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Here's a real example, using a test image that has a lot of different alpha values in it, and a 24-bit alpha channel: Original: 87047 bytes optipng

by beej71 16y ago

Here's a real example, using a test image that has a lot of different alpha values in it, and a 24-bit alpha channel: Original: 87047 bytes optipng -o7: 82489 bytes this tool: 22585 bytes The main difference, other than size, is that optipng produced an exact copy of the the original, while this tool is lossy, and threw away a bunch of data from the alpha channel. The win is that for the majority of all web cases, 8 bits of alpha is plenty.

ck2 16y ago
Note that it's not perfect for all cases, but it's certainly acceptable for small icons. Here's a 32x32 icon I converted down to 8-bit alpha and then 1-bit, and then resized the whole group so they could be more closely examined: http://i.imgur.com/XTksy.png http://i.imgur.com/XTksy.png
- pornel 16y ago
  I was able to convert it without noticeable degradation down to 7 bit: http://geekhood.net/bitcoin-32bit.png http://geekhood.net/bitcoin-32bit.png http://geekhood.net/bitcoin-7bit.png http://geekhood.net/bitcoin-7bit.png (using my pngquant)
  - ck2 16y ago
    Yah, after looking at the source I saw they are only using php's gd2 library on 8bitalpha.com which is definitely inferior to your work. The problem is lack of Windows binaries, this is the most modern I can find: http://sites.google.com/site/bitdepthtools/downloads http://sites.google.com/site/bitdepthtools/downloads BTW this is the original image I am resampling down to icon size, it seems very challenging because of the multi-layered round edges and many gradients internally: http://en.wikipedia.org/wiki/File:Bitcoin.png http://en.wikipedia.org/wiki/File:Bitcoin.png
    - pornel 16y ago
      For me OS X screenshots are the toughest — lots of shades in transparent shadow, smooth gray gradients (can't use posterization) AND three small gradients in close/minimize/zoom and couple of odd pixels for anti-alias of corners (can't use pick-most-popular algorithm).
pornel 16y ago
Actually, making good palette for PNG8+a is very difficult, because in PNG8 alpha is not an additional channel, but property of palette entries. Instead of 256 RGB colors in palette you have 265 RGBA colors, so e.g. red and semitransparent red are two distinct entries.

by beej71 16y ago

ck2 16y ago

Note that it's not perfect for all cases, but it's certainly acceptable for small icons. Here's a 32x32 icon I converted down to 8-bit alpha and then 1-bit, and then resized the whole group so they could be more closely examined: http://i.imgur.com/XTksy.png http://i.imgur.com/XTksy.png

pornel 16y ago
I was able to convert it without noticeable degradation down to 7 bit: http://geekhood.net/bitcoin-32bit.png http://geekhood.net/bitcoin-32bit.png http://geekhood.net/bitcoin-7bit.png http://geekhood.net/bitcoin-7bit.png (using my pngquant)
- ck2 16y ago
  Yah, after looking at the source I saw they are only using php's gd2 library on 8bitalpha.com which is definitely inferior to your work. The problem is lack of Windows binaries, this is the most modern I can find: http://sites.google.com/site/bitdepthtools/downloads http://sites.google.com/site/bitdepthtools/downloads BTW this is the original image I am resampling down to icon size, it seems very challenging because of the multi-layered round edges and many gradients internally: http://en.wikipedia.org/wiki/File:Bitcoin.png http://en.wikipedia.org/wiki/File:Bitcoin.png
  - pornel 16y ago
    For me OS X screenshots are the toughest — lots of shades in transparent shadow, smooth gray gradients (can't use posterization) AND three small gradients in close/minimize/zoom and couple of odd pixels for anti-alias of corners (can't use pick-most-popular algorithm).

pornel 16y ago

Actually, making good palette for PNG8+a is very difficult, because in PNG8 alpha is not an additional channel, but property of palette entries. Instead of 256 RGB colors in palette you have 265 RGBA colors, so e.g. red and semitransparent red are two distinct entries.