Created attachment 185063 [details]
Sample files.

Some PNG files optimized with OptiPNG (<http://optipng.sourceforge.net/>) are open empty in GIMP. I'll attach a sample file that always reproduces this bug:

1) Open sample.xcf. It contains a big symbol π with DejaVu Sans Mono and opacity of 4.7%.

2) Create a PNG with default settings. Result is sample-gimp.png, also attached to this report.

3) Optimize this PNG (result is also attached, see sample-optipng.png):


$ optipng -keep -o7 sample-gimp.png 
OptiPNG 0.6.4: Advanced PNG optimizer.
Copyright (C) 2001-2010 Cosmin Truta.

** Processing: sample-gimp.png
695x800 pixels, 4x8 bits/pixel, RGB+alpha
Reducing image to 4 bits/pixel, 14 colors (13 transparent) in palette
Input IDAT size = 4288 bytes
Input file size = 4436 bytes

Trying:
  zc = 9  zm = 9  zs = 0  f = 0		IDAT size = 2002
  zc = 9  zm = 8  zs = 0  f = 0		IDAT size = 2002
  zc = 8  zm = 9  zs = 0  f = 0		IDAT size = 1964
  zc = 8  zm = 9  zs = 1  f = 0		IDAT size = 1964
  zc = 9  zm = 9  zs = 0  f = 5		IDAT size = 1872
                               
Selecting parameters:
  zc = 9  zm = 9  zs = 0  f = 5		IDAT size = 1872

Output IDAT size = 1872 bytes (2416 bytes decrease)
Output file size = 2094 bytes (2342 bytes = 52.80% decrease)


4) Open sample-optipng.png in GIMP.

Actual result: GIMP opens it, but the file is empty, just a 695x800 transparent image.

Expected result: see the translucent symbol π.

More information: no problem to use sample-optipng.png with other software like Inkscape, Eye of GNOME, OpenOffice.org Draw, Firefox, etc.