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EXPERIMENTAL: add support for alpha channel

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This is a (minor) bitstream change: if the 'color_space' bit is set to '1'
(which is normally an undefined/invalid behaviour), we add extra data at the
end of partition #0 (so-called 'extensions')

Namely, we add the size of the extension data as 3 bytes (little-endian),
followed by a set of bits telling which extensions we're incorporating.
The data then _preceeds_ this trailing tags.

This is all experimental, and you'll need to have
'#define WEBP_EXPERIMENTAL_FEATURES' in webp/types.h to enable this code
(at your own risk! :))

Still, this hack produces almost-valid WebP file for decoders that don't
check this color_space bit. In particular, previous 'dwebp' (and for instance
Chrome) will recognize this files and decode them, but without the alpha
of course. Other decoder will just see random extra stuff at the end of
partition #0.

To experiment with the alpha-channel, you need to compile on Unix platform
and use PNGs for input/output.

If 'alpha.png' is a source with alpha channel, then you can try (on Unix):

  cwebp alpha.png -o alpha.webp
  dwebp alpha.webp -o test.png

cwebp now has a '-noalpha' flag to ignore any alpha information from the
source, if present.

More hacking and experimenting welcome!

Change-Id: I3c7b1fd8411c9e7a9f77690e898479ad85c52f3e
This commit is contained in:
Pascal Massimino
2011-04-25 16:58:04 -07:00
parent cfbf88a6c4
commit 2ab4b72f53
27 changed files with 477 additions and 59 deletions
Download Patch File Download Diff File Expand all files Collapse all files

