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Date: Mon, 11 Oct 2004 15:17:27 +0300 (EEST) From: Hannu Savolainen Subject: Re: SDL uses obsolete OSS features I did some work on getting OSS to work better with SDL. There have been some problems with select which should be fixed now. I'm having some problems in understanding what is the purpose of the DSP_WaitAudio() routine. I added a return to the very beginning of this routine and commendted out the define for USE_BLOCKING_WRITES. At least lbreakout2 seems to work as well as earlier. The latencies are the same. An ordinary blocking write does exactly the same thing than DSP_WaitAudio does. So I would recommend using the USE_BLOCKING_WRITES approach and removing everything from the DSP_WaitAudio routine. Also enabling USE_BLOCKING_WRITES makes it possible to simplify DSP_PlayAudio() because you don't need to handle the partial writes (the do-while loop). Attached is a patch against SDL-1.2.7. After these changes SDL will use OSS as it's designed to be used (make it as simple as possible). This code should work with all OSS implementations because it uses only the very fundamental features that have been there since the jurassic times.
author Sam Lantinga <slouken@libsdl.org>
date Fri, 12 Nov 2004 21:39:04 +0000
parents 355632dca928
children
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<HTML
><HEAD
><TITLE
>SDL_SetAlpha</TITLE
><META
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><HR
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><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
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