
+		if (gif_data[0] != 0x2c) return GIF_FRAME_DATA_ERROR;
+
+		/*	Do some simple boundary checking
+		*/
+		offset_x = gif_data[1] | (gif_data[2] << 8);
+		offset_y = gif_data[3] | (gif_data[4] << 8);
+		width = gif_data[5] | (gif_data[6] << 8);
+		height = gif_data[7] | (gif_data[8] << 8);
+
+		/*	Set up the redraw characteristics. We have to check for extending the area
+			due to multi-image frames.
+		*/
+		if (!first_image) {
+			if (gif->frames[frame].redraw_x > offset_x) {
+				gif->frames[frame].redraw_width += (gif->frames[frame].redraw_x - offset_x);
+				gif->frames[frame].redraw_x = offset_x;
+			}
+			if (gif->frames[frame].redraw_y > offset_y) {
+				gif->frames[frame].redraw_height += (gif->frames[frame].redraw_y - offset_y);
+				gif->frames[frame].redraw_y = offset_y;
+			}
+			if ((offset_x + width) > (gif->frames[frame].redraw_x + gif->frames[frame].redraw_width))
+				gif->frames[frame].redraw_width = (offset_x + width) - gif->frames[frame].redraw_x;
+			if ((offset_y + height) > (gif->frames[frame].redraw_y + gif->frames[frame].redraw_height))
+				gif->frames[frame].redraw_height = (offset_y + height) - gif->frames[frame].redraw_y;
+		} else {
+			first_image = false;
+			gif->frames[frame].redraw_x = offset_x;
+			gif->frames[frame].redraw_y = offset_y;
+			gif->frames[frame].redraw_width = width;
+			gif->frames[frame].redraw_height = height;
+		}
+
+		/*	if we are clearing the background then we need to redraw enough to cover the previous
+			frame too
+		*/
+		gif->frames[frame].redraw_required = ((background_action == 2) || (background_action == 3));
+
+		/*	Boundary checking - shouldn't ever happen except with junk data
+		*/
+		if (gif_initialise_sprite(gif, (offset_x + width), (offset_y + height)))
+			return GIF_INSUFFICIENT_MEMORY;
+
+		/*	Decode the flags
+		*/
+		flags = gif_data[9];
+		colour_table_size = 2 << (flags & 0x07);
+
+		/*	Move our data onwards and remember we've got a bit of this frame
+		*/
+		gif_data += 10;
+		gif_bytes = (gif_end - gif_data);
+		gif->frame_count_partial = frame + 1;
+
+		/*	Skip the local colour table
+		*/
+		if (flags & 0x80) {
+			gif_data += 3 * colour_table_size;
+			if ((gif_bytes = (gif_end - gif_data)) < 0)
+				return GIF_INSUFFICIENT_FRAME_DATA;
+		}
+
+		/*	Ensure we have a correct code size
+		*/
+		if (gif_data[0] > GIF_MAX_LZW)
+			return GIF_DATA_ERROR;
+
+		/*	Move our data onwards
+		*/
+		gif_data++;
+		if (--gif_bytes < 0)
+			return GIF_INSUFFICIENT_FRAME_DATA;
+
+		/*	Repeatedly skip blocks until we get a zero block or run out of data
+		*/
+		block_size = 0;
+		while (block_size != 1) {
+			/*	Skip the block data
+			*/
+			block_size = gif_data[0] + 1;
+			if ((gif_bytes -= block_size) < 0)
+				return GIF_INSUFFICIENT_FRAME_DATA;
+			gif_data += block_size;
+		}
+
+		/*	Check for end of data
+		*/
+		more_images &= !((gif_bytes < 1) || (gif_data[0] == 0x3b));
+	}
+
+	/*	Check if we've finished
+	*/
+	if (gif_bytes < 1)
+		return GIF_INSUFFICIENT_FRAME_DATA;
+	else {
+		gif->buffer_position = gif_data - gif->gif_data;
+		gif->frame_count = frame + 1;
+		if (gif_data[0] == 0x3b) return 1;
+	}
+	return 0;
+}
+
+
+/**	Decodes a GIF frame.
