A best way to compress a png atlas with transparency

Question

I have an atlas of assets in a transparent png with 8-bit alpha (660kb) and I want to compress it.

Could you suggest what options do I have? When using a pngquant (I'm using SuperPNG plugin for Photoshop), I'm getting a 58kb file but color is transformed to 8bit palette and there's not enough colors to have smooth gradients (balloons are a good example). The original atlas I'm working with has two times more assets so compression is even more visible...

Is it possible to get something like in-between result? I'd be happy with 150-200kb file with more colors and alpha. Maybe I can use imagemagick with some funny options?

Answer 1

pngquant has option to set quality. From pngquant man page

-Q min-max, --quality min-max
              min and max are numbers in range 0 (worst) to 100 (perfect), similar to JPEG.  pngquant will
              use the least amount of colors required to meet or exceed the max quality. If conversion re-
              sults in quality below the min quality the image won't be saved (or if outputting to stdin,
              24-bit original will be output) and pngquant will exit with status code 99

Here is two example with -Q 0 and -Q 100. The file was originally 232 K, used pngcrush to reduce to 224 K (no noticeable improvement).

Lowest quality

pngquant --speed 1 -f --quality 0 --strip pngout.png 8 K

Highest quality

pngquant --speed 1 -f --quality 100 --strip pngout.png 52 K

Also I listed size of all possible output from 0 to 100

for i in {0..100}; do pngquant --speed 1 -f --quality $i --strip pngout.png ; echo "Quality $i " >> pngq.txt ; du -b pngout-fs8.png  >> pngq.txt ; done
 

--strip option remove some metadata.

Size in Byte

Quality 0 
5644    pngout-fs8.png
Quality 1 
33327   pngout-fs8.png
Quality 2 
41913   pngout-fs8.png
Quality 3 
47641   pngout-fs8.png
Quality 4 
50614   pngout-fs8.png
Quality 5 
51664   pngout-fs8.png
Quality 6 
51771   pngout-fs8.png
Quality 7 
51807   pngout-fs8.png
Quality 8 
51816   pngout-fs8.png
Quality 9 
51797   pngout-fs8.png
Quality 10 
51739   pngout-fs8.png
Quality 11 
51704   pngout-fs8.png
Quality 12 
51729   pngout-fs8.png

Rest all are 5129, so 12 is the maximum level achived. Using optipng, -Q 100 was reduced to 51431 Byte (0.58% reduced)

Further reading ImageMagick: Lossless max compression for PNG? at Stackoverflow.