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imgcodecs: bmp: support 32bpp BMP with IMREAD_UNCHANGED #27559

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imgcodecs: bmp: support 32bpp BMP with IMREAD_UNCHANGED #27559

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Kumataro
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Close #27555

Pull Request Readiness Checklist

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

  • I agree to contribute to the project under Apache 2 License.
  • 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
  • The PR is proposed to the proper branch
  • There is a reference to the original bug report and related work
  • There is accuracy test, performance test and test data in opencv_extra repository, if applicable
    Patch to opencv_extra has the same branch name.
  • The feature is well documented and sample code can be built with the project CMake

@asmorkalov
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asmorkalov commented Jul 21, 2025 •
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The PR actually reverts some changes introduced in #20916 and modify related test. I'm looking on documentation and use cases.

@asmorkalov asmorkalov added this to the 4.13.0 milestone Jul 21, 2025
@Kumataro
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Thank you for your comment!

The fact that OpenCV's imwrite(CV_8UC4) outputs BI_RGB(RAW) 32bit format was one of the reasons why I thought it would be better to revert the previous patch #20916 .

@asmorkalov
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FFmpeg implementation:
https://github.com/FFmpeg/FFmpeg/blob/7f150dc4b715708bd6b5a4161f5115c3f9df8518/libavcodec/bmp.c#L143

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@asmorkalov asmorkalov Awaiting requested review from asmorkalov

@mshabunin mshabunin Awaiting requested review from mshabunin

@opencv-alalek opencv-alalek Awaiting requested review from opencv-alalek

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Successfully merging this pull request may close these issues.

imread with IMREAD_UNCHANGED : 32Bit Bmp will not read alpha channel, 3 channel image returned
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@Kumataro @asmorkalov