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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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you can open this png in pixlr.com/editor and save, resulting png can be opened in photoshop –  kreker Aug 14 '13 at 23:22

4 Answers 4

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.

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Photoshop CS4 is no better either, though Fireworks CS4 opens it perfectly. No clue what gives. –  Darth Android Jun 12 '10 at 4:01
    
I guess it's good to know I'm not just doing something wrong :-) –  Daniel Beardsley Jun 12 '10 at 7:29
up vote 7 down vote accepted

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.

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Indeed, see sitepoint.com/blogs/2007/09/18/png8-the-clear-winner: “I can certainly confirm that none of PhotoShop, Gimp, Paintshop Pro, or Xara has this functionality [PNG8 alpha transparency] built-in.” –  Marcel Korpel Aug 14 '10 at 11:56
    
Updated URL for that article: PNG8 – The Clear Winner –  jeffbyrnes Mar 18 '13 at 14:55

SuperPNG is able to correctly read this file.

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Simple solution (in Photoshop CC):

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

The file will now be editable as usual.

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