// This function always returns an initialized 'bw' object, even upon error.
static int EncodeAlphaInternal(const uint8_t* const data, int width, int height,
int method, int filter, int reduce_levels,
int effort_level, // in [0..6] range
uint8_t* const tmp_alpha,
FilterTrial* result) {
int ok = 0;
const uint8_t* alpha_src;
WebPFilterFunc filter_func;
uint8_t header;
const size_t data_size = width * height;
const uint8_t* output = NULL;
size_t output_size = 0;
VP8LBitWriter tmp_bw;
assert((uint64_t)data_size == (uint64_t)width * height); // as per spec
assert(filter >= 0 && filter < WEBP_FILTER_LAST);
assert(method >= ALPHA_NO_COMPRESSION);
assert(method <= ALPHA_LOSSLESS_COMPRESSION);
assert(sizeof(header) == ALPHA_HEADER_LEN);
// TODO(skal): have a common function and #define's to validate alpha params.
filter_func = WebPFilters[filter];
if (filter_func != NULL) {
filter_func(data, width, height, width, tmp_alpha);
alpha_src = tmp_alpha;
} else {
alpha_src = data;
}
if (method != ALPHA_NO_COMPRESSION) {
ok = VP8LBitWriterInit(&tmp_bw, data_size >> 3);
ok = ok && EncodeLossless(alpha_src, width, height, effort_level,
&tmp_bw, &result->stats);
if (ok) {
output = VP8LBitWriterFinish(&tmp_bw);
output_size = VP8LBitWriterNumBytes(&tmp_bw);
if (output_size > data_size) {
// compressed size is larger than source! Revert to uncompressed mode.
method = ALPHA_NO_COMPRESSION;
VP8LBitWriterWipeOut(&tmp_bw);
}
} else {
VP8LBitWriterWipeOut(&tmp_bw);
return 0;
}
}
static int EncodeLossless(const uint8_t* const data, int width, int height,
int effort_level, // in [0..6] range
VP8LBitWriter* const bw,
WebPAuxStats* const stats) {
int ok = 0;
WebPConfig config;
WebPPicture picture;
WebPPictureInit(&picture);
picture.width = width;
picture.height = height;
picture.use_argb = 1;
picture.stats = stats;
if (!WebPPictureAlloc(&picture)) return 0;
// Transfer the alpha values to the green channel.
WebPDispatchAlphaToGreen(data, width, picture.width, picture.height,
picture.argb, picture.argb_stride);
WebPConfigInit(&config);
config.lossless = 1;
// Enable exact, or it would alter RGB values of transparent alpha, which is
// normally OK but not here since we are not encoding the input image but an
// internal encoding-related image containing necessary exact information in
// RGB channels.
config.exact = 1;
config.method = effort_level; // impact is very small
// Set a low default quality for encoding alpha. Ensure that Alpha quality at
// lower methods (3 and below) is less than the threshold for triggering
// costly 'BackwardReferencesTraceBackwards'.
config.quality = 8.f * effort_level;
assert(config.quality >= 0 && config.quality <= 100.f);
// TODO(urvang): Temporary fix to avoid generating images that trigger
// a decoder bug related to alpha with color cache.
// See: https://code.google.com/p/webp/issues/detail?id=239
// Need to re-enable this later.
ok = (VP8LEncodeStream(&config, &picture, bw, 0 /*use_cache*/) == VP8_ENC_OK);
WebPPictureFree(&picture);
ok = ok && !bw->error_;
if (!ok) {
VP8LBitWriterWipeOut(bw);
return 0;
}
return 1;
}