png transparency fails for fully transparent image

[lvdlinden]

A fully transparent image is rendered as black (opaque).

When a single pixel is changed, the image is rendered correctly.

I've added a screenshot. The black square should be transparent.

I'm using wxGtk 2.8.0 on linux 2.6.20 (kubuntu)

[lvdlinden]

screenshot, black square should be transparent

[lvdlinden]

File Added: png-test.cpp

[lvdlinden]

minimal test application

[lvdlinden]

File Added: fully_transparent.png

[lvdlinden]

fully transparent png image (32x32 pixels)

[lvdlinden]

File Added: almost_transparent.png

[lvdlinden]

almost fully transparent png image (32x32 pixels)

[dickhamill]

Reproduced this issue under Windows and Linux. Can't reproduce under Mac.

Here's what's happening:

When your png file is loaded (common/imagpng.cpp), the CheckTransparency function is used to determine if your image will require an alpha channel (Transparency_Alpha), or if you can get away with just using a mask (Transparency_Mask).

If your image only has 0x00 and 0xFF for alpha values, an entire alpha channel isn't needed, because the draw function can use a simple mask to draw. So, in this case, it creates your image without an alpha channel, and rewrites the image data to turn any pixels with alpha=0x00 into the transparent color. It chooses a transparent color by scanning the image data and choosing the first unused RGB value it can find. It then sets the attribute on the bitmap to specify that it has a mask, and specifies the transparent color.

When you call
dc.DrawBitmap( bitmap1_, 4, 4 );
there's a default parameter of "false" for the fourth parameter, which is "useMask". Because your image only contains 0x00 for the alpha channel, it falls under the special case mentioned above - which is that it's going to try to draw it with a mask instead of a full alpha channel.
And, since the "useMask" defaults to false, it draws the image without the mask it created.

So, although not obvious, the solution is easy. Change your DrawBitmap line to this:
dc.DrawBitmap( bitmap1_, 4, 4, true );
and you'll force the bitmap to be drawn with transparency.

This fixed the issue under Windows and Linux.

[wojdyr]

dickhamill: thanks for this analysis.
The same problem was reported before as #3019, so I'm closing this ticket.