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Up Topic Gosu / Gosu Exchange / Incorrectly handled image alpha channel
- - By Jamer Date 2011-11-10 07:00
Hello everyone, :)

I've noticed that when I open a specific PNG image up in Gimp, it appears to have a transparent background.  However, when I open that same image up in Gosu, it has an opaque blue background.  Can somebody help me understand why this happens and how I might avoid it?

I'm running Debian Linux and the latest Gosu from git.

Thanks in advance,

Image file in question:
Screenshot of image in Gimp:
Screenshot of image in Gosu:
Parent - - By disciplex Date 2011-11-10 10:24
I just ran a test with Ruby 1.9.3 and Gosu 0.7.38 using the code below and it worked fine.

image =, 'forest.png', false)
image.draw(0, 0, 1)

Maybe some code would help?
Parent - By Jamer Date 2011-11-10 10:50
I'm using C++, but I would expect the Ruby gem to perform the same. The following code invokes the bug:

#include <Gosu/Gosu.hpp>

class GameWindow : public Gosu::Window
    Gosu::Image image;

    :   Window(640, 480, false),
        image(graphics(), L"forest.png")

    void update()

    void draw()
        image.draw(0, 0, 0);

int main()
    GameWindow window;;
Parent - - By jlnr (dev) Date 2011-11-10 10:48
Oh, it's a 8-bit (palette) PNG. It sounds as if FreeImage has the same bug that Gosu had when it used libpng directly. Maybe you can check if there has been anything in the FreeImage changelog recently.

24-bit PNGs should be safe until then. :)
Parent - - By jlnr (dev) Date 2011-11-10 10:49
I was curious and checked myself:

Version 3.11.0 last mentions palette images and transparency. What version of FreeImage do you use?
Parent - - By Jamer Date 2011-11-10 10:54
Ooh, I'm using the packaged 3.10.0 in Debian Testing. Thanks for the response, I'll investigate newer versions. In the mean time, I'll be sure to convert to 24-bit PNGs. :)
Parent - - By jlnr (dev) Date 2011-11-10 13:46
Good grief, the fix is from July 28th, 2008. :D
Parent - - By erisdiscord Date 2011-11-10 15:04
See what I mean about Linux distributions dragging their asses? I barely expect Ruby 1.9 to become the standard version across distributions by 2013. C:
Parent - By RavensKrag Date 2011-11-16 07:17
It'll become the standard version by the time we're ready to all use 2.0 :D
Parent - By Jamer Date 2011-11-10 10:59
Thanks, jlnr!
Parent - - By Jamer Date 2011-11-11 02:20
Unfortunately, I cannot get even the latest version of FreeImage to process it correctly.  Maybe they just don't handle transparency in indexed PNGs?
Parent - - By jlnr (dev) Date 2011-11-11 05:18 Edited 2011-11-11 05:31
Right, reproduced it on the latest Ubuntu. It actually contains transparency handling code, see this diff:

The code has been cleaned up in the meantime (look for png_get_tRNS):

This is the FreeImage function they use, not sure if you have to activate it with a flag or something?
(see code snippet on following page)

The crappy docs of libpng don't say much either:

It looks like FreeImage is doing everything exactly as the very last version of Gosu's custom PNG loader was doing it, and that one was fixed to work with GIMP's files:

Can you easily pry into FreeImage with gdb?
Parent - - By Jamer Date 2011-11-20 02:36
I'm not sure if it's a bug in their code, but they don't reference the transparency table when just calling FreeImage_ConvertToRawBits.  Here's a patch that calls FreeImage_ConvertTo32Bits first, which acknowledges the transparency table.  It requires a version of FreeImage that contains the proper transparency handling code as you've identified above.
Parent - By jlnr (dev) Date 2011-11-20 10:16
Thanks for the analysis, I merged the commit. I guess the world were too easy if libraries just had a Give_Me_The_R8G8B8A8_Buffer_Of_This_Image(const char*) function. :)
Up Topic Gosu / Gosu Exchange / Incorrectly handled image alpha channel

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