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Bug 1638

Summary: Blit Alpha surface bug
Product: SDL Reporter: Sylvain <sylvain.becker>
Component: videoAssignee: Sam Lantinga <slouken>
Status: RESOLVED FIXED QA Contact: Sam Lantinga <slouken>
Severity: normal    
Priority: P2 Keywords: target-2.0.0
Version: HG 2.0   
Hardware: x86_64   
OS: Linux   
Attachments: example file

Description Sylvain 2012-11-09 23:32:05 UTC 
On linux + SDL-2.0 HG (6589)

When blitting an Alpha surface to an opaque surface, the output is not as expected !


It can be solved by switch to 32 bpp, or using an alpha of 128 instead of 192.


See the exemple in attachment.
Comment 1 Sylvain 2012-11-09 23:32:47 UTC 
Created attachment 986 [details]
example file
Comment 2 Ryan C. Gordon 2013-07-12 22:15:34 UTC 
(Sorry if you get a lot of copies of this email, we're touching dozens of bug reports right now.)

Tagging a bunch of bugs as target-2.0.0, Priority 2.

This means we're in the final stretch for an official SDL 2.0.0 release! These are the bugs we really want to fix before shipping if humanly possible.

That being said, we don't promise to fix them because of this tag, we just want to make sure we don't forget to deal with them before we bless a final 2.0.0 release, and generally be organized about what we're aiming to ship.

Hopefully you'll hear more about this bug soon. If you have more information (including "this got fixed at some point, nevermind"), we would love to have you come add more information to the bug report when you have a moment.

Thanks!
--ryan.
Comment 3 Sam Lantinga 2013-07-23 11:15:39 UTC 
Can you try the latest snapshot and see if that gives you the results you expect?
http://www.libsdl.org/tmp/SDL-2.0.zip

If it doesn't, can you describe the results that you expect and the results you get?

Thanks!
Comment 4 Sam Lantinga 2013-07-27 05:40:41 UTC 
I believe this is fixed by the alpha blend formula change in this commit:
http://hg.libsdl.org/SDL/rev/6ff02ff3cf06

Please reopen the bug if that's not the case.

Thanks!
Comment 5 Sylvain 2013-07-29 15:40:23 UTC 
Hi,

I reopen the issue as it seems not to be fixed with the current latest version of today.

in the attached file, I constructed to surface : surf1 and surf2.
surf2 is black+alpha, and is blitted several time to surf1. It's done in a *disjoint* way so that surf1 is fully colored by surf2.

It appears black diagonals. like if some line pitch was never taken into account.

Sylvain
Comment 6 Sam Lantinga 2013-08-01 04:30:56 UTC 
Fixed, thanks for the great test case!
http://hg.libsdl.org/SDL/rev/5287c82340e3

Also, the reason you needed this:
surf2 = SDL_ConvertSurface(surf2, surf2->format, surf2->flags);
was because as a side effect it set up the surface for alpha blending.

You can add this line instead:
SDL_SetSurfaceBlendMode(surf2, SDL_BLENDMODE_BLEND);

Cheers!