Question

So I asked this question earlier:

CSS box shadow not truly transparent?

And I realize that I do not really know what a .png file is. .bmps are just uncompressed bitmaps, .jpgs are bitmaps compressed using the special jpeg algorithm, and I thought .pngs were just bitmaps compressed losslessly using some special png algorithm.

However, it turns out .pngs can be indexed colors like gifs (still losslessly?) and Adobe Fireworks can make special “Fireworks PNG”s which are editable, letting the user drag and drop stuff around the image a.l.a. MS Word document but still allowing them to be readable by “standard” image processing stuff (browsers, paint.net, etc.) as a normal .png.

What gives? Clearly there is way more to the .png format than just losslessly compressed bitmaps.

Answer 1

Standard PNGs don’t support editing. Simplifying it a bit, they are just what you said they are: losslessly compressed bitmaps (vs JPGs, which employ lossy compression, or GIFs which are also bitmaps, but only support up to a 256 color palette).

Fireworks PNGs contain a special header and extra data that allows them to retain vector and layer information. But they are not standard PNGs.

Adobe’s reference page:

Fireworks PNG files contain a second “chunk” of data that other applications can’t read, which contains proprietary information about things like slicing, interactivity, and any Live Effects that may have been applied.

And finally, to address this:

What gives? Clearly there is way more to the .png format than just losslessly compressed bitmaps.

Yes, there is more to PNG than the standard. Extensions have been added to standard PNG that allow for animation, for example.