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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/include/opencv2/imgcodecs.hpp
+10 -1
View File
@@ -102,7 +102,8 @@ enum ImwriteFlags {
IMWRITE_EXR_TYPE = (3 << 4) + 0 /* 48 */, //!< override EXR storage type (FLOAT (FP32) is default)
IMWRITE_EXR_COMPRESSION = (3 << 4) + 1 /* 49 */, //!< override EXR compression type (ZIP_COMPRESSION = 3 is default)
IMWRITE_EXR_DWA_COMPRESSION_LEVEL = (3 << 4) + 2 /* 50 */, //!< override EXR DWA compression level (45 is default)
IMWRITE_WEBP_QUALITY = 64, //!< For WEBP, it can be a quality from 1 to 100 (the higher is the better). By default (without any parameter) and for quality above 100 the lossless compression is used.
IMWRITE_WEBP_QUALITY = 64, //!< For WEBP, it can be a lossy quality from 1 to 100 (the higher is the better) for IMWRITE_WEBP_LOSSLESS_OFF. By default (without this parameter) or if quality > 100, IMWRITE_WEBP_LOSSLESS_ON is used instead.
IMWRITE_WEBP_LOSSLESS_MODE = 65, //!< For WEBP, it can be a lossless compression strategy. See cv::ImwriteWEBPLosslessMode. Default is IMWRITE_WEBP_LOSSLESS_OFF. For Animated WEBP, it is not supported.
IMWRITE_HDR_COMPRESSION = (5 << 4) + 0 /* 80 */, //!< specify HDR compression
IMWRITE_PAM_TUPLETYPE = 128,//!< For PAM, sets the TUPLETYPE field to the corresponding string value that is defined for the format
IMWRITE_TIFF_RESUNIT = 256,//!< For TIFF, use to specify which DPI resolution unit to set. See ImwriteTiffResolutionUnitFlags. Default is IMWRITE_TIFF_RESOLUTION_UNIT_INCH.
@@ -240,6 +241,14 @@ enum ImwritePAMFlags {
IMWRITE_PAM_FORMAT_RGB_ALPHA = 5
};
//! Imwrite WEBP specific values for IMWRITE_WEBP_LOSSLESS_MODE parameter key.
enum ImwriteWEBPLosslessMode {
IMWRITE_WEBP_LOSSLESS_OFF = 0, //!< Lossy compression mode. Uses IMWRITE_WEBP_QUALITY to control compression. (Default)
//!< @note If IMWRITE_WEBP_QUALITY is not specified, it falls back to IMWRITE_WEBP_LOSSLESS_ON to maintain backward compatibility.
IMWRITE_WEBP_LOSSLESS_ON = 1, //!< Standard lossless compression. May modify or discard RGB values of fully transparent pixels to improve compression ratio.
IMWRITE_WEBP_LOSSLESS_PRESERVE_COLOR = 2, //!< Exact lossless compression. Preserves all RGB data even for pixels with 0 alpha (equivalent to WebP's exact flag).
