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Merged
merged 8 commits into from
Jul 14, 2025
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Merged

imgcodecs: OpenEXR multispectral read/write support #27485

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292ca1d
OpenEXR multispectral read/write support
xaos-cz Jun 26, 2025
3b8f1d0
Update grfmt_exr.cpp
xaos-cz Jun 26, 2025
1103a0d
Merge branch 'opencv:4.x' into exr_multispectral
xaos-cz Jun 27, 2025
f8c2f76
better checks
xaos-cz Jun 27, 2025
fb73fe9
Update grfmt_exr.cpp
xaos-cz Jun 27, 2025
a8320fb
Update test_exr.impl.hpp
xaos-cz Jun 30, 2025
ca820ad
test fix
xaos-cz Jul 1, 2025
b30f59b
Update test_exr.impl.hpp
xaos-cz Jul 10, 2025
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2 changes: 1 addition & 1 deletion modules/imgcodecs/include/opencv2/imgcodecs.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -464,7 +464,7 @@ filename extension (see cv::imread for the list of extensions). In general, only
single-channel or 3-channel (with 'BGR' channel order) images
can be saved using this function, with these exceptions:

- With OpenEXR encoder, only 32-bit float (CV_32F) images can be saved.
- With OpenEXR encoder, only 32-bit float (CV_32F) images can be saved. More than 4 channels can be saved. (imread can load it then.)
- 8-bit unsigned (CV_8U) images are not supported.
- With Radiance HDR encoder, non 64-bit float (CV_64F) images can be saved.
- All images will be converted to 32-bit float (CV_32F).
Expand Down
61 changes: 52 additions & 9 deletions modules/imgcodecs/src/grfmt_exr.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -118,7 +118,8 @@ ExrDecoder::ExrDecoder()
m_ischroma = false;
m_hasalpha = false;
m_native_depth = false;

m_multispectral = false;
m_channels = 0;
}


Expand All @@ -140,7 +141,7 @@ void ExrDecoder::close()

int ExrDecoder::type() const
{
return CV_MAKETYPE((m_isfloat ? CV_32F : CV_32S), ((m_iscolor && m_hasalpha) ? 4 : m_iscolor ? 3 : m_hasalpha ? 2 : 1));
return CV_MAKETYPE((m_isfloat ? CV_32F : CV_32S), (m_multispectral ? m_channels : (m_iscolor && m_hasalpha) ? 4 : m_iscolor ? 3 : m_hasalpha ? 2 : 1));
}


Expand Down Expand Up @@ -169,6 +170,7 @@ bool ExrDecoder::readHeader()
m_green = channels.findChannel( "G" );
m_blue = channels.findChannel( "B" );
m_alpha = channels.findChannel( "A" );
m_multispectral = channels.findChannel( "0" ) != nullptr;

if( m_alpha ) // alpha channel supported in RGB, Y, and YC scenarios
m_hasalpha = true;
Expand All @@ -179,6 +181,23 @@ bool ExrDecoder::readHeader()
m_ischroma = false;
result = true;
}
else if( m_multispectral )
{
m_channels = 0;
for( auto it = channels.begin(); it != channels.end(); it++ )
m_channels++;

m_iscolor = true; // ??? false
m_ischroma = false;
m_hasalpha = false;
result = m_channels <= CV_CN_MAX;

for ( int i = 1; result && i < m_channels; i++ ) // channel 0 was found previously
{
const Channel *ch = channels.findChannel( std::to_string(i) );
result = ch && ch->xSampling == 1 && ch->ySampling == 1; // subsampling is not supported
}
}
else
{
m_green = channels.findChannel( "Y" );
Expand Down Expand Up @@ -214,8 +233,9 @@ bool ExrDecoder::readHeader()
bool ExrDecoder::readData( Mat& img )
{
m_native_depth = CV_MAT_DEPTH(type()) == img.depth();
bool multispectral = img.channels() > 4;
bool color = img.channels() > 2; // output mat has 3+ channels; Y or YA are the 1 and 2 channel scenario
bool alphasupported = ( img.channels() % 2 == 0 ); // even number of channels indicates alpha
bool alphasupported = !multispectral && ( img.channels() % 2 == 0 ); // even number of channels indicates alpha
int channels = 0;
uchar* data = img.ptr();
size_t step = img.step;
Expand All @@ -231,10 +251,13 @@ bool ExrDecoder::readData( Mat& img )
const size_t floatsize = sizeof(float);
size_t xstep = m_native_depth ? floatsize : 1; // 4 bytes if native depth (FLOAT), otherwise converting to 1 byte U8 depth
size_t ystep = 0;
const int channelstoread = ( (m_iscolor && alphasupported) ? 4 :
const int channelstoread = ( multispectral ? img.channels() : (m_iscolor && alphasupported) ? 4 :
( (m_iscolor && !m_ischroma) || color) ? 3 : alphasupported ? 2 : 1 ); // number of channels to read may exceed channels in output img
size_t xStride = floatsize * channelstoread;

