Mercurial > sdl-ios-xcode

view docs/html/sdlsetalpha.html @ 3069:caefe2344f65

Find changesets by keywords (author, files, the commit message), revision number or hash, or revset expression.
Date: Thu, 27 Dec 2007 07:38:25 +0000 From: John Bartholomew Subject: [SDL] SDL Semaphore implementation broken on Windows? Hi, Over the past couple of days, I've been battling with SDL, SDL_Mixer and SMPEG to try to find an audio hang bug. I believe I've found the problem, which I think is a race condition inside SDL's semaphore implementation (at least the Windows implementation). The semaphore code uses Windows' built in semaphore functions, but it also maintains a separate count value. This count value is updated with bare increment and decrement operations in SemPost and SemWaitTimeout - no locking primitives to protect them. In tracking down the apparent audio bug, I found that at some point a semaphore's count value was being decremented to -1, which is clearly not a valid value for it to take. I'm still not certain exactly what sequence of operations is occuring for this to happen, but I believe that overall it's a race condition between a thread calling SemPost (which increments the count) and the thread on the other end calling SemWait (which decrements it). I will try to make a test case to verify this, but I'm not sure if I'll be able to (threading errors being difficult to reproduce even in the best circumstances). However, assuming this is the cause of my problems, there is a very simple fix: Windows provides InterlockedIncrement() and InterlockedDecrement() functions to perform increments and decrements which are guaranteed to be atomic. So the fix is in thread/win32/SDL_syssem.c: replace occurrences of --sem->count with InterlockedDecrement(&sem->count); and replace occurrences of ++sem->count with InterlockedIncrement(&sem->count); This is using SDL v1.2.12, built with VC++ 2008 Express, running on a Core 2 duo processor.
author Sam Lantinga <slouken@libsdl.org>
date Tue, 17 Feb 2009 05:39:18 +0000
parents 355632dca928
children
line wrap: on
line source