};
//! Imwrite HDR specific values for IMWRITE_HDR_COMPRESSION parameter key
enum ImwriteHDRCompressionFlags {
IMWRITE_HDR_COMPRESSION_NONE = 0,
modules/imgcodecs/src/grfmt_webp.cpp
+131 -26
View File
@@ -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
modules/imgcodecs/test/test_webp.cpp
+148
View File
@@ -114,6 +114,154 @@ TEST(Imgcodecs_WebP, encode_decode_with_alpha_webp)
EXPECT_EQ(512, img_webp_bgr.rows);
}
// See https://github.com/opencv/opencv/issues/28503
TEST(Imgcodecs_WebP, encode_decode_LOSSLESS_MODE)
{
cv::Mat img(cv::Size(64,4), CV_8UC4, cv::Scalar(124,64,67,0) );
for(int ix = 0; ix < img.size().width; ix++)
{
img.at<Vec4b>(0, ix)[3] = 0; // Transpacency pixel
img.at<Vec4b>(1, ix)[3] = 1;
img.at<Vec4b>(2, ix)[3] = 254;
img.at<Vec4b>(3, ix)[3] = 255;
}
std::vector<uint8_t> work;
EXPECT_NO_THROW(cv::imencode(".webp", img, work, {IMWRITE_WEBP_LOSSLESS_MODE, IMWRITE_WEBP_LOSSLESS_ON}));
cv::Mat img_ON = cv::imdecode(work, IMREAD_UNCHANGED);
EXPECT_NO_THROW(cv::imencode(".webp", img, work, {IMWRITE_WEBP_LOSSLESS_MODE, IMWRITE_WEBP_LOSSLESS_PRESERVE_COLOR}));
cv::Mat img_PRESERVE_COLOR = cv::imdecode(work, IMREAD_UNCHANGED);
for(int ix = 0; ix < img.size().width; ix++)
{
EXPECT_EQ(img_ON.at<Vec4b>(0, ix), Vec4b(0, 0, 0, 0)); // LOSSLESS_ON -> COLOR will be optimized/dropped.
EXPECT_EQ(img_PRESERVE_COLOR.at<Vec4b>(0, ix), Vec4b(124,64,67,0)); // PRESERVE_COLOR
EXPECT_EQ(img_ON.at<Vec4b>(1, ix), img_PRESERVE_COLOR.at<Vec4b>(1, ix) );
EXPECT_EQ(img_ON.at<Vec4b>(2, ix), img_PRESERVE_COLOR.at<Vec4b>(2, ix) );
EXPECT_EQ(img_ON.at<Vec4b>(3, ix), img_PRESERVE_COLOR.at<Vec4b>(3, ix) );
}
}
// Expected result categories for WebP encoding tests.
enum ImencodeLosslessResult {
LOSSY, // Expect lossy compression (pixel differences allowed)
LOSSLESS, // Expect standard lossless compression (pixel values match)
EXACT // Expect exact lossless (preserving RGB values of transparent pixels)
};
typedef std::tuple<int, int, ImencodeLosslessResult> WebPModePriorityParams;
class Imgcodecs_WebP_Mode_Priority : public testing::TestWithParam<WebPModePriorityParams> {};
TEST_P(Imgcodecs_WebP_Mode_Priority, encode_webp_mode_priority)
{
const int mode = std::get<0>(GetParam());
const int quality = std::get<1>(GetParam());
const ImencodeLosslessResult expected = std::get<2>(GetParam());
// Generate a 100x100 RGBA test image.
// Set a transparent pixel with specific color (Blue) to verify EXACT mode.
Mat src(100, 100, CV_8UC4, Scalar(255, 255, 255, 255));
src.at<Vec4b>(0, 0) = Vec4b(255, 0, 0, 0); // Transparent Blue (B:255, G:0, R:0, A:0)
// Build the imwrite parameter vector dynamically.
std::vector<int> params;
if (mode != -1) {
params.push_back(IMWRITE_WEBP_LOSSLESS_MODE);
params.push_back(mode);
}
if (quality != -1) {
params.push_back(IMWRITE_WEBP_QUALITY);
params.push_back(quality);
}
// Encode to memory and decode back.
std::vector<uchar> buf;
ASSERT_TRUE(imencode(".webp", src, buf, params));
Mat dst = imdecode(buf, IMREAD_UNCHANGED);
ASSERT_FALSE(dst.empty());
// Validation logic
if (expected == LOSSY) {
// We expect some differences in lossy mode
double diff = cv::norm(src, dst, NORM_INF);
EXPECT_GT(diff, 0) << "Should be lossy (Quality: " << quality << ")";
}
else if (expected == LOSSLESS) {
// Standard lossless: we allow the library to modify RGB values
// of fully transparent pixels (A=0) to improve compression ratio.