+
+	@return GIF_FRAME_DATA_ERROR for GIF frame data error
+		GIF_INSUFFICIENT_FRAME_DATA for insufficient data to complete the frame
+		GIF_DATA_ERROR for GIF error (invalid frame header)
+		GIF_INSUFFICIENT_DATA for insufficient data to do anything
+		0 for successful decoding
+*/
+int gif_decode_frame(struct gif_animation *gif, unsigned int frame) {
+	unsigned int index = 0;
+	unsigned char *gif_data;
+	unsigned char *gif_end;
+	int gif_bytes;
+	unsigned int width, height, offset_x, offset_y;
+	unsigned int flags, colour_table_size, interlace;
+	unsigned int *colour_table;
+	unsigned int *frame_data = 0;	// Set to 0 for no warnings
+	unsigned int *frame_scanline;
+	unsigned int extension_size;
+	unsigned int background_action;
+	int transparency_index = -1;
+	unsigned int save_buffer_position;
+	unsigned int return_value = 0;
+	unsigned int x, y, decode_y, burst_bytes;
+	unsigned int block_size;
+	register int colour;
+	bool more_images = true;
+
+	/*	Ensure we have a frame to decode
+	*/
+	if (frame > gif->frame_count_partial)
+		return GIF_INSUFFICIENT_DATA;
+	if ((!clear_image) && ((int)frame == gif->decoded_frame))
+		return 0;
+
+	/*	If the previous frame was dirty, remove it
+	*/
+	if (!clear_image) {
+	  	if (frame == 0)
+	  		gif->dirty_frame = -1;
+		if (gif->decoded_frame == gif->dirty_frame) {
+			clear_image = true;
+			if (frame != 0)
+				gif_decode_frame(gif, gif->dirty_frame);
+			clear_image = false;
+		}
+		gif->dirty_frame = -1;
+	}
+
+	/*	Get the start of our frame data and the end of the GIF data
+	*/
+	gif_data = gif->gif_data + gif->frames[frame].frame_pointer;
+	gif_end = gif->gif_data + gif->buffer_size;
+	gif_bytes = (gif_end - gif_data);
+
+	/*	Check we have enough data for the header
+	*/
+	if (gif_bytes < 9)
+		return GIF_INSUFFICIENT_DATA;
+
+	/*	Clear the previous frame totally. We can't just pretend we've got a smaller
+		sprite and clear what we need as some frames have multiple images which would
+		produce errors.
+	*/
+	frame_data = (unsigned int *)bitmap_get_buffer(gif->frame_image);
+	if (!frame_data)
+		return GIF_INSUFFICIENT_MEMORY;
+	if (!clear_image) {
+		if ((frame == 0) || (gif->decoded_frame == -1))
+			memset((char*)frame_data, 0x00, gif->width * gif->height * sizeof(int));
+		gif->decoded_frame = frame;
+	}
+
+	/*	Save the buffer position
+	*/
+	save_buffer_position = gif->buffer_position;
+	gif->buffer_position = gif_data - gif->gif_data;
+
+	/*	We've got to do this more than one time if we've got multiple images
+	*/
+	while (more_images) {
+		background_action = 0;
+
+		/*	Ensure we have some data
+		*/
+		gif_data = gif->gif_data + gif->buffer_position;
+		if ((gif_end - gif_data) < 10) {
+			return_value = GIF_INSUFFICIENT_FRAME_DATA;
+			break;
+		}
+
+		/*	Decode the extensions
+		*/
+		while (gif_data[0] == 0x21) {
+
+			/*	Get the extension size
+			*/
+			extension_size = gif_data[2];
+
+			/*	Check we've enough data for the extension then header
+			*/
+			if ((gif_end - gif_data) < (int)(extension_size + 13)) {
+				return_value = GIF_INSUFFICIENT_FRAME_DATA;
+				break;
+			}
+
+			/*	Graphic control extension - store the frame delay.
