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Merge pull request #28519 from Kumataro:fix28503

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imgcodecs(webp): improve IMWRITE_WEBP_LOSSLESS_MODE to support exact lossless compression #28519

Close #28503 

Add IMWRITE_WEBP_LOSSLESS_MODE enum to control lossless strategy:
- IMWRITE_WEBP_LOSSLESS_OFF: Lossy compression.
- IMWRITE_WEBP_LOSSLESS_ON:  Optimize/drop color in transparent pixels.
- IMWRITE_WEBP_LOSSLESS_PRESERVE_COLOR: Preserve color in transparent pixels.

Note: Currently limited to still images; animated WebP support is deferred per YAGNI.

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [x] The feature is well documented and sample code can be built with the project CMake
This commit is contained in:
Kumataro
2026-02-26 17:01:36 +09:00
committed by GitHub
parent 1d6b8bdef7
commit 6c374bebee
3 changed files with 289 additions and 27 deletions
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modules/imgcodecs/src/grfmt_webp.cpp
+131 -26
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@@ -338,7 +338,7 @@ WebPEncoder::WebPEncoder()
m_support_metadata[IMAGE_METADATA_EXIF] = true;
m_support_metadata[IMAGE_METADATA_XMP] = true;
m_support_metadata[IMAGE_METADATA_ICCP] = true;
m_supported_encode_key = {IMWRITE_WEBP_QUALITY};
m_supported_encode_key = {IMWRITE_WEBP_QUALITY, IMWRITE_WEBP_LOSSLESS_MODE};
}
WebPEncoder::~WebPEncoder() { }
@@ -348,32 +348,121 @@ ImageEncoder WebPEncoder::newEncoder() const
return makePtr<WebPEncoder>();
}
// Simple API style
static size_t cvEncodeLosslessExactBGRA(const uint8_t* rgba, int width, int height, int stride, uint8_t** output)
{
WebPConfig config;
WebPPicture pic;
WebPMemoryWriter wrt;
// 6 is the default value for speed/compression balance in lossless mode.
// It doesn't affect visual quality, only file size and encoding time.
if (!WebPConfigInit(&config) || !WebPConfigLosslessPreset(&config, 6))
{
return 0;
}
config.exact = 1;
if (!WebPPictureInit(&pic))
{
return 0;
}
pic.width = width;
pic.height = height;
pic.use_argb = 1; // BGRA
if (!WebPPictureImportBGRA(&pic, rgba, stride))
{
return 0;
}
WebPMemoryWriterInit(&wrt);
pic.writer = WebPMemoryWrite;
pic.custom_ptr = &wrt;
if (!WebPEncode(&config, &pic))
{
WebPMemoryWriterClear(&wrt);
WebPPictureFree(&pic);
return 0;
}
*output = wrt.mem;
size_t size = wrt.size;
WebPPictureFree(&pic);
return size;
}
bool WebPEncoder::write(const Mat& img, const std::vector<int>& params)
{
CV_CheckDepthEQ(img.depth(), CV_8U, "WebP codec supports 8U images only");
const int width = img.cols, height = img.rows;
bool comp_lossless = true;
float quality = 100.0f;
int lossless_mode = -1; // not specified
float quality = 0.0f; // not specified
for(size_t i = 0; i < params.size(); i += 2)
{
const int value = params[i+1];
if (params[i] == IMWRITE_WEBP_QUALITY)
if (params[i] == IMWRITE_WEBP_LOSSLESS_MODE)
{
comp_lossless = false;
quality = static_cast<float>(value);
if (quality < 1.0f)
switch(value)
{
quality = 1.0f;
CV_LOG_WARNING(nullptr, cv::format("The value(%d) for IMWRITE_WEBP_QUALITY must be between 1 to 100(lossy) or more(lossless). It is fallbacked to 1", value));
}
if (quality > 100.0f)
{
comp_lossless = true;