// Thus, we compare only visible pixels or check with a slightly relaxed condition.
// Option A: If you want to allow RGB changes on A=0:
// We can't use cv::norm directly if A=0 pixels are modified.
// Let's check if they are identical except for the transparent pixel.
Mat diff;
absdiff(src, dst, diff);
Scalar total_diff = sum(diff);
// If only the (0,0) pixel changed from (255,0,0,0) to (0,0,0,0),
// total_diff will be 255.
EXPECT_LE(total_diff[0] + total_diff[1] + total_diff[2], 255)
<< "Standard lossless should not have significant pixel differences";
EXPECT_EQ(src.at<Vec4b>(0,0)[3], dst.at<Vec4b>(0,0)[3]) << "Alpha must be preserved";
}
else if (expected == EXACT) {
// Exact lossless: Every single bit must match, including transparent pixels.
double diff = cv::norm(src, dst, NORM_INF);
EXPECT_EQ(0, diff) << "Exact mode must preserve all pixel values perfectly";
EXPECT_EQ(src.at<Vec4b>(0, 0), dst.at<Vec4b>(0, 0))
<< "RGB values of transparent pixels must be preserved in EXACT mode";
}
else {
FAIL() << "Unknown expectation type";
}
}
/**
* Helper to generate human-readable test names in gtest output.
*/
static std::string getModeStr(int m) {
if (m == -1) return "OMIT";
if (m == IMWRITE_WEBP_LOSSLESS_OFF) return "OFF";
if (m == IMWRITE_WEBP_LOSSLESS_ON) return "ON";
if (m == IMWRITE_WEBP_LOSSLESS_PRESERVE_COLOR) return "PRESERVE";
return "UNKNOWN";
}
static std::string getExpectStr(ImencodeLosslessResult r) {
return (r == LOSSY) ? "LOSSY" : (r == EXACT) ? "EXACT" : "LOSSLESS";
}
INSTANTIATE_TEST_CASE_P(Imgcodecs, Imgcodecs_WebP_Mode_Priority,
testing::Values(
// Default (OMIT mode) cases
WebPModePriorityParams(-1, -1, LOSSLESS),
WebPModePriorityParams(-1, 80, LOSSY),
WebPModePriorityParams(-1, 101, LOSSLESS),
// LOSSLESS_OFF (Explicitly off)
WebPModePriorityParams(IMWRITE_WEBP_LOSSLESS_OFF, -1, LOSSLESS),
WebPModePriorityParams(IMWRITE_WEBP_LOSSLESS_OFF, 80, LOSSY),
WebPModePriorityParams(IMWRITE_WEBP_LOSSLESS_OFF, 101, LOSSLESS),
// LOSSLESS_ON (Force lossless)
WebPModePriorityParams(IMWRITE_WEBP_LOSSLESS_ON, -1, LOSSLESS),
WebPModePriorityParams(IMWRITE_WEBP_LOSSLESS_ON, 80, LOSSLESS),
WebPModePriorityParams(IMWRITE_WEBP_LOSSLESS_ON, 101, LOSSLESS),
// PRESERVE_COLOR (Exact lossless)
WebPModePriorityParams(IMWRITE_WEBP_LOSSLESS_PRESERVE_COLOR, -1, EXACT),
WebPModePriorityParams(IMWRITE_WEBP_LOSSLESS_PRESERVE_COLOR, 80, EXACT),
WebPModePriorityParams(IMWRITE_WEBP_LOSSLESS_PRESERVE_COLOR, 101, EXACT)
),
[](const testing::TestParamInfo<WebPModePriorityParams>& info_) {
std::string mode = getModeStr(std::get<0>(info_.param));
int q = std::get<1>(info_.param);
std::string q_str = (q == -1) ? "omit" : std::to_string(q);
return mode + "_q" + q_str + "_" + getExpectStr(std::get<2>(info_.param));
}
);
#endif // HAVE_WEBP
}} // namespace