+			*/
+			if (gif_data[1] == 0xf9) {
+				flags = gif_data[3];
+				if (flags & 0x01) transparency_index = gif_data[6];
+				background_action = ((flags & 0x1c) >> 2);
+				more_images = false;
+			}
+			/*	Move to the first sub-block
+			*/
+			gif_data += 2;
+
+			/*	Skip all the sub-blocks
+			*/
+			while (gif_data[0] != 0x00) {
+				gif_data += gif_data[0] + 1;
+				if ((gif_end - gif_data) < 10) {
+					return_value = GIF_INSUFFICIENT_FRAME_DATA;
+					break;
+				}
+			}
+			gif_data++;
+		}
+
+		/*	Decode the header
+		*/
+		if (gif_data[0] != 0x2c) {
+			return_value = GIF_DATA_ERROR;
+			break;
+		}
+		offset_x = gif_data[1] | (gif_data[2] << 8);
+		offset_y = gif_data[3] | (gif_data[4] << 8);
+		width = gif_data[5] | (gif_data[6] << 8);
+		height = gif_data[7] | (gif_data[8] << 8);
+
+		/*	Boundary checking - shouldn't ever happen except unless the data has been
+			modified since initialisation.
+		*/
+		if ((offset_x + width > gif->width) || (offset_y + height > gif->height)) {
+			return_value = GIF_DATA_ERROR;
+			break;
+		}
+
+		/*	Decode the flags
+		*/
+		flags = gif_data[9];
+		colour_table_size = 2 << (flags & 0x07);
+		interlace = flags & 0x40;
+
+		/*	Move through our data
+		*/
+		gif_data += 10;
+		gif_bytes = (int)(gif_end - gif_data);
+
+		/*	Set up the colour table
+		*/
+		if (flags & 0x80) {
+			if (gif_bytes < (int)(3 * colour_table_size)) {
+				return_value = GIF_INSUFFICIENT_FRAME_DATA;
+				break;
+			}
+			colour_table = gif->local_colour_table;
+			if (!clear_image) {
+				for (index = 0; index < colour_table_size; index++) {
+					char colour[] = {gif_data[0], gif_data[1],
+							gif_data[2], 0xff};
+					colour_table[index] = *((int *) colour);
+					gif_data += 3;
+				}
+			} else {
+				gif_data += 3 * colour_table_size;
+			}
+			gif_bytes = (int)(gif_end - gif_data);
+		} else {
+			colour_table = gif->global_colour_table;
+		}
+
+		/*	If we are clearing the image we just clear, if not decode
+		*/
+		if (!clear_image) {
+			/*	Set our dirty status
+			*/
+			if ((background_action == 2) || (background_action == 3))
+				gif->dirty_frame = frame;
+
+			/*	Initialise the LZW decoding
+			*/
+			set_code_size = gif_data[0];
+			gif->buffer_position = (gif_data - gif->gif_data) + 1;
+
+			/*	Set our code variables
+			*/
+			code_size = set_code_size + 1;
+			clear_code = (1 << set_code_size);
+			end_code = clear_code + 1;
+			max_code_size = clear_code << 1;
+			max_code = clear_code + 2;
+			curbit = lastbit = 0;
+			last_byte = 2;
+			get_done = false;
+			direct = buf;
+			gif_init_LZW(gif);
+
+			/*	Decompress the data
+			*/
+			for (y = 0; y < height; y++) {
+				if (interlace)
+					decode_y = gif_interlaced_line(height, y) + offset_y;
+				else
+					decode_y = y + offset_y;
+				frame_scanline = frame_data + offset_x + (decode_y * gif->width);
+
+				/*	Rather than decoding pixel by pixel, we try to burst out streams
+					of data to remove the need for end-of data checks every pixel.