<HTML
><HEAD
><TITLE
>SDL_SetAlpha</TITLE
><META
NAME="GENERATOR"
CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+
"><LINK
REL="HOME"
TITLE="SDL Library Documentation"
HREF="index.html"><LINK
REL="UP"
TITLE="Video"
HREF="video.html"><LINK
REL="PREVIOUS"
TITLE="SDL_SetColorKey"
HREF="sdlsetcolorkey.html"><LINK
REL="NEXT"
TITLE="SDL_SetClipRect"
HREF="sdlsetcliprect.html"></HEAD
><BODY
CLASS="REFENTRY"
BGCOLOR="#FFF8DC"
TEXT="#000000"
LINK="#0000ee"
VLINK="#551a8b"
ALINK="#ff0000"
><DIV
CLASS="NAVHEADER"
><TABLE
SUMMARY="Header navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TH
COLSPAN="3"
ALIGN="center"
>SDL Library Documentation</TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="bottom"
><A
HREF="sdlsetcolorkey.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="80%"
ALIGN="center"
VALIGN="bottom"
></TD
><TD
WIDTH="10%"
ALIGN="right"
VALIGN="bottom"
><A
HREF="sdlsetcliprect.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><H1
><A
NAME="SDLSETALPHA"
></A
>SDL_SetAlpha</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN2096"
></A
><H2
>Name</H2
>SDL_SetAlpha&nbsp;--&nbsp;Adjust the alpha properties of a surface</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN2099"
></A
><H2
>Synopsis</H2
><DIV
CLASS="FUNCSYNOPSIS"
><A
NAME="AEN2100"
></A
><P
></P
><PRE
CLASS="FUNCSYNOPSISINFO"
>#include "SDL.h"</PRE
><P
><CODE
><CODE
CLASS="FUNCDEF"
>int <B
CLASS="FSFUNC"
>SDL_SetAlpha</B
></CODE
>(SDL_Surface *surface, Uint32 flag, Uint8 alpha);</CODE
></P
><P
></P
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN2106"
></A
><H2
>Description</H2
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>This function and the semantics of SDL alpha blending have changed since version 1.1.4. Up until version 1.1.5, an alpha value of 0 was considered opaque and a value of 255 was considered transparent. This has now been inverted: 0 (<TT
CLASS="LITERAL"
>SDL_ALPHA_TRANSPARENT</TT
>) is now considered transparent and 255 (<TT
CLASS="LITERAL"
>SDL_ALPHA_OPAQUE</TT
>) is now considered opaque.</P
></BLOCKQUOTE
></DIV
><P
><TT
CLASS="FUNCTION"
>SDL_SetAlpha</TT
> is used for setting the per-surface alpha
value and/or enabling and disabling alpha blending.</P
><P
>The<TT
CLASS="PARAMETER"
><I
>surface</I
></TT
> parameter specifies which surface whose alpha
attributes you wish to adjust. <TT
CLASS="PARAMETER"
><I
>flags</I
></TT
> is used to specify
whether alpha blending should be used (<TT
CLASS="LITERAL"
>SDL_SRCALPHA</TT
>) and
whether the surface should use RLE acceleration for blitting
(<TT
CLASS="LITERAL"
>SDL_RLEACCEL</TT
>). <TT
CLASS="PARAMETER"
><I
>flags</I
></TT
> can be an OR'd
combination of these two options, one of these options or 0. If
<TT
CLASS="LITERAL"
>SDL_SRCALPHA</TT
> is not passed as a flag then all alpha
information is ignored when blitting the surface. The
<TT
CLASS="PARAMETER"
><I
>alpha</I
></TT
> parameter is the per-surface alpha value; a
surface need not have an alpha channel to use per-surface alpha and blitting
can still be accelerated with <TT
CLASS="LITERAL"
>SDL_RLEACCEL</TT
>.</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>The per-surface alpha value of 128 is considered a special case and
is optimised, so it's much faster than other per-surface values.</P
></BLOCKQUOTE
></DIV
><P
>Alpha effects surface blitting in the following ways:</P
><DIV
CLASS="INFORMALTABLE"
><A
NAME="AEN2126"
></A
><P
></P
><TABLE
BORDER="0"
CLASS="CALSTABLE"
><TBODY
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>RGBA-&#62;RGB with <TT
CLASS="LITERAL"
>SDL_SRCALPHA</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The source is alpha-blended with the destination, using the alpha channel. <TT
CLASS="LITERAL"
>SDL_SRCCOLORKEY</TT
> and the per-surface alpha are ignored.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>RGBA-&#62;RGB without <TT
CLASS="LITERAL"
>SDL_SRCALPHA</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The RGB data is copied from the source. The source alpha channel and the per-surface alpha value are ignored.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>RGB-&#62;RGBA with <TT
CLASS="LITERAL"
>SDL_SRCALPHA</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The source is alpha-blended with the destination using the per-surface alpha
value. If <TT
CLASS="LITERAL"
>SDL_SRCCOLORKEY</TT
> is set, only the pixels not
matching the colorkey value are copied. The alpha channel of the copied pixels
is set to opaque.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>RGB-&#62;RGBA without <TT
CLASS="LITERAL"
>SDL_SRCALPHA</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The RGB data is copied from the source and the alpha value of the copied pixels
is set to opaque. If <TT
CLASS="LITERAL"
>SDL_SRCCOLORKEY</TT
> is set, only the pixels
not matching the colorkey value are copied. </P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>RGBA-&#62;RGBA with <TT
CLASS="LITERAL"
>SDL_SRCALPHA</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The source is alpha-blended with the destination using the source alpha
channel. The alpha channel in the destination surface is left untouched.
<TT
CLASS="LITERAL"
>SDL_SRCCOLORKEY</TT
> is ignored.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>RGBA-&#62;RGBA without <TT
CLASS="LITERAL"
>SDL_SRCALPHA</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The RGBA data is copied to the destination surface. If <TT
CLASS="LITERAL"
>SDL_SRCCOLORKEY</TT
> is set, only the pixels not matching the colorkey value are copied.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>RGB-&#62;RGB with <TT
CLASS="LITERAL"
>SDL_SRCALPHA</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The source is alpha-blended with the destination using the per-surface alpha value. If <TT
CLASS="LITERAL"
>SDL_SRCCOLORKEY</TT
> is set, only the pixels not matching the colorkey value are copied.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>RGB-&#62;RGB without <TT
CLASS="LITERAL"
>SDL_SRCALPHA</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The RGB data is copied from the source. If <TT
CLASS="LITERAL"
>SDL_SRCCOLORKEY</TT
> is set, only the pixels not matching the colorkey value are copied.</P
></TD
></TR
></TBODY
></TABLE
><P
></P
></DIV
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> Note that RGBA-&#62;RGBA blits (with SDL_SRCALPHA set) keep the alpha
of the destination surface. This means that you cannot compose two arbitrary
RGBA surfaces this way and get the result you would expect from "overlaying"
them; the destination alpha will work as a mask.</P
><P
>Also note that per-pixel and per-surface alpha cannot be combined;
the per-pixel alpha is always used if available</P
></BLOCKQUOTE
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN2179"
></A
><H2
>Return Value</H2
><P
>This function returns <SPAN
CLASS="RETURNVALUE"
>0</SPAN
>, or
<SPAN
CLASS="RETURNVALUE"
>-1</SPAN
> if there was an error.</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN2184"
></A
><H2
>See Also</H2
><P
><A
HREF="sdlmaprgba.html"
><TT
CLASS="FUNCTION"
>SDL_MapRGBA</TT
></A
>,
<A
HREF="sdlgetrgba.html"
><TT
CLASS="FUNCTION"
>SDL_GetRGBA</TT
></A
>,
<A
HREF="sdldisplayformatalpha.html"
><TT
CLASS="FUNCTION"
>SDL_DisplayFormatAlpha</TT
></A
>,
<A
HREF="sdlblitsurface.html"
><TT
CLASS="FUNCTION"
>SDL_BlitSurface</TT
></A
></P
></DIV
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="sdlsetcolorkey.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="index.html"
ACCESSKEY="H"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="sdlsetcliprect.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>SDL_SetColorKey</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="video.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>SDL_SetClipRect</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>