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Bug 3159

Summary: SDL_blit_slow with BLENDING does not work
Product: SDL Reporter: Fritzor <fhattendorf>
Component: videoAssignee: Sam Lantinga <slouken>
Status: RESOLVED FIXED QA Contact: Sam Lantinga <slouken>
Severity: normal    
Priority: P2 CC: chli.hug, philipp.wiesemann
Version: 2.0.3   
Hardware: x86_64   
OS: Linux   
Attachments: Fixes the alpha blending in SDL_blit_slow.

Description Fritzor 2015-10-27 12:17:25 UTC 
Source Suface is ABGR and Destination Surface is ABGR. We use software blending. In the Switch-Case statement for SDL_COPY_BLEND (Line 126) the alpha-channel is not calculated like in every SDL_blit_auto - function. So if the destination Surface has alpha - channel of zero the resulting surface has zero as well. 
Add:  “dstA = srcA + ((255 - srcA) * dstA) / 255;” to code and everything is okay.
Comment 1 Fritzor 2015-10-27 12:18:21 UTC 
I forget: The code in the SDL_blit_slow.c file is meant.
Comment 2 Simon Hug 2016-06-11 19:34:25 UTC 
Created attachment 2490 [details]
Fixes the alpha blending in SDL_blit_slow.

I stumbled over the same thing. Here's a patch.
Comment 3 Sam Lantinga 2016-10-01 20:40:18 UTC 
Fixed, thanks!
https://hg.libsdl.org/SDL/rev/eeb3847d81a6