第一章 基础
初始化
SDL由八个子系统组成——音频、CDROM、事件处理、文件I/O、游戏杆、线程、记时器和视频。使用前必须调用SDL_Init或 SDL_InitSubSystem初始化。SDL_Init必须早于其他所有SDL调用，它将自动初始化事件处理、文件I/O和线程子系统，并根据参数 选择启动其他子系统。例如，初始化缺省和视频子系统：
SDL_Init(SDL_INIT_VIDEO);初始化缺省、视频和记时器子系统：
SDL_Init(SDL_INIT_VIDEO | SDL_INIT_TIMER);SDL_Init对应SDL_Quit（和SDL_QuitSubSystem）。SDL_Quit关闭所有子系统，必须在程序关闭前调用。
除此之外，我们还必须进行错误处理。很多SDL函数返回一个值指示成功与否。例如SDL_Init失败时返回-1。每当SDL出错时，错误信息被保存，并可用SDL_GetError取得。
例1-1 初始化SDL
#include "SDL.h" /* All SDL App's need this */
#include <stdio.h>
int main() {
printf("Initializing SDL.\n");
/* Initialize defaults, Video and Audio */
if((SDL_Init(SDL_INIT_VIDEO|SDL_INIT_AUDIO)==-1)) {
printf("Could not initialize SDL: %s.\n", SDL_GetError());
exit(-1);
}
printf("SDL initialized.\n");
printf("Quiting SDL.\n");
/* Shutdown all subsystems */
SDL_Quit();
printf("Quiting....\n");
exit(0);
}
第二章 图像和视频
SDL Vidow显示
初始化SDL Video显示
视频是最常用的部分，也是SDL最完整的子系统。下面的初始化过程是每个SDL程序都要做的，即使可能有些不同。
例2-1 初始化视频
SDL_Surface *screen;
/* Initialize the SDL library */
if( SDL_Init(SDL_INIT_VIDEO) < 0 ) {
fprintf(stderr,
"Couldn't initialize SDL: %s\n", SDL_GetError());
exit(1);
}
/* Clean up on exit */
atexit(SDL_Quit);
/*
* Initialize the display in a 640x480 8-bit palettized mode,
* requesting a software surface
*/
screen = SDL_SetVideoMode(640, 480, 8, SDL_SWSURFACE);
if ( screen == NULL ) {
fprintf(stderr, "Couldn't set 640x480x8 video mode: %s\n",
SDL_GetError());
exit(1);
}
初始化最佳视频模式
如果你希望某种色深（颜色数）但如果用户的显示器不支持也可以接受其他色深，使用加SDL_ANYFORMAT参数的SDL_SetVideoMode。您还可以用SDL_VideoModeOK来找到与请求模式最接近的模式。
例2-2 初始化最佳视频模式
/* Have a preference for 8-bit, but accept any depth */
screen = SDL_SetVideoMode(640, 480, 8, SDL_SWSURFACE|SDL_ANYFORMAT);
if ( screen == NULL ) {
fprintf(stderr, "Couldn't set 640x480x8 video mode: %s\n",
SDL_GetError());
exit(1);
}
printf("Set 640x480 at %d bits-per-pixel mode\n",
screen->format->BitsPerPixel);
读取并显示BMP文件
当SDL已经初始化，视频模式已经选择，下面的函数将读取并显示指定的BMP文件。
例2-3 读取并显示BMP文件
void display_bmp(char *file_name)
{
SDL_Surface *image;
/* Load the BMP file into a surface */
image = SDL_LoadBMP(file_name);
if (image == NULL) {
fprintf(stderr, "Couldn't load %s: %s\n", file_name, SDL_GetError());
return;
}
/*
* Palettized screen modes will have a default palette (a standard
* 8*8*4 colour cube), but if the image is palettized as well we can
* use that palette for a nicer colour matching
*/
if (image->format->palette && screen->format->palette) {
SDL_SetColors(screen, image->format->palette->colors, 0,
image->format->palette->ncolors);
}
/* Blit onto the screen surface */
if(SDL_BlitSurface(image, NULL, screen, NULL) < 0)
fprintf(stderr, "BlitSurface error: %s\n", SDL_GetError());
SDL_UpdateRect(screen, 0, 0, image->w, image->h);
/* Free the allocated BMP surface */
SDL_FreeSurface(image);
}
直接在显示上绘图
下面两个函数实现在图像平面的像素读写。它们被细心设计成可以用于所有色深。记住在使用前要先锁定图像平面，之后要解锁。
在像素值和其红、绿、蓝值间转换，使用SDL_GetRGB()和SDL_MapRGB()。
例2-4 getpixel()
/*
* Return the pixel value at (x, y)
* NOTE: The surface must be locked before calling this!
*/
Uint32 getpixel(SDL_Surface *surface, int x, int y)
{
int bpp = surface->format->BytesPerPixel;
/* Here p is the address to the pixel we want to retrieve */
Uint8 *p = (Uint8 *)surface->pixels + y * surface->pitch + x * bpp;
switch(bpp) {
case 1:
return *p;
case 2:
return *(Uint16 *)p;
case 3:
if(SDL_BYTEORDER == SDL_BIG_ENDIAN)
return p[0] << 16 | p[1] << 8 | p[2];
else
return p[0] | p[1] << 8 | p[2] << 16;
case 4:
return *(Uint32 *)p;
default:
return 0; /* shouldn't happen, but avoids warnings */
}
}
例2-5 putpixel()
/*
* Set the pixel at (x, y) to the given value
* NOTE: The surface must be locked before calling this!