CV_Assert( m_multispectral == multispectral );
CV_Assert( justcopy || !multispectral ); // multispectral needs justcopy

// See https://github.com/opencv/opencv/issues/26705
// If ALGO_HINT_ACCURATE is set, read BGR and swap to RGB.
// If ALGO_HINT_APPROX is set, read RGB directly.
Expand Down Expand Up @@ -312,6 +335,15 @@ bool ExrDecoder::readData( Mat& img )
xsample[0] = m_green->xSampling;
}
}
else if( m_multispectral )
{
for ( int i = 0; i < m_channels; i++ )
{
frame.insert( std::to_string(i), Slice( m_type,
buffer - m_datawindow.min.x * xStride - m_datawindow.min.y * ystep + (floatsize * i),
xStride, ystep, 1, 1, 0.0 ));
}
}
else
{
if( m_blue )
Expand Down Expand Up @@ -382,7 +414,7 @@ bool ExrDecoder::readData( Mat& img )
{
m_file->readPixels( m_datawindow.min.y, m_datawindow.max.y );

if( m_iscolor )
if( m_iscolor && !m_multispectral )
{
if (doReadRGB)
{
Expand Down Expand Up @@ -804,13 +836,19 @@ bool ExrEncoder::write( const Mat& img, const std::vector<int>& params )
header.channels().insert( "B", Channel( type ) );
//printf("bunt\n");
}
else
else if( channels == 1 || channels == 2 )
{
header.channels().insert( "Y", Channel( type ) );
//printf("gray\n");
}
else if( channels > 4 )
{
for ( int i = 0; i < channels; i++ )
header.channels().insert( std::to_string(i), Channel( type ) );
//printf("multi-channel\n");
}

if( channels % 2 == 0 )
if( channels % 2 == 0 && channels <= 4)
{ // even number of channels indicates Alpha
header.channels().insert( "A", Channel( type ) );
}
Expand Down Expand Up @@ -843,10 +881,15 @@ bool ExrEncoder::write( const Mat& img, const std::vector<int>& params )
frame.insert( "G", Slice( type, buffer + size, size * channels, bufferstep ));
frame.insert( "R", Slice( type, buffer + size * 2, size * channels, bufferstep ));
}
else
else if( channels == 1 || channels == 2 )
frame.insert( "Y", Slice( type, buffer, size * channels, bufferstep ));
else if( channels > 4 )
{
for ( int i = 0; i < channels; i++ )
frame.insert( std::to_string(i), Slice( type, buffer + size * i, size * channels, bufferstep ));
}

if( channels % 2 == 0 )
if( channels % 2 == 0 && channels <= 4 )
{ // even channel count indicates Alpha channel
frame.insert( "A", Slice( type, buffer + size * (channels - 1), size * channels, bufferstep ));
}
Expand Down
2 changes: 2 additions & 0 deletions modules/imgcodecs/src/grfmt_exr.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -100,6 +100,8 @@ class ExrDecoder CV_FINAL : public BaseImageDecoder
bool m_iscolor;
bool m_isfloat;
bool m_hasalpha;
bool m_multispectral;
int m_channels;

private:
ExrDecoder(const ExrDecoder &); // copy disabled
Expand Down
9 changes: 9 additions & 0 deletions modules/imgcodecs/src/loadsave.cpp
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Expand Up @@ -960,7 +960,12 @@ static bool imwrite_( const String& filename, const std::vector<Mat>& img_vec,
Mat image = img_vec[page];
CV_Assert(!image.empty());

#ifdef HAVE_OPENEXR
CV_Assert( image.channels() == 1 || image.channels() == 3 || image.channels() == 4 || encoder.dynamicCast<ExrEncoder>() );
#else
CV_Assert( image.channels() == 1 || image.channels() == 3 || image.channels() == 4 );
#endif


