Compilation not working

[ojwoodford]

Compilation was broken using R2025b on MacOS 15 and Apple Silicon. I suspect this is down to the new MATLAB C++ mex API changes. I couldn't see any C++ specific features in savepng.cpp, so a simple fix was to rename savepng.cpp to savepng.c, and edit savepng.m to reflect this. Then it compiled and worked correctly. This should be an entirely backwards compatible and cross-platform fix.

Thanks for the tool!

[ojwoodford]

I did however notice that imwrite now produces a file size almost half that of savepng with a compression level of 10, so the tool may no longer be of relevance, unless there are some much better open-source png compression engines available now.

[stefslon]

Thanks. I don't have access to R2025b so can't test out new imwrite performance, but either way I guess it's time to look around for a faster PNG compressors.

I saw this one sometime ago: https://github.com/richgel999/fpng from the same guy who wrote miniz, which is at the core of this library. So I might give it a shot to see how to compares.

Do you mind posting your results of benchmark_savepng.m here?

[ojwoodford]

Save Time [sec]

Compression	IMWRITE	PNGENCODER	SAVEPNG	Improvement
1	0.187583	0.000000	0.076866	2.8
2	0.192996	0.000000	0.065924	2.9
3	0.189150	0.000000	0.076347	2.5
4	0.189545	0.000000	0.071309	2.7
5	0.191632	0.000000	0.075988	2.5
6	0.189045	0.000000	0.102534	1.9
7	0.193224	0.000000	0.120946	1.6
8	0.187564	0.000000	0.159519	1.2
9	0.190612	0.000000	0.191152	1.0
10	0.190533	0.000000	0.266019	0.7

File Size [bytes]

Compression	IMWRITE	PNGENCODER	SAVEPNG	Improvement
1	666857.30	0.00	745048.30	111.70%
2	670970.40	0.00	650205.43	97.10%
3	670970.40	0.00	548045.83	81.92%
4	670970.40	0.00	578519.83	86.43%
5	670970.40	0.00	548045.83	81.92%
6	670970.40	0.00	510920.00	76.33%
7	670970.40	0.00	504917.23	75.40%
8	670970.40	0.00	502311.77	75.03%
9	670970.40	0.00	499965.97	74.70%
10	670970.40	0.00	496883.87	74.25%

It really depends what image you're encoding. savepng might be better for indexed colormaps, but not full color images. I was testing on MATLAB's peppers.png.