Why are some PNGs open in Photoshop with Indexed Colors and no transparency?

Posted by Daniel Beardsley on Super User See other posts from Super User or by Daniel Beardsley
Published on 2010-06-12T02:56:31Z Indexed on 2010/06/12 3:03 UTC
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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 left:png opened in Photoshop, right:screen grab of png from browser

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

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