+				*/
+				x = width;
+				while (x > 0) {
+					burst_bytes = (stack_pointer - stack);
+					if (burst_bytes > 0) {
+						if (burst_bytes > x)
+							burst_bytes = x;
+						x -= burst_bytes;
+						while (burst_bytes-- > 0) {
+							if ((colour = *--stack_pointer) != transparency_index)
+								  *frame_scanline = colour_table[colour];
+							frame_scanline++;
+						}
+					} else {
+					  	if (!gif_next_LZW(gif)) {
+					  		return_value = gif->current_error;
+					  		goto gif_decode_frame_exit;
+					  	}
+					}
+				}
+			}
+		} else {
+			/*	Clear our frame
+			*/
+			if ((background_action == 2) || (background_action == 3)) {
+				for (y = 0; y < height; y++) {
+					frame_scanline = frame_data + offset_x + ((offset_y + y) * gif->width);
+					memset(frame_scanline, 0x00, width * 4);
+				}
+			}
+
+			/*	Repeatedly skip blocks until we get a zero block or run out of data
+			*/
+			gif_bytes = gif->buffer_size - gif->buffer_position;
+			gif_data = gif->gif_data + gif->buffer_size;
+			block_size = 0;
+			while (block_size != 1) {
+				/*	Skip the block data
+				*/
+				block_size = gif_data[0] + 1;
+				if ((gif_bytes -= block_size) < 0) {
+					return_value = GIF_INSUFFICIENT_FRAME_DATA;
+					goto gif_decode_frame_exit;
+				}
+				gif_data += block_size;
+			}
+		}
+gif_decode_frame_exit:
+
+		/*	Check for end of data
+		*/
+		gif_bytes = gif->buffer_size - gif->buffer_position;
+		more_images &= !((gif_bytes < 1) || (gif_data[0] == 0x3b));
+		gif->buffer_position++;
+	}
+
+	/*	Check if we should test for optimisation
+	*/
+	if (gif->frames[frame].virgin) {
+		gif->frames[frame].opaque = bitmap_test_opaque(gif->frame_image);
+		gif->frames[frame].virgin = false;
+	}
+	bitmap_set_opaque(gif->frame_image, gif->frames[frame].opaque);
+	bitmap_modified(gif->frame_image);
+
+	/*	Restore the buffer position
+	*/
+	gif->buffer_position = save_buffer_position;
+
+	/*	Success!
+	*/
+	return return_value;
+
+}
+
+static unsigned int gif_interlaced_line(int height, int y) {
+	if ((y << 3) < height) return (y << 3);
+	y -= ((height + 7) >> 3);
+	if ((y << 3) < (height - 4)) return (y << 3) + 4;
+	y -= ((height + 3) >> 3);
+	if ((y << 2) < (height - 2)) return (y << 2) + 2;
+	y -= ((height + 1) >> 2);
+	return (y << 1) + 1;
+}
+
+/*	Releases any workspace held by the animation
+*/
+void gif_finalise(struct gif_animation *gif) {
+	/*	Release all our memory blocks
+	*/
+	if (gif->frame_image)
+		bitmap_destroy(gif->frame_image);
+	gif->frame_image = NULL;
+	free(gif->frames);
+	gif->frames = NULL;
+	free(gif->local_colour_table);
+	gif->local_colour_table = NULL;
+	free(gif->global_colour_table);
+	gif->global_colour_table = NULL;
+}
+
+/**
+ * Initialise LZW decoding
+ */
+void gif_init_LZW(struct gif_animation *gif) {
+	int i;
+
+	gif->current_error = 0;
+	if (clear_code >= (1 << GIF_MAX_LZW)) {
+		stack_pointer = stack;
+		gif->current_error = GIF_FRAME_DATA_ERROR;
+		return;
+	}
+
+	/* initialise our table */
+	memset(table, 0x00, (1 << GIF_MAX_LZW) * 8);
+	for (i = 0; i < clear_code; ++i)
+		table[1][i] = i;
+
+	/* update our LZW parameters */