43
examples/cwebp.c
View File

@ -162,7 +162,8 @@ static HRESULT ReadPictureWithWIC(const char* filename,
return hr;
}
static int ReadPicture(const char* const filename, WebPPicture* const pic) {
static int ReadPicture(const char* const filename, WebPPicture* const pic,
int keep_alpha) {
int ok;
if (pic->width != 0 && pic->height != 0) {
// If image size is specified, infer it as YUV format.
@ -282,10 +283,11 @@ static void PNGAPI error_function(png_structp png, png_const_charp dummy) {
longjmp(png_jmpbuf(png), 1);
}
static int ReadPNG(FILE* in_file, WebPPicture* const pic) {
static int ReadPNG(FILE* in_file, WebPPicture* const pic, int keep_alpha) {
png_structp png;
png_infop info;
int color_type, bit_depth, interlaced;
int has_alpha;
int num_passes;
int p;
int ok = 0;
@ -327,12 +329,16 @@ static int ReadPNG(FILE* in_file, WebPPicture* const pic) {
if (png_get_valid(png, info, PNG_INFO_tRNS)) {
png_set_tRNS_to_alpha(png);
}
has_alpha = !!(color_type & PNG_COLOR_MASK_ALPHA);
if (!keep_alpha) {
png_set_strip_alpha(png);
has_alpha = 0;
}
// TODO(skal): Strip Alpha for now (till Alpha is supported).
png_set_strip_alpha(png);
num_passes = png_set_interlace_handling(png);
png_read_update_info(png, info);
stride = 3 * width * sizeof(*rgb);
stride = (has_alpha ? 4 : 3) * width * sizeof(*rgb);
rgb = (uint8_t*)malloc(stride * height);
if (rgb == NULL) goto Error;
for (p = 0; p < num_passes; ++p) {
@ -346,14 +352,15 @@ static int ReadPNG(FILE* in_file, WebPPicture* const pic) {
pic->width = width;
pic->height = height;
ok = WebPPictureImportRGB(pic, rgb, stride);
ok = has_alpha ? WebPPictureImportRGBA(pic, rgb, stride)
: WebPPictureImportRGB(pic, rgb, stride);
free(rgb);
End:
return ok;
}
#else
static int ReadPNG(FILE* in_file, WebPPicture* const pic) {
static int ReadPNG(FILE* in_file, WebPPicture* const pic, int keep_alpha) {
printf("PNG support not compiled. Please install the libpng development "
"package before building.\n");
return 0;
@ -385,7 +392,8 @@ static InputFileFormat GetImageType(FILE* in_file) {
return format;
}
static int ReadPicture(const char* const filename, WebPPicture* const pic) {
static int ReadPicture(const char* const filename, WebPPicture* const pic,
int keep_alpha) {
int ok = 0;
FILE* in_file = fopen(filename, "rb");
if (in_file == NULL) {
@ -397,7 +405,7 @@ static int ReadPicture(const char* const filename, WebPPicture* const pic) {
// If no size specified, try to decode it as PNG/JPEG (as appropriate).
const InputFileFormat format = GetImageType(in_file);
if (format == PNG) {
ok = ReadPNG(in_file, pic);
ok = ReadPNG(in_file, pic, keep_alpha);
} else if (format == JPEG) {
ok = ReadJPEG(in_file, pic);
}
@ -475,6 +483,10 @@ static void PrintExtraInfo(const WebPPicture* const pic, int short_output) {
100.f * stats->header_bytes[0] / stats->coded_size,
stats->header_bytes[1],
100.f * stats->header_bytes[1] / stats->coded_size);
if (stats->alpha_data_size) {
fprintf(stderr, " transparency: %6d\n",
stats->alpha_data_size);
}
fprintf(stderr, " Residuals bytes "
"|segment 1|segment 2|segment 3"
"|segment 4| total\n");
@ -590,6 +602,8 @@ static void HelpLong(void) {
printf(" -sharpness <int> ....... "
"filter sharpness (0:most .. 7:least sharp)\n");
printf(" -strong ................ use strong filter instead of simple.\n");
printf(" -alpha_comp <int> ...... set the transparency-compression\n");
printf(" -noalpha ............... discard any transparency information.\n");
printf(" -pass <int> ............ analysis pass number (1..10)\n");
printf(" -crop <x> <y> <w> <h> .. crop picture with the given rectangle\n");
printf(" -map <int> ............. print map of extra info.\n");
@ -618,12 +632,15 @@ int main(int argc, const char *argv[]) {
int c;
int short_output = 0;
int quiet = 0;
int keep_alpha = 0;
int crop = 0, crop_x = 0, crop_y = 0, crop_w = 0, crop_h = 0;
WebPPicture picture;
WebPConfig config;
WebPAuxStats stats;
Stopwatch stop_watch;
#ifdef WEBP_EXPERIMENTAL_FEATURES
keep_alpha = 1;
#endif
if (!WebPPictureInit(&picture) || !WebPConfigInit(&config)) {
fprintf(stderr, "Error! Version mismatch!\n");
goto Error;
@ -675,6 +692,10 @@ int main(int argc, const char *argv[]) {
config.preprocessing = strtol(argv[++c], NULL, 0);
} else if (!strcmp(argv[c], "-segments") && c < argc - 1) {
config.segments = strtol(argv[++c], NULL, 0);
} else if (!strcmp(argv[c], "-alpha_comp") && c < argc - 1) {
config.alpha_compression = strtol(argv[++c], NULL, 0);
} else if (!strcmp(argv[c], "-noalpha")) {
keep_alpha = 0;
} else if (!strcmp(argv[c], "-map") && c < argc - 1) {
picture.extra_info_type = strtol(argv[++c], NULL, 0);
} else if (!strcmp(argv[c], "-crop") && c < argc - 4) {
@ -734,7 +755,7 @@ int main(int argc, const char *argv[]) {
// Read the input
if (verbose)
StopwatchReadAndReset(&stop_watch);
if (!ReadPicture(in_file, &picture)) {
if (!ReadPicture(in_file, &picture, keep_alpha)) {
fprintf(stderr, "Error! Cannot read input picture\n");
goto Error;
}