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492 lines
7.7 KiB
HTML
492 lines
7.7 KiB
HTML
<HTML
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><HEAD
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><TITLE
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>SDL_SetAlpha</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.64
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TITLE="SDL_SetColorKey"
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><TR
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><TH
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COLSPAN="3"
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ALIGN="center"
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>SDL Library Documentation</TH
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><TR
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><TD
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WIDTH="10%"
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><H1
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><A
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NAME="SDLSETALPHA"
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>SDL_SetAlpha</A
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></H1
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><DIV
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CLASS="REFNAMEDIV"
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><A
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NAME="AEN1681"
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></A
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><H2
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>Name</H2
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>SDL_SetAlpha -- Adjust the alpha properties of a surface</DIV
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><DIV
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CLASS="REFSYNOPSISDIV"
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><A
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NAME="AEN1684"
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></A
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><H2
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>Synopsis</H2
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><DIV
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CLASS="FUNCSYNOPSIS"
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><A
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NAME="AEN1685"
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></A
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><P
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></P
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><PRE
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CLASS="FUNCSYNOPSISINFO"
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>#include "SDL.h"</PRE
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>int <B
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CLASS="FSFUNC"
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>SDL_SetAlpha</B
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></CODE
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>(SDL_Surface *surface, Uint32 flag, Uint8 alpha);</CODE
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></P
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><P
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></P
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></DIV
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="AEN1691"
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></A
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><H2
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>Description</H2
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><DIV
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CLASS="NOTE"
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><BLOCKQUOTE
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CLASS="NOTE"
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><P
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><B
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>Note: </B
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>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
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CLASS="LITERAL"
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>SDL_ALPHA_TRANSPARENT</TT
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>) is now considered transparent and 255 (<TT
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CLASS="LITERAL"
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>SDL_ALPHA_OPAQUE</TT
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>) is now considered opaque.</P
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></BLOCKQUOTE
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></DIV
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><P
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><TT
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CLASS="FUNCTION"
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>SDL_SetAlpha</TT
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> is used for setting the per-surface alpha
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value and/or enabling and disabling alpha blending.</P
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><P
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>The<TT
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CLASS="PARAMETER"
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><I
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>surface</I
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></TT
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> parameter specifies which surface whose alpha
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attributes you wish to adjust. <TT
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CLASS="PARAMETER"
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><I
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>flags</I
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></TT
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> is used to specify
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whether alpha blending should be used (<TT
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CLASS="LITERAL"
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>SDL_SRCALPHA</TT
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>) and
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whether the surface should use RLE acceleration for blitting
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(<TT
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CLASS="LITERAL"
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>SDL_RLEACCEL</TT
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>). <TT
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CLASS="PARAMETER"
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><I
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>flags</I
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></TT
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> can be an OR'd
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combination of these two options, one of these options or 0. If
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<TT
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CLASS="LITERAL"
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>SDL_SRCALPHA</TT
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> is not passed as a flag then all alpha
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information is ignored when blitting the surface. The
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<TT
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CLASS="PARAMETER"
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><I
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>alpha</I
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></TT
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> parameter is the per-surface alpha value; a
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surface need not have an alpha channel to use per-surface alpha and blitting
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can still be accelerated with <TT
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CLASS="LITERAL"
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>SDL_RLEACCEL</TT
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>.</P
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><DIV
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CLASS="NOTE"
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><BLOCKQUOTE
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CLASS="NOTE"
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><P
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><B
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>Note: </B
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>The per-surface alpha value of 128 is considered a special case and
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is optimised, so it's much faster than other per-surface values.</P
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></BLOCKQUOTE
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></DIV
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><P
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>Alpha effects surface blitting in the following ways:</P
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><DIV
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CLASS="INFORMALTABLE"
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><A
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NAME="AEN1711"
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></A
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><P
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></P
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><TABLE
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BORDER="0"
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CLASS="CALSTABLE"
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><TBODY
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>RGBA->RGB with <TT
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CLASS="LITERAL"
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>SDL_SRCALPHA</TT
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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><P
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>The source is alpha-blended with the destination, using the alpha channel. <TT
