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Fixed assembler command line for older yasm releases and documented nasm switches. Fixes Bugzilla #440.
author Ryan C. Gordon <icculus@icculus.org>
date Sat, 16 Jun 2007 05:51:51 +0000
parents e867f327aa54
children
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<HTML
><HEAD
><TITLE
>SDL_PixelFormat</TITLE
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><H1
><A
NAME="SDLPIXELFORMAT"
></A
>SDL_PixelFormat</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN3178"
></A
><H2
>Name</H2
>SDL_PixelFormat&nbsp;--&nbsp;Stores surface format information</DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN3181"
></A
><H2
>Structure Definition</H2
><PRE
CLASS="PROGRAMLISTING"
>typedef struct SDL_PixelFormat {
  SDL_Palette *palette;
  Uint8  BitsPerPixel;
  Uint8  BytesPerPixel;
  Uint8  Rloss, Gloss, Bloss, Aloss;
  Uint8  Rshift, Gshift, Bshift, Ashift;
  Uint32 Rmask, Gmask, Bmask, Amask;
  Uint32 colorkey;
  Uint8  alpha;
} SDL_PixelFormat;</PRE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN3184"
></A
><H2
>Structure Data</H2
><DIV
CLASS="INFORMALTABLE"
><A
NAME="AEN3186"
></A
><P
></P
><TABLE
BORDER="0"
CLASS="CALSTABLE"
><TBODY
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="STRUCTFIELD"
><I
>palette</I
></TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Pointer to the <A
HREF="sdlpalette.html"
>palette</A
>, or <TT
CLASS="LITERAL"
>NULL</TT
> if the <TT
CLASS="STRUCTFIELD"
><I
>BitsPerPixel</I
></TT
>&#62;8</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="STRUCTFIELD"
><I
>BitsPerPixel</I
></TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>The number of bits used to represent each pixel in a surface. Usually 8, 16, 24 or 32.</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="STRUCTFIELD"
><I
>BytesPerPixel</I
></TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>The number of bytes used to represent each pixel in a surface. Usually one to four.</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="STRUCTFIELD"
><I
>[RGBA]mask</I
></TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Binary mask used to retrieve individual color values</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="STRUCTFIELD"
><I
>[RGBA]loss</I
></TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Precision loss of each color component (2<SUP
>[RGBA]loss</SUP
>)</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="STRUCTFIELD"
><I
>[RGBA]shift</I
></TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Binary left shift of each color component in the pixel value</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="STRUCTFIELD"
><I
>colorkey</I
></TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Pixel value of transparent pixels</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="STRUCTFIELD"
><I
>alpha</I
></TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Overall surface alpha value</TD
></TR
></TBODY
></TABLE
><P
></P
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN3225"
></A
><H2
>Description</H2
><P
>A <SPAN
CLASS="STRUCTNAME"
>SDL_PixelFormat</SPAN
> describes the format of the pixel data stored at the <TT
CLASS="STRUCTFIELD"
><I
>pixels</I
></TT
> field of a <A
HREF="sdlsurface.html"
><SPAN
CLASS="STRUCTNAME"
>SDL_Surface</SPAN
></A
>. Every surface stores a <SPAN
CLASS="STRUCTNAME"
>SDL_PixelFormat</SPAN
> in the <TT
CLASS="STRUCTFIELD"
><I
>format</I
></TT
> field.</P
><P
>If you wish to do pixel level modifications on a surface, then understanding how SDL stores its color information is essential.</P
><P
>8-bit pixel formats are the easiest to understand. Since its an 8-bit format, we have 8 <TT
CLASS="STRUCTFIELD"
><I
>BitsPerPixel</I
></TT
> and 1 <TT
CLASS="STRUCTFIELD"
><I
>BytesPerPixel</I
></TT
>. Since <TT
CLASS="STRUCTFIELD"
><I
>BytesPerPixel</I
></TT
> is 1, all pixels are represented by a Uint8 which contains an index into <TT
CLASS="STRUCTFIELD"
><I
>palette</I
></TT
>-&#62;<TT
CLASS="STRUCTFIELD"
><I
>colors</I
></TT
>. So, to determine the color of a pixel in a 8-bit surface: we read the color index from <SPAN
CLASS="STRUCTNAME"
>surface</SPAN
>-&#62;<TT
CLASS="STRUCTFIELD"
><I
>pixels</I
></TT
> and we use that index to read the <A
HREF="sdlcolor.html"
><SPAN
CLASS="STRUCTNAME"
>SDL_Color</SPAN
></A
> structure from <SPAN
CLASS="STRUCTNAME"
>surface</SPAN
>-&#62;<TT
CLASS="STRUCTFIELD"
><I
>format</I
></TT
>-&#62;<TT
CLASS="STRUCTFIELD"
><I
>palette</I
></TT
>-&#62;<TT
CLASS="STRUCTFIELD"
><I
>colors</I
></TT
>. Like so:
<PRE
CLASS="PROGRAMLISTING"
>SDL_Surface *surface;
SDL_PixelFormat *fmt;
SDL_Color *color;
Uint8 index;

