Looks good to me in general. Couple of questions:

• How the expected size is calculated? It will be different for PNG versions and SPNG?
• Why do you need the same test for 3 images and all compression levels. I propose to reduce the test suite to safe execution time.

Looks good to me in general. Couple of questions:

• How the expected size is calculated? It will be different for PNG versions and SPNG?

to find expected sizes, i run the test with some values and corrected manually with values that the test pass.

[ RUN      ] Imgcodecs_Png_ImwriteFlags.compression_level/0, where GetParam() = ("gifsuite/gif_big.png", 0, 7262971)
/build/precommit_linux64/4.x/opencv/modules/imgcodecs/test/test_png.cpp:345: Failure
Expected equality of these values:
  buf.size()
    Which is: 7263071
  compression_level_output_size
    Which is: 7262971

PNG and SPNG gives same values. when compiling with zlib-ng the resuls are various.

• Why do you need the same test for 3 images and all compression levels. I propose to reduce the test suite to safe execution time.

i faced different result on linux and windows for the following file. i intended to test with different files. sure i will reduce the list.

[ RUN      ] Imgcodecs_Png_ImwriteFlags.compression_level/0, where GetParam() = ("gifsuite/gif_big.png", 0, 7262971)
/build/precommit_linux64/4.x/opencv/modules/imgcodecs/test/test_png.cpp:345: Failure
Expected equality of these values:
  buf.size()
    Which is: 7263071
  compression_level_output_size
    Which is: 7262971

Tested several libpng versions, SPNG, ARMv7. Works well. To be merged after CI.