Why are PNG-8 files mangled when opened in Photoshop?

Question

Why are some 32 bit PNGs opened in Photoshop with Indexed Colors and no transparency?

For instance, I grabbed a png icon file of the Stack Overflow logo at: http://blog.stackoverflow.com/wp-content/uploads/icon-so.png When opening it in Photoshop CS3, it apparently treats it as indexed color and gets rid of the alpha channel. The image on the right is a screen grab of the icon. Changing the Image mode in Photoshop to RGB doesn't change the image at all. I've tried this with a few other PNGs and it seems hit or miss.

When viewed in other programs, it displays fine.

left:png opened in Photoshop, right:screen grab of png from browser

What gives?, does Photoshop not interpret the PNG file format correctly?

Answer 1

It looks like the problem is that Photoshop doesn't support part of the PNG-8 standard. PNG-8 like GIF uses a 256 color pallet. Both formats support using one of those 256 indexed colors to represent transparency, PNG-8 also supports setting an alpha-value for each of those 256 colors on the pallete. Photoshop (apparently CS3 and CS4) doesn't support this and renders every pixel opaque.

Answer 2

Simple solution (in Photoshop CC):

Image -> Mode -> Check "RGB Color" (Not Indexed Color)

The file will now be editable as usual.

Answer 3

There are two types of PNG files in common use. PNG8 uses indexed colors and has a single color for transparency, which means no alpha channel. PNG24 supports a separate alpha channel. The image you link to is a PNG8.

As for why Photoshop CS3 is mauling PNG8s so badly... that I don't know.