caedes' Notes


Comparing png compression ratios of stb_image_write, LodePNG, miniz and libpng

Filed under: Programming — caedes @ 04:29
Tags: , , , , , , ,

Because of I was wondering how good/bad stb_image_write’s PNG compression really is in comparison to other encoders.

So I did a quick comparison between stb_image_write (v0.98) LodePNG (version 20150418), miniz’s tdefl_write_image_to_png_file_in_memory_ex() (v1.15) and libpng (version 1.2.50 from Ubuntu 14.04), always with the highest possible compression I could configure.

I used four different test images, all 24bit RGB:
A screenshot from Quake2: Zaero in 1680×1050, a screenshot from RBDoom3BFG in 1392×920, the classic “Lenna” test image in 512×512 and a colorful wallpaper with several parrots in 2560×1600.
Update: I also tested a screenshot of Snakebird


I tested libpng with highest compression (png_set_compression_level(png_ptr, 9);) and default compression.
libpng with highest compression is used as 100%, as it almost always yields the smallest image.

Doom3BFG Screenshot:

Bytes Percent
libpng high 1133340 100%
libpng default 1167675 103%
LodePNG 1198051 106%
miniz 1569473 138%
stb 1608577 142%

Quake2 Screenshot:

Bytes Percent
libpng high 2082772 100%
libpng default 2125051 102%
LodePNG 2243547 108%
miniz 2683565 129%
stb 3146951 151%


Bytes Percent
libpng high 5062358 100%
libpng default 5195689 103%
LodePNG 5328474 105%
miniz 9000478 178%
stb 7439122 147%


Bytes Percent
libpng high 476195 100%
libpng default 476196 100%
LodePNG 499061 105%
miniz 729880 153%
stb 723394 152%


Bytes Percent
libpng high 259338 100%
libpng default 265571 102%
LodePNG 291126 112%
miniz 226136 87%
stb 315422 122%


Sean was right with “It looks to me like the miniz doesn’t use PNG filters, so it’s just compressing the raw image data as is, which isn’t going to be very good compression either, but along a different axis.”.
Sometimes the compression of miniz (or more correctly tdefl_write_image_to_png_file_in_memory_ex()) is better and sometimes stbi_write_png() is better – so it doesn’t matter which one you use, if you wanna use one of them.

However, compared to libpng and LodePNG both don’t compress very well – the resulting images are 29%-78% bigger.
LodePNG on the other hand produces almost as good results as libpng (only 5%-8% bigger) and is significantly easier to use – integrating it in your project is easy (just drop the source and the header file to your project) and using it is about as easy as stbi_write_png().
(For image loading however I found stb_image much better than LodePNG, as it loads PNGs much faster, see my other article)

UPDATE: A kinda strange case is the Snakebird-Screenshot: Sean said that “something more computer-arty — flat colors or gradients” will yield different results, because “that’s one of the places where the PNG filters are beneficial”.
And he was right: In that case the stbi_write_png() output was only 22% bigger than the libpng output – but for some reason miniz, which does not seem to do PNG filtering, compressed even better than libpng (the resulting file was 13% smaller).

The Code:

Can be found at:

* stb_image.h and stb_image_write.h from
* lodepng.c and lodepng.h from
* miniz.c from
* libpng from or your Linux distro or whatever (tested v1.2.50)

For Reference: The images

(click for full size)





And this wallpaper of parrots:


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