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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

47
examples/dwebp.c
View File

@ -109,7 +109,8 @@ static HRESULT WriteUsingWIC(const char* out_file_name, REFGUID container_guid,
}
static int WritePNG(const char* out_file_name, unsigned char* rgb, int stride,
uint32_t width, uint32_t height) {
uint32_t width, uint32_t height, int has_alpha) {
assert(!has_alpha); // TODO(mikolaj)
return SUCCEEDED(WriteUsingWIC(out_file_name,
MAKE_REFGUID(GUID_ContainerFormatPng), rgb, stride, width,
height));
@ -122,7 +123,7 @@ static void PNGAPI error_function(png_structp png, png_const_charp dummy) {
}
static int WritePNG(FILE* out_file, unsigned char* rgb, int stride,
png_uint_32 width, png_uint_32 height) {
png_uint_32 width, png_uint_32 height, int has_alpha) {
png_structp png;
png_infop info;
png_uint_32 y;
@ -142,7 +143,8 @@ static int WritePNG(FILE* out_file, unsigned char* rgb, int stride,
return 0;
}
png_init_io(png, out_file);
png_set_IHDR(png, info, width, height, 8, PNG_COLOR_TYPE_RGB,
png_set_IHDR(png, info, width, height, 8,
has_alpha ? PNG_COLOR_TYPE_RGBA : PNG_COLOR_TYPE_RGB,
PNG_INTERLACE_NONE, PNG_COMPRESSION_TYPE_DEFAULT,
PNG_FILTER_TYPE_DEFAULT);
png_write_info(png, info);
@ -159,7 +161,7 @@ static int WritePNG(FILE* out_file, unsigned char* rgb, int stride,
typedef uint32_t png_uint_32;
static int WritePNG(FILE* out_file, unsigned char* rgb, int stride,
png_uint_32 width, png_uint_32 height) {
png_uint_32 width, png_uint_32 height, int has_alpha) {
printf("PNG support not compiled. Please install the libpng development "
"package before building.\n");
printf("You can run with -ppm flag to decode in PPM format.\n");
@ -167,12 +169,28 @@ static int WritePNG(FILE* out_file, unsigned char* rgb, int stride,
}
#endif
static int WritePPM(FILE* fout, unsigned char* rgb,
static int WritePPM(FILE* fout, const unsigned char* rgb,
uint32_t width, uint32_t height) {
fprintf(fout, "P6\n%d %d\n255\n", width, height);
return (fwrite(rgb, width * height, 3, fout) == 3);
}
static int WriteAlphaPlane(FILE* fout, const unsigned char* rgba,
uint32_t width, uint32_t height) {
uint32_t y;
fprintf(fout, "P5\n%d %d\n255\n", width, height);
for (y = 0; y < height; ++y) {
const unsigned char* line = rgba + y * (width * 4);
uint32_t x;
for (x = 0; x < width; ++x) {
if (fputc(line[4 * x + 3], fout) == EOF) {
return 0;
}
}
}
return 1;
}
static int WritePGM(FILE* fout,
unsigned char* y_plane, unsigned char *u, unsigned char* v,
int y_stride, int uv_stride,
@ -202,6 +220,7 @@ typedef enum {
PNG = 0,
PPM,
PGM,
ALPHA_PLANE_ONLY // this is for experimenting only
} OutputFileFormat;
static void Help(void) {
@ -222,6 +241,7 @@ int main(int argc, const char *argv[]) {
const char *out_file = NULL;
int width, height, stride, uv_stride;
int has_alpha = 0;
uint8_t* out = NULL, *u = NULL, *v = NULL;
OutputFileFormat format = PNG;
Stopwatch stop_watch;
@ -232,6 +252,8 @@ int main(int argc, const char *argv[]) {
return 0;
} else if (!strcmp(argv[c], "-o") && c < argc - 1) {
out_file = argv[++c];
} else if (!strcmp(argv[c], "-alpha")) {
format = ALPHA_PLANE_ONLY;
} else if (!strcmp(argv[c], "-ppm")) {
format = PPM;
} else if (!strcmp(argv[c], "-version")) {
@ -284,8 +306,12 @@ int main(int argc, const char *argv[]) {
case PNG:
#ifdef _WIN32
out = WebPDecodeBGR((const uint8_t*)data, data_size, &width, &height);
stride = 3 * width;
has_alpha = 0;
#else
out = WebPDecodeRGB((const uint8_t*)data, data_size, &width, &height);
out = WebPDecodeRGBA((const uint8_t*)data, data_size, &width, &height);
stride = 4 * width;
has_alpha = 1;
#endif
break;
case PPM:
@ -295,6 +321,9 @@ int main(int argc, const char *argv[]) {
out = WebPDecodeYUV((const uint8_t*)data, data_size, &width, &height,
&u, &v, &stride, &uv_stride);
break;
case ALPHA_PLANE_ONLY:
out = WebPDecodeRGBA((const uint8_t*)data, data_size, &width, &height);
break;
default:
free(data);
return -1;
@ -331,14 +360,16 @@ int main(int argc, const char *argv[]) {
int ok = 1;
if (format == PNG) {
#ifdef _WIN32
ok &= WritePNG(out_file, out, 3 * width, width, height);
ok &= WritePNG(out_file, out, stride, width, height, has_alpha);
#else
ok &= WritePNG(fout, out, 3 * width, width, height);
ok &= WritePNG(fout, out, stride, width, height, has_alpha);
#endif
} else if (format == PPM) {
ok &= WritePPM(fout, out, width, height);
} else if (format == PGM) {
ok &= WritePGM(fout, out, u, v, stride, uv_stride, width, height);
} else if (format == ALPHA_PLANE_ONLY) {
ok &= WriteAlphaPlane(fout, out, width, height);
}
if (fout)
fclose(fout);