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CLASS="LITERAL"
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>SDL_SRCCOLORKEY</TT
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> and the per-surface alpha are ignored.</P
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></TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>RGBA->RGB without <TT
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CLASS="LITERAL"
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>SDL_SRCALPHA</TT
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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><P
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>The RGB data is copied from the source. The source alpha channel and the per-surface alpha value are ignored.</P
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></TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>RGB->RGBA with <TT
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CLASS="LITERAL"
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>SDL_SRCALPHA</TT
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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><P
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>The source is alpha-blended with the destination using the per-surface alpha
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value. If <TT
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CLASS="LITERAL"
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>SDL_SRCCOLORKEY</TT
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> is set, only the pixels not
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matching the colorkey value are copied. The alpha channel of the copied pixels
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is set to opaque.</P
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></TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>RGB->RGBA without <TT
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CLASS="LITERAL"
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>SDL_SRCALPHA</TT
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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><P
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>The RGB data is copied from the source and the alpha value of the copied pixels
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is set to opaque. If <TT
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CLASS="LITERAL"
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>SDL_SRCCOLORKEY</TT
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> is set, only the pixels
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not matching the colorkey value are copied. </P
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></TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>RGBA->RGBA with <TT
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CLASS="LITERAL"
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>SDL_SRCALPHA</TT
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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><P
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>The source is alpha-blended with the destination using the source alpha
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channel. The alpha channel in the destination surface is left untouched.
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<TT
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CLASS="LITERAL"
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>SDL_SRCCOLORKEY</TT
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> is ignored.</P
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></TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>RGBA->RGBA without <TT
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CLASS="LITERAL"
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>SDL_SRCALPHA</TT
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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><P
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>The RGBA data is copied to the destination surface. If <TT
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CLASS="LITERAL"
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>SDL_SRCCOLORKEY</TT
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> is set, only the pixels not matching the colorkey value are copied.</P
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></TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>RGB->RGB with <TT
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CLASS="LITERAL"
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>SDL_SRCALPHA</TT
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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><P
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>The source is alpha-blended with the destination using the per-surface alpha value. If <TT
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CLASS="LITERAL"
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>SDL_SRCCOLORKEY</TT
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> is set, only the pixels not matching the colorkey value are copied.</P
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></TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>RGB->RGB without <TT
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CLASS="LITERAL"
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>SDL_SRCALPHA</TT
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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><P
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>The RGB data is copied from the source. If <TT
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CLASS="LITERAL"
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>SDL_SRCCOLORKEY</TT
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> is set, only the pixels not matching the colorkey value are copied.</P
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></TD
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></TR
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></TBODY
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></TABLE
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><P
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></P
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></DIV
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><DIV
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CLASS="NOTE"
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><BLOCKQUOTE
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CLASS="NOTE"
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><P
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><B
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>Note: </B
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> Note that RGBA->RGBA blits (with SDL_SRCALPHA set) keep the alpha
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of the destination surface. This means that you cannot compose two arbitrary
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RGBA surfaces this way and get the result you would expect from "overlaying"
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them; the destination alpha will work as a mask.</P
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><P
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>Also note that per-pixel and per-surface alpha cannot be combined;
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the per-pixel alpha is always used if available</P
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></BLOCKQUOTE
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></DIV
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="AEN1764"
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></A
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><H2
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>Return Value</H2
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><P
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>This function returns <SPAN
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CLASS="RETURNVALUE"
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>0</SPAN
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>, or
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<SPAN
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CLASS="RETURNVALUE"
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>-1</SPAN
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> if there was an error.</P
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="AEN1769"
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></A
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><H2
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>See Also</H2
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><P
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><A
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HREF="sdlmaprgba.html"
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><TT
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CLASS="FUNCTION"
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>SDL_MapRGBA</TT
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></A
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>,
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<A
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HREF="sdlgetrgba.html"
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><TT
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CLASS="FUNCTION"
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>SDL_GetRGBA</TT
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></A
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>,
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<A
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HREF="sdldisplayformatalpha.html"
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><TT
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CLASS="FUNCTION"
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>SDL_DisplayFormatAlpha</TT
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></A
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>,
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<A
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HREF="sdlblitsurface.html"
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><TT
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CLASS="FUNCTION"
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>SDL_BlitSurface</TT
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></A
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></P
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