case IMWRITE_WEBP_LOSSLESS_ON:
case IMWRITE_WEBP_LOSSLESS_OFF:
case IMWRITE_WEBP_LOSSLESS_PRESERVE_COLOR:
lossless_mode = value;
break;
default:
CV_LOG_WARNING(nullptr, cv::format("The value(%d) for IMWRITE_WEBP_LOSSLESS_MODE must be one of ImwriteWEBPLosslessMode. It is ignored", value));
break;
}
}
if (params[i] == IMWRITE_WEBP_QUALITY)
{
if (value < 1)
{
CV_LOG_WARNING(nullptr, cv::format("The value(%d) for IMWRITE_WEBP_QUALITY must be between 1 to 100(lossy) or more(lossless). It is fallbacked to 1", value));
quality = 1.0f;
}
else if (value > 100)
{
quality = 101.0f;
}
else // value is 1 to 100
{
quality = static_cast<float>(value);
}
}
}
switch(lossless_mode)
{
case -1: // not specified by user
case IMWRITE_WEBP_LOSSLESS_OFF:
// Fallback to lossless if quality is not specified (-1.0f) or out of lossy range (>100.0f).
// This maintains backward compatibility where WebP defaults to lossless.
if ((quality < 1.0f) || (quality > 100.0f))
{
lossless_mode = IMWRITE_WEBP_LOSSLESS_ON;
quality = 101.0f;
}
else
{
lossless_mode = IMWRITE_WEBP_LOSSLESS_OFF;
// Use specified quality for lossy compression.
}
break;
case IMWRITE_WEBP_LOSSLESS_ON:
case IMWRITE_WEBP_LOSSLESS_PRESERVE_COLOR:
// Force quality value to lossless range when explicit lossless mode is selected.
quality = 101.0f;
break;
default:
CV_Error(Error::StsError, cv::format("Unexpected lossless_mode(%d)", lossless_mode));
break;
}
int channels = img.channels();
@@ -391,29 +480,45 @@ bool WebPEncoder::write(const Mat& img, const std::vector<int>& params)
uint8_t *encoder_out = NULL;
size_t size = 0;
if (comp_lossless)
if (channels == 3)
{
if (channels == 3)
switch(lossless_mode)
{
size = WebPEncodeLosslessBGR(image->ptr(), width, height, (int)image->step, &encoder_out);
}
else if (channels == 4)
{
size = WebPEncodeLosslessBGRA(image->ptr(), width, height, (int)image->step, &encoder_out);
case IMWRITE_WEBP_LOSSLESS_OFF:
size = WebPEncodeBGR(image->ptr(), width, height, (int)image->step, quality, &encoder_out);
break;
case IMWRITE_WEBP_LOSSLESS_ON:
case IMWRITE_WEBP_LOSSLESS_PRESERVE_COLOR:
size = WebPEncodeLosslessBGR(image->ptr(), width, height, (int)image->step, &encoder_out);
break;
default:
CV_Error(Error::StsError, cv::format("Unexcepted lossless_mode(%d)", lossless_mode));
break;
}
}
else
{
if (channels == 3)
CV_CheckEQ(channels, 4, "Unexpected channels is used");
switch(lossless_mode)
{
size = WebPEncodeBGR(image->ptr(), width, height, (int)image->step, quality, &encoder_out);
}
else if (channels == 4)
{
size = WebPEncodeBGRA(image->ptr(), width, height, (int)image->step, quality, &encoder_out);
case IMWRITE_WEBP_LOSSLESS_OFF:
size = WebPEncodeBGRA(image->ptr(), width, height, (int)image->step, quality, &encoder_out);
break;
case IMWRITE_WEBP_LOSSLESS_ON:
size = WebPEncodeLosslessBGRA(image->ptr(), width, height, (int)image->step, &encoder_out);
break;
case IMWRITE_WEBP_LOSSLESS_PRESERVE_COLOR:
size = cvEncodeLosslessExactBGRA(image->ptr(), width, height, (int)image->step, &encoder_out);
break;
}
}
if (size == 0)
{
CV_LOG_ERROR(NULL, cv::format("WebP encoding failed with lossless_mode=%d", lossless_mode));
return false;
}
#if WEBP_DECODER_ABI_VERSION >= 0x0206
Ptr<uint8_t> out_cleaner(encoder_out, WebPFree);
#else