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?
Best Answer
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.