.
.

/* Create surface */
.
.
fmt=surface-&#62;format;

/* Check the bitdepth of the surface */
if(fmt-&#62;BitsPerPixel!=8){
  fprintf(stderr, "Not an 8-bit surface.\n");
  return(-1);
}

/* Lock the surface */
SDL_LockSurface(surface);

/* Get the topleft pixel */
index=*(Uint8 *)surface-&#62;pixels;
color=fmt-&#62;palette-&#62;colors[index];

/* Unlock the surface */
SDL_UnlockSurface(surface);
printf("Pixel Color-&#62; Red: %d, Green: %d, Blue: %d. Index: %d\n",
          color-&#62;r, color-&#62;g, color-&#62;b, index);
.
.</PRE
></P
><P
>Pixel formats above 8-bit are an entirely different experience. They are
considered to be "TrueColor" formats and the color information is stored in the
pixels themselves, not in a palette. The mask, shift and loss fields tell us
how the color information is encoded. The mask fields allow us to isolate each
color component, the shift fields tell us the number of bits to the right of
each component in the pixel value and the loss fields tell us the number of
bits lost from each component when packing 8-bit color component in a pixel.
<PRE
CLASS="PROGRAMLISTING"
>/* Extracting color components from a 32-bit color value */
SDL_PixelFormat *fmt;
SDL_Surface *surface;
Uint32 temp, pixel;
Uint8 red, green, blue, alpha;
.
.
.
fmt=surface-&#62;format;
SDL_LockSurface(surface);
pixel=*((Uint32*)surface-&#62;pixels);
SDL_UnlockSurface(surface);

/* Get Red component */
temp=pixel&#38;fmt-&#62;Rmask; /* Isolate red component */
temp=temp&#62;&#62;fmt-&#62;Rshift;/* Shift it down to 8-bit */
temp=temp&#60;&#60;fmt-&#62;Rloss; /* Expand to a full 8-bit number */
red=(Uint8)temp;

/* Get Green component */
temp=pixel&#38;fmt-&#62;Gmask; /* Isolate green component */
temp=temp&#62;&#62;fmt-&#62;Gshift;/* Shift it down to 8-bit */
temp=temp&#60;&#60;fmt-&#62;Gloss; /* Expand to a full 8-bit number */
green=(Uint8)temp;

/* Get Blue component */
temp=pixel&#38;fmt-&#62;Bmask; /* Isolate blue component */
temp=temp&#62;&#62;fmt-&#62;Bshift;/* Shift it down to 8-bit */
temp=temp&#60;&#60;fmt-&#62;Bloss; /* Expand to a full 8-bit number */
blue=(Uint8)temp;

/* Get Alpha component */
temp=pixel&#38;fmt-&#62;Amask; /* Isolate alpha component */
temp=temp&#62;&#62;fmt-&#62;Ashift;/* Shift it down to 8-bit */
temp=temp&#60;&#60;fmt-&#62;Aloss; /* Expand to a full 8-bit number */
alpha=(Uint8)temp;

printf("Pixel Color -&#62; R: %d,  G: %d,  B: %d,  A: %d\n", red, green, blue, alpha);
.
.
.</PRE
></P
></DIV
><DIV
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><A
NAME="AEN3252"
></A
><H2
>See Also</H2
><P
><A
HREF="sdlsurface.html"
><SPAN
CLASS="STRUCTNAME"
>SDL_Surface</SPAN
></A
>,
<A
HREF="sdlmaprgb.html"
><TT
CLASS="FUNCTION"
>SDL_MapRGB</TT
></A
></P
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