This bug report was migrated from our old Bugzilla tracker.

Reported in version: 1.2.9
Reported for operating system, platform: Linux, x86

Comments on the original bug report:

On 2006-01-23 04:32:53 +0000, Davide Coppola wrote:

I have tried to blit a PNG with an alpha channel on a transparent surface created using format data from the image, the result of SDL_BlitSurface() is just nothing!

I have solved my problem using memcpy() if the image has the same dimension of the surface and using getpixel()/putpixel() if the image is smaller than the surface (or if I need a partial blit).

this source shows the problem:
http://mars.sourceforge.net/SDL/trans_bug/trans_test1.c
this are my solutions:
http://mars.sourceforge.net/SDL/trans_bug/trans_test2.c
and this is the package with all the sources and data:
http://mars.sourceforge.net/SDL/trans_bug.tar.gz

I hope this could help you.

On 2006-01-27 11:23:21 +0000, Ryan C. Gordon wrote:

Setting Sam as "QA Contact" on all bugs (even resolved ones) so he'll definitely be in the loop to any further discussion here about SDL.

--ryan.

On 2006-03-20 02:40:09 +0000, Sam Lantinga wrote:

This is not technically a bug.

According to the documentation:

• RGBA->RGBA:

• SDL_SRCALPHA set:
  

•  alpha-blend (using the source alpha channel) the RGB values;
  

•  leave destination alpha untouched. [Note: is this correct?]
  

•  SDL_SRCCOLORKEY ignored.
  

• SDL_SRCALPHA not set:
  

•  copy all of RGBA to the destination.
  

SDL_DisplayFormatAlpha() turns on SDL_SRCALPHA by default (to enable blending)

What I did to fix this in your test program was just add SDL_SetAlpha(img, 0, 0) before the blit onto window. Of course a manual memcpy works just as well. :)

Thanks for the great test case!

On 2006-03-20 04:20:11 +0000, Davide Coppola wrote:

(In reply to comment # 2)

I confirm that your solution works well, and I'm glad of this ;)

Just a note for someone interested:
if you want to blit img on screen after you have set SDL_SetAlpha(img, 0, 0), you have to set SDL_SetAlpha(img, SDL_SRCALPHA, 0).

Best regards.