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Date: Tue, 12 Aug 2003 14:26:19 +0200 (MEST)
From: "Mattias Engdeg?rd"
Subject: bug in SDL_GetRGB/GetRGBA
There's an embarrassing bug in GetRGB/GetRGBA which apparently has been there
for years. It incorrectly converts colours with < 8 bits/channel.
It came to my attention today in #sdl.
What it does now is (for each channel):
rv = (pixel & fmt->Rmask) >> fmt->Rshift;
*r = (rv << fmt->Rloss) + (rv >> (8 - fmt->Rloss));
which is wrong; the last line should be
*r = (rv << fmt->Rloss) + (rv >> (8 - (fmt->Rloss << 1)));
author | Sam Lantinga <slouken@libsdl.org> |
---|---|
date | Tue, 12 Aug 2003 15:17:20 +0000 |
parents | e5bc29de3f0a |
children | 355632dca928 |
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<HTML ><HEAD ><TITLE >SDL_MapRGBA</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.64 "><LINK REL="HOME" TITLE="SDL Library Documentation" HREF="index.html"><LINK REL="UP" TITLE="Video" HREF="video.html"><LINK REL="PREVIOUS" TITLE="SDL_MapRGB" HREF="sdlmaprgb.html"><LINK REL="NEXT" TITLE="SDL_GetRGB" HREF="sdlgetrgb.html"></HEAD ><BODY CLASS="REFENTRY" BGCOLOR="#FFF8DC" TEXT="#000000" LINK="#0000ee" VLINK="#551a8b" ALINK="#ff0000" ><DIV CLASS="NAVHEADER" ><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="sdlmaprgb.html" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="sdlgetrgb.html" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><H1 ><A NAME="SDLMAPRGBA" >SDL_MapRGBA</A ></H1 ><DIV CLASS="REFNAMEDIV" ><A NAME="AEN1303" ></A ><H2 >Name</H2 >SDL_MapRGBA -- Map a RGBA color value to a pixel format.</DIV ><DIV CLASS="REFSYNOPSISDIV" ><A NAME="AEN1306" ></A ><H2 >Synopsis</H2 ><DIV CLASS="FUNCSYNOPSIS" ><A NAME="AEN1307" ></A ><P ></P ><PRE CLASS="FUNCSYNOPSISINFO" >#include "SDL.h"</PRE ><P ><CODE ><CODE CLASS="FUNCDEF" >Uint32 <B CLASS="FSFUNC" >SDL_MapRGBA</B ></CODE >(SDL_PixelFormat *fmt, Uint8 r, Uint8 g, Uint8 b, Uint8 a);</CODE ></P ><P ></P ></DIV ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN1313" ></A ><H2 >Description</H2 ><P >Maps the RGBA color value to the specified pixel format and returns the pixel value as a 32-bit int.</P ><P >If the format has a palette (8-bit) the index of the closest matching color in the palette will be returned.</P ><P >If the specified pixel format has no alpha component the alpha value will be ignored (as it will be in formats with a palette).</P ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN1318" ></A ><H2 >Return Value</H2 ><P >A pixel value best approximating the given RGBA color value for a given pixel format. If the pixel format bpp (color depth) is less than 32-bpp then the unused upper bits of the return value can safely be ignored (e.g., with a 16-bpp format the return value can be assigned to a <SPAN CLASS="TYPE" >Uint16</SPAN >, and similarly a <SPAN CLASS="TYPE" >Uint8</SPAN > for an 8-bpp format).</P ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN1323" ></A ><H2 >See Also</H2 ><P ><A HREF="sdlgetrgb.html" >SDL_GetRGB</A >, <A HREF="sdlgetrgba.html" >SDL_GetRGBA</A >, <A HREF="sdlmaprgb.html" >SDL_MapRGB</A >, <A HREF="sdlpixelformat.html" >SDL_PixelFormat</A ></P ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="sdlmaprgb.html" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="sdlgetrgb.html" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >SDL_MapRGB</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="video.html" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >SDL_GetRGB</TD ></TR ></TABLE ></DIV ></BODY ></HTML >