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Bug 3159 - SDL_blit_slow with BLENDING does not work
Summary: SDL_blit_slow with BLENDING does not work
Status: RESOLVED FIXED
Alias: None
Product: SDL
Classification: Unclassified
Component: video (show other bugs)
Version: 2.0.3
Hardware: x86_64 Linux
: P2 normal
Assignee: Sam Lantinga
QA Contact: Sam Lantinga
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2015-10-27 12:17 UTC by Fritzor
Modified: 2016-10-01 20:40 UTC (History)
2 users (show)

See Also:

Attachments
Fixes the alpha blending in SDL_blit_slow. (526 bytes, patch)
2016-06-11 19:34 UTC, Simon Hug 		Details | Diff
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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