Best practice for PNG optimization?

Question

I 'd like to prepare my PNGs for the best optimization, so I can get the best image quality (lossless if possible) and the smallest size.

From what I understand, I should use: PNG, 72 dpi, RGB, but what else?

Here is what we find in the iPhone HIG:

Note:*The standard bit depth for icons and images is 24 bits (8 bits each for red, green, and blue), plus an 8-bit alpha channel. The PNG format is recommended, because it preserves color depth and supports an embedded alpha channel.

I guess this mean we should save the image as PNG 24 and create them in 8 bits mode? But I also read about 32 bits for best quality ?

The interlacing scheme (witch add to the file size) allows for the PNGs to display faster. Does this applies to the iPhone?

Thanks.

Answer 1

I suggest using ImageAlpha (lossy) on as many images as you can, because it greatly reduces their size.

Optimize all images with ImageOptim — it will remove all invisible junk and re-compress the data.

Disable Xcode conversion, because it undoes other optimisations and can make images much slower to load.

Answer 2

24 bit is red, green and blue with 8 bits each. 32 bits is RGB plus an 8-bit alpha channel. So if you need (semi-)transparent images, you should go for 32bit PNG, otherwise 24bit.

You don't have to compress/crush the PNGs yourself, Xcode's build steps will automatically use pngcrush and re-order the color channels for the iPhone's BGR memory alignment.