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Tag Info
Tag Number: 
Tag Type: 
Tag Flash Version: 
Unknown SWF Tag: 
This tag is defined by the Flash documentation by Adobe
Brief Description: 

A bitmap compressed using ZLIB (similar to the PNG format).

Tag Structure: 
struct swf_definebitslossless {
	swf_long_tag		f_tag;		/* 20 or 36 */
	unsigned short		f_image_id;
	unsigned char		f_format;	/* 3, 4 or 5 */
	unsigned short		f_width;
	unsigned short		f_height;
	if(f_format == 3) {
		unsigned char	f_colormap_count;
		if(f_tag == DefineBitsLossless) {
			swf_rgb		f_colormap[f_colormap_count];
		else {
			swf_rgba	f_colormap[f_colormap_count];
		unsigned char	f_indices[((f_width + 3) & -4) * f_height];
	else {
		if(f_tag == DefineBitsLossless) {
			swf_xrgb	f_bitmap[f_width * f_height];
		else {
			swf_argb	f_bitmap[f_width * f_height];

These tags declares a loss-less image bitmap. It has a small header followed by an optional colormap and the bitmap data. When we have a colormap, the bitmap data is an array of indices in the colormap aligned to 4 bytes on a per row basis.

There are three supported formats:

Color Format Comments
(8 bits(1))
RGB RGBA Uses a colormap with up to 256 entries of 24 or 32 bits colors.
(16 bits(1))
RGB555 RGB555 There is no alpha available in this format. The data is saved in big endian (it is NOT a U16 like some documentations say it is). The colors looks like this (most significant bit first): 0RRRRRGGGGGBBBBB. You should certainly always use the DefineBitsLossless tag for this format.
(32 bits)
XRGB ARGB Uses a strange order for the components. Most probably because the alpha was added later and thus inserted in place of the X to keep some backward compatibility with older versions.

(1) the data must be 32 bits aligned (4 bytes) on a per row basis. In 8 bits, you may have to add up to three bytes at the end of each row ( 4 - width & 3 when width & 3 is not zero.). In 16 bits, you need to add two bytes at the end of each row when the width of the image is odd.

The f_colormap, f_indices and f_bitmap are all compressed with the ZLIB scheme.

WATCH OUT: the f_colormap and f_indices are compressed as one large block.

WARNING: These tags require you to save the swf_tag in long format (i.e. f_tag_and_size & 0x3F == 0x3F even if the size is smaller than 63.)

WARNING: An image cannot always be scaled more than 64×. Trying to enlarge it more may result in a rectangle of one color. The 64× is cumulative. So a sprite of an image × 3 inside another sprite × 10 inside another sprite × 4 results in scaling of 120 and this is likely to break the image. This seems to be true mainly when there is a rotation or skew.


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