There are more than 30 different filters in vcl/source/bitmap/ to manipulate bitmaps. Some of these filters are already covered with a test in vcl/qa/cppunit/BitmapFilterTest.cxx but most of them don't have a corresponding test. The idea for this easyhack is to check which tests are missing and implement them for N24_BPP and N32_BPP bitmaps
Looking into BitmapFilterTest.cxx, at the moment it contains 22 tests for 12 distinct filters: $ git grep -h BitmapFilterTest vcl/qa/cppunit/BitmapFilterTest.cxx|awk '{print substr($2, 1, length($2)-1)}' testClampAlpha_24_BPP testClampAlpha_32_BPP testBlurCorrectness_24_BPP testBlurCorrectness_32_BPP testBasicMorphology testPerformance_24_BPP testPerformance_32_BPP testGenerateStripRanges testMultiplyBlendFilter_24_BPP testMultiplyBlendFilter_32_BPP testNormalBlendFilter_24_BPP testNormalBlendFilter_32_BPP testDarkenBlendFilter_24_BPP testDarkenBlendFilter_32_BPP testLightenBlendFilter_24_BPP testLightenBlendFilter_32_BPP testScreenBlendFilter_24_BPP testScreenBlendFilter_32_BPP testArithmeticBlendFilter_24_BPP testArithmeticBlendFilter_32_BPP testDuoToneFilter_24_BPP testDuoToneFilter_32_BPP 32 Bitmap filters can be be found in vcl/source/bitmap/: $ ls vcl/source/bitmap/*Filter.cxx|wc -l 32 You may compare the names in these two, and find the missing ones. For each of the missing tests, first you should understand the purpose of the filter, and then come up with an idea to test the filter. Then, you should create two functions for 24 and 32 bit bitmap. In the implementation(s), you should make sure that the filter generates an expected output when it modifies the bitmap. This is usually done by checking the modified bitmap after the filter is applied with BitmapFilter::Filter().
Xisco Fauli committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/core/commit/ceafe6ac7423ddc49489f8c0a87572b08b036474 tdf#170384: test BitmapColorizeFilter It will be available in 26.8.0. The patch should be included in the daily builds available at https://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More information about daily builds can be found at: https://wiki.documentfoundation.org/Testing_Daily_Builds Affected users are encouraged to test the fix and report feedback.