Mat temp;
if( !encoder->isFormatSupported(image.depth()) )
Expand Down Expand Up @@ -1473,7 +1478,11 @@ bool imencode( const String& ext, InputArray _img,
CV_Assert(!image.empty());

const int channels = image.channels();
#ifdef HAVE_OPENEXR
CV_Assert( channels == 1 || channels == 3 || channels == 4 || encoder.dynamicCast<ExrEncoder>() );
#else
CV_Assert( channels == 1 || channels == 3 || channels == 4 );
#endif

Mat temp;
if( !encoder->isFormatSupported(image.depth()) )
Expand Down
26 changes: 26 additions & 0 deletions modules/imgcodecs/test/test_exr.impl.hpp
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Expand Up @@ -68,6 +68,32 @@ TEST(Imgcodecs_EXR, readWrite_32FC3)
EXPECT_EQ(0, remove(filenameOutput.c_str()));
}

TEST(Imgcodecs_EXR, readWrite_32FC7)
{ // 0-6 channels (nultispectral)
const string root = cvtest::TS::ptr()->get_data_path();
const string filenameInput = root + "readwrite/test32FC7.exr";
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imho using a generated cv::Mat here instead of adding an extra test file to opencv_extra is better.

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    TEST(Imgcodecs_EXR, readWrite_32FC7)
    { // 0-6 channels (multispectral)
        const string filename = cv::tempfile(".exr");

        const Size sz(3, 5);
        Mat img(sz, CV_MAKETYPE(CV_32F, 7));
        img.at<cv::Vec<float, 7>>(0, 0)[0] = 101.125;
        img.at<cv::Vec<float, 7>>(2, 1)[3] = 203.500;
        img.at<cv::Vec<float, 7>>(4, 2)[6] = 305.875;

        ASSERT_TRUE(cv::imwrite(filename, img));
        const Mat img2 = cv::imread(filename, IMREAD_UNCHANGED);
        EXPECT_EQ(img2.type(), img.type());
        EXPECT_EQ(img2.size(), img.size());
        EXPECT_LE(cvtest::norm(img, img2, NORM_INF | NORM_RELATIVE), 1e-3);

        const Mat img3 = cv::imread(filename, IMREAD_GRAYSCALE);
        ASSERT_TRUE(img3.empty());
        const Mat img4 = cv::imread(filename, IMREAD_COLOR);
        ASSERT_TRUE(img4.empty());
        EXPECT_EQ(0, remove(filename.c_str()));
    }

output

[ RUN      ] Imgcodecs_EXR.readWrite_32FC7
[ERROR:0@3.966] global loadsave.cpp:594 cv::imread_ imread_('C:\Users\aidata\AppData\Local\Temp\ocvBA1A.tmp.exr'): can't read data: OpenCV(4.12.0-pre) C:\projects\opencv\modules\imgcodecs\src\grfmt_exr.cpp:260: error: (-2:Unspecified error) in function 'bool __cdecl cv::ExrDecoder::readData(class cv::Mat &)'
> OpenCV EXR decoder needs more number of channels for multispectral images. Use cv::IMREAD_UNCHANGED mode for imread. (expected: 'img.channels() == CV_MAT_CN(type())'), where
>     'img.channels()' is 1
> must be equal to
>     'CV_MAT_CN(type())' is 7

[ERROR:0@3.966] global loadsave.cpp:594 cv::imread_ imread_('C:\Users\aidata\AppData\Local\Temp\ocvBA1A.tmp.exr'): can't read data: OpenCV(4.12.0-pre) C:\projects\opencv\modules\imgcodecs\src\grfmt_exr.cpp:260: error: (-2:Unspecified error) in function 'bool __cdecl cv::ExrDecoder::readData(class cv::Mat &)'
> OpenCV EXR decoder needs more number of channels for multispectral images. Use cv::IMREAD_UNCHANGED mode for imread. (expected: 'img.channels() == CV_MAT_CN(type())'), where
>     'img.channels()' is 3
> must be equal to
>     'CV_MAT_CN(type())' is 7

[       OK ] Imgcodecs_EXR.readWrite_32FC7 (5 ms)

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        EXPECT_EQ(0, remove(filename.c_str()));
        const Mat img3 = cv::imread(filename, IMREAD_GRAYSCALE);
        ASSERT_TRUE(img3.empty());
        const Mat img4 = cv::imread(filename, IMREAD_COLOR);
        ASSERT_TRUE(img4.empty());
    }

Removing file is before testing grayscale and color in current branch. It prints error message as you can't load multispectral image as GRAYSCALE or COLOR.