+	code_size = set_code_size + 1;
+	max_code_size = clear_code << 1;
+	max_code = clear_code + 2;
+	stack_pointer = stack;
+	do {
+		firstcode = oldcode = gif_next_code(gif, code_size);
+	} while (firstcode == clear_code);
+	*stack_pointer++ =firstcode;
+}
+
+
+static bool gif_next_LZW(struct gif_animation *gif) {
+	int code, incode;
+	int block_size;
+	int new_code;
+
+	code = gif_next_code(gif, code_size);
+	if (code < 0) {
+	  	gif->current_error = code;
+		return false;
+	} else if (code == clear_code) {
+		gif_init_LZW(gif);
+		return true;
+	} else if (code == end_code) {
+		/* skip to the end of our data so multi-image GIFs work */
+		if (zero_data_block) {
+			gif->current_error = GIF_FRAME_DATA_ERROR;
+			return false;
+		}
+		block_size = 0;
+		while (block_size != 1) {
+			block_size = gif->gif_data[gif->buffer_position] + 1;
+			gif->buffer_position += block_size;
+		}
+		gif->current_error = GIF_FRAME_DATA_ERROR;
+		return false;
+	}
+
+	incode = code;
+	if (code >= max_code) {
+		*stack_pointer++ = firstcode;
+		code = oldcode;
+	}
+
+	/* The following loop is the most important in the GIF decoding cycle as every
+	 * single pixel passes through it.
+	 *
+	 * Note: our stack is always big enough to hold a complete decompressed chunk. */
+	while (code >= clear_code) {
+		*stack_pointer++ = table[1][code];
+		new_code = table[0][code];
+		if (new_code < clear_code) {
+			code = new_code;
+			break;
+		}
+		*stack_pointer++ = table[1][new_code];
+		code = table[0][new_code];
+		if (code == new_code) {
+		  	gif->current_error = GIF_FRAME_DATA_ERROR;
+			return false;
+		}
+	}
+
+	*stack_pointer++ = firstcode = table[1][code];
+
+	if ((code = max_code) < (1 << GIF_MAX_LZW)) {
+		table[0][code] = oldcode;
+		table[1][code] = firstcode;
+		++max_code;
+		if ((max_code >= max_code_size) && (max_code_size < (1 << GIF_MAX_LZW))) {
+			max_code_size = max_code_size << 1;
+			++code_size;
+		}
+	}
+	oldcode = incode;
+	return true;
+}
+
+static int gif_next_code(struct gif_animation *gif, int code_size) {
+	int i, j, end, count, ret;
+	unsigned char *b;
+
+	end = curbit + code_size;
+	if (end >= lastbit) {
+		if (get_done)
+			return GIF_INSUFFICIENT_FRAME_DATA;
+		buf[0] = direct[last_byte - 2];
+		buf[1] = direct[last_byte - 1];
+
+		/* get the next block */
+		direct = gif->gif_data + gif->buffer_position;
+		zero_data_block = ((count = direct[0]) == 0);
+		if ((gif->buffer_position + count) >= gif->buffer_size)
+			return GIF_INSUFFICIENT_FRAME_DATA;
+		if (count == 0)
+			get_done = true;
+		else {
+			direct -= 1;
+			buf[2] = direct[2];
+			buf[3] = direct[3];
+		}
+		gif->buffer_position += count + 1;
+
+		/* update our variables */
+		last_byte = 2 + count;
+		curbit = (curbit - lastbit) + 16;
+		lastbit = (2 + count) << 3;
+		end = curbit + code_size;
+	}
+
+	i = curbit >> 3;
+	if (i < 2)
+		b = buf;
+	else
+		b = direct;
+
+	ret = b[i];
+	j = (end >> 3) - 1;
+	if (i <= j) {
+		ret |= (b[i + 1] << 8);
+		if (i < j)
+			ret |= (b[i + 2] << 16);
+	}
+	ret = (ret >> (curbit % 8)) & maskTbl[code_size];
+	curbit += code_size;
+	return ret;
+}