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evilbite 2012-02-19 09:38:21 PST There is only one Altivec accelerated blit function (ConvertAltivec32to32_prefetch() or ConvertAltivec32to32_noprefetch(), depending on the CPU used) that is supposed to handle all alpha combinations. This works as follows for every pixel line: 1. Blit single pixels until an aligned address is reached 2. Accelerated blit as far as possible 3. Blit single remaining pixels Part 2. is set up correctly to handle different combinations of the alpha channels of the participating surfaces. Parts 1. and 3. only do a simple copy of all the pixel's components from souce to destination. But when the source surface has no alpha channel (Amask is 0, e.g. the video surface) the surface's alpha value must be used instead. Otherwise crap (uninitialized data) is being copied to the destiniation's alpha channel. The attached patch is a quick'n'dirty solution to the problem. A more sophisticated solution might require separate functions for different combinations of the alpha channels of the participating surfaces.
Simple DirectMedia Layer
(SDL)
Version 2.0
---
http://www.libsdl.org/
This is the Simple DirectMedia Layer, a general API that provides low
level access to audio, keyboard, mouse, joystick, 3D hardware via OpenGL,
and 2D framebuffer across multiple platforms.
The current version supports Windows, Windows CE, Mac OS X, Linux, FreeBSD,
NetBSD, OpenBSD, BSD/OS, Solaris, iOS, and Android. The code contains
support for other operating systems but those are not officially supported.
SDL is written in C, but works with C++ natively, and has bindings to
several other languages, including Ada, C#, Eiffel, Erlang, Euphoria,
Go, Guile, Haskell, Java, Lisp, Lua, ML, Objective C, Pascal, Perl, PHP,
Pike, Pliant, Python, Ruby, and Smalltalk.
This library is distributed under the zlib license, which can be found
in the file "COPYING".
The best way to learn how to use SDL is to check out the header files in
the "include" subdirectory and the programs in the "test" subdirectory.
The header files and test programs are well commented and always up to date.
More documentation is available in HTML format in "docs/index.html", and
a documentation wiki is available online at:
http://www.libsdl.org/cgi/docwiki.cgi
The test programs in the "test" subdirectory are in the public domain.
Frequently asked questions are answered online:
http://www.libsdl.org/faq.php
If you need help with the library, or just want to discuss SDL related
issues, you can join the developers mailing list:
http://www.libsdl.org/mailing-list.php
Enjoy!
Sam Lantinga (slouken@libsdl.org)