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        EXPECT_EQ(0, remove(filename.c_str()));
        const Mat img3 = cv::imread(filename, IMREAD_GRAYSCALE);
        ASSERT_TRUE(img3.empty());
        const Mat img4 = cv::imread(filename, IMREAD_COLOR);
        ASSERT_TRUE(img4.empty());
    }

Removing file is before testing grayscale and color in current branch. It prints error message as you can't load multispectral image as GRAYSCALE or COLOR.

i fixed it on my code

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i suggest using this

    TEST(Imgcodecs_EXR, readWrite_32FC7)
    { // 0-6 channels (multispectral)
        const string filename = cv::tempfile(".exr");

        const Size sz(3, 5);
        Mat img = Mat::zeros(sz, CV_MAKETYPE(CV_32F, 7));
        img.at<cv::Vec<float, 7>>(0, 0)[0] = 101.125;
        img.at<cv::Vec<float, 7>>(2, 1)[3] = 203.500;
        img.at<cv::Vec<float, 7>>(4, 2)[6] = 305.875;

        ASSERT_TRUE(cv::imwrite(filename, img));
        const Mat img2 = cv::imread(filename, IMREAD_UNCHANGED);
        EXPECT_EQ(img2.type(), img.type());
        EXPECT_EQ(img2.size(), img.size());
        EXPECT_LE(cvtest::norm(img, img2, NORM_INF | NORM_RELATIVE), 1e-3);

        const Mat img3 = cv::imread(filename, IMREAD_GRAYSCALE);
        ASSERT_TRUE(img3.empty());
        const Mat img4 = cv::imread(filename, IMREAD_COLOR);
        ASSERT_TRUE(img4.empty());
        EXPECT_EQ(0, remove(filename.c_str()));
    }

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        const Size sz(3, 5);
        Mat img = Mat::zeros(sz, CV_MAKETYPE(CV_32F, 7));
        img.at<cv::Vec<float, 7>>(0, 0)[0] = 101.125;
        img.at<cv::Vec<float, 7>>(2, 1)[3] = 203.500;
        img.at<cv::Vec<float, 7>>(4, 2)[6] = 305.875;
        ASSERT_TRUE(cv::imwrite(filename, img));

generates the same file at opencv/opencv_extra#1262

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I use the same pattern as tests readWrite_32FC1, readWrite_32FC3, readWrite_32FC1_half, readWrite_32FC3_half. It's generate image file upon GENERATE_DATA define. I don't know when it was called with this define so I do the new test readWrite_32FC7 same way.

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@xaos-cz thank you for your contribution. i am also a common contributor ( a bit more experienced). I think this test is to verify that the change you made is working correctly and to show if a later change breaks your code. It doesn't matter if it's similar in form to the other tests.

const string filenameOutput = cv::tempfile(".exr");
#ifndef GENERATE_DATA
const Mat img = cv::imread(filenameInput, IMREAD_UNCHANGED);
#else
const Size sz(3, 5);
Mat img(sz, CV_32FC7);
img.at<cv::Vec<float, 7>>(0, 0)[0] = 101.125;
img.at<cv::Vec<float, 7>>(2, 1)[3] = 203.500;
img.at<cv::Vec<float, 7>>(4, 2)[6] = 305.875;
ASSERT_TRUE(cv::imwrite(filenameInput, img));
#endif
ASSERT_FALSE(img.empty());
ASSERT_EQ(CV_MAKETYPE(CV_32F, 7), img.type());

ASSERT_TRUE(cv::imwrite(filenameOutput, img));
const Mat img2 = cv::imread(filenameOutput, IMREAD_UNCHANGED);
ASSERT_EQ(img2.type(), img.type());
ASSERT_EQ(img2.size(), img.size());
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@mshabunin mshabunin Jul 7, 2025 •
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Temporary file will not be removed if test fail on this or previous asserts. We need to use EXPECT.

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I am new to gtest so I don't know that EXPECT should be used. I copied the pattern which was in previous tests: readWrite_32FC1, readWrite_32FC3.

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    EXPECT_EQ(img2.type(), img.type());
    EXPECT_EQ(img2.size(), img.size());

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I changed that only in my test readWrite_32FC7, but the same is in several other tests.

EXPECT_LE(cvtest::norm(img, img2, NORM_INF | NORM_RELATIVE), 1e-3);
EXPECT_EQ(0, remove(filenameOutput.c_str()));
}


TEST(Imgcodecs_EXR, readWrite_32FC1_half)
{
Expand Down
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