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Bug 2878

Summary: SDL_LoadBMP masks alpha channel
Product: SDL Reporter: Keith Campbell <keith>
Component: videoAssignee: Ryan C. Gordon <icculus>
Status: RESOLVED FIXED QA Contact: Sam Lantinga <slouken>
Severity: normal    
Priority: P2 CC: icculus
Version: 2.0.3Keywords: triage-2.0.4
Hardware: x86_64   
OS: Mac OS X 10.8   
Attachments: Reproduction program.
Reproduction BMP image with alpha channel.

Description Keith Campbell 2015-02-12 02:05:57 UTC 
Created attachment 2030 [details]
Reproduction program.

This is on Mac OS 10.10.2

The problem is exemplified through the attached reproduction program, which reports the pixel format for the SDL_Surface created by SDL_LoadBMP when loading the attached bitmap file. The expected result is a nonzero alpha mask, instead here's what I observer:

> clang bug.c -framework SDL2 -o bug
> ./bug bug.bmp
format: 0x16161804
palette: 0x0
BitsPerPixel: 32
BytesPerPixel: 4
Rmask: 0x00ff0000
Gmask: 0x0000ff00
Bmask: 0x000000ff
Amask: 0x00000000

A known workaround (which works for me at least) is to insert the following code after the surface is created:
> surface->format->format = SDL_PIXELFORMAT_ARGB8888;
This seems to trick SDL into doing proper alpha blending with the surface as the alpha bits are already there.
Comment 1 Keith Campbell 2015-02-12 02:07:36 UTC 
Created attachment 2031 [details]
Reproduction BMP image with alpha channel.
Comment 2 Keith Campbell 2015-02-12 02:13:37 UTC 
FYI: The attached image was created with ImageMagick
Comment 3 Ryan C. Gordon 2015-02-19 06:32:14 UTC 
Marking a large number of bugs with the "triage-2.0.4" keyword at once. Sorry
if you got a lot of email from this. This is to help me sort through some bugs
in regards to a 2.0.4 release. We may or may not fix this bug for 2.0.4,
though!
Comment 4 Ryan C. Gordon 2015-03-24 00:02:44 UTC 
We fixed this recently, outside of this bug report:

[icculus@caridad ~]$ ./bug bug.bmp 
format: 0x16362004
palette: 0x0
BitsPerPixel: 32
BytesPerPixel: 4
Rmask: 0x00ff0000
Gmask: 0x0000ff00
Bmask: 0x000000ff
Amask: 0xff000000

So you should be good to go in 2.0.4.

--ryan.