*/
void putpixel(SDL_Surface *surface, int x, int y, Uint32 pixel)
{
int bpp = surface->format->BytesPerPixel;
/* Here p is the address to the pixel we want to set */
Uint8 *p = (Uint8 *)surface->pixels + y * surface->pitch + x * bpp;
switch(bpp) {
case 1:
*p = pixel;
break;
case 2:
*(Uint16 *)p = pixel;
break;
case 3:
if(SDL_BYTEORDER == SDL_BIG_ENDIAN) {
p[0] = (pixel >> 16) & 0xff;
p[1] = (pixel >> 8) & 0xff;
p[2] = pixel & 0xff;
} else {
p[0] = pixel & 0xff;
p[1] = (pixel >> 8) & 0xff;
p[2] = (pixel >> 16) & 0xff;
}
break;
case 4:
*(Uint32 *)p = pixel;
break;
}
}
例2-6 使用上面的putpixel()在屏幕中心画一个黄点
/* Code to set a yellow pixel at the center of the screen */
int x, y;
Uint32 yellow;
/* Map the color yellow to this display (R=0xff, G=0xFF, B=0x00)
Note: If the display is palettized, you must set the palette first.
*/
yellow = SDL_MapRGB(screen->format, 0xff, 0xff, 0x00);
x = screen->w / 2;
y = screen->h / 2;
/* Lock the screen for direct access to the pixels */
if ( SDL_MUSTLOCK(screen) ) {
if ( SDL_LockSurface(screen) < 0 ) {
fprintf(stderr, "Can't lock screen: %s\n", SDL_GetError());
return;
}
}
putpixel(screen, x, y, yellow);
if ( SDL_MUSTLOCK(screen) ) {
SDL_UnlockSurface(screen);
}
/* Update just the part of the display that we've changed */
SDL_UpdateRect(screen, x, y, 1, 1);
return;
并用SDL和OpenGL
SDL可以在多种平台（Linux/X11, Win32, BeOS, MacOS Classic/Toolbox, MacOS X, FreeBSD/X11 and Solaris/X11）上创建和使用OpenGL上下文。这允许你在OpenGL程序中使用SDL的音频、事件、线程和记时器，而这些通常是GLUT的任务。
初始化
和普通的初始化类似，但有三点不同：必须传SDL_OPENGL参数给SDL_SetVideoMode；必须使用SDL_GL_SetAttribute指定一些GL属性（深度缓冲区位宽，帧缓冲位宽等）；如果您想使用双缓冲，必须作为GL属性指定
例2-7 初始化SDL加OpenGL
/* Information about the current video settings. */
const SDL_VideoInfo* info = NULL;
/* Dimensions of our window. */
int width = 0;
int height = 0;
/* Color depth in bits of our window. */
int bpp = 0;
/* Flags we will pass into SDL_SetVideoMode. */
int flags = 0;
/* First, initialize SDL's video subsystem. */
if( SDL_Init( SDL_INIT_VIDEO ) < 0 ) {
/* Failed, exit. */
fprintf( stderr, "Video initialization failed: %s\n",
SDL_GetError( ) );
quit_tutorial( 1 );
}
/* Let's get some video information. */
info = SDL_GetVideoInfo( );
if( !info ) {
/* This should probably never happen. */
fprintf( stderr, "Video query failed: %s\n",
SDL_GetError( ) );
quit_tutorial( 1 );
}
/*
* Set our width/height to 640/480 (you would
* of course let the user decide this in a normal
* app). We get the bpp we will request from
* the display. On X11, VidMode can't change
* resolution, so this is probably being overly
* safe. Under Win32, ChangeDisplaySettings
* can change the bpp.
*/
width = 640;
height = 480;
bpp = info->vfmt->BitsPerPixel;
/*
* Now, we want to setup our requested
* window attributes for our OpenGL window.
* We want *at least* 5 bits of red, green
* and blue. We also want at least a 16-bit
* depth buffer.
*
* The last thing we do is request a double
* buffered window. '1' turns on double
* buffering, '0' turns it off.
*
* Note that we do not use SDL_DOUBLEBUF in
* the flags to SDL_SetVideoMode. That does
* not affect the GL attribute state, only
* the standard 2D blitting setup.
*/
SDL_GL_SetAttribute( SDL_GL_RED_SIZE, 5 );
SDL_GL_SetAttribute( SDL_GL_GREEN_SIZE, 5 );
SDL_GL_SetAttribute( SDL_GL_BLUE_SIZE, 5 );
SDL_GL_SetAttribute( SDL_GL_DEPTH_SIZE, 16 );
SDL_GL_SetAttribute( SDL_GL_DOUBLEBUFFER, 1 );
/*
* We want to request that SDL provide us
* with an OpenGL window, in a fullscreen
* video mode.
*
* EXERCISE:
* Make starting windowed an option, and
* handle the resize events properly with
* glViewport.
*/
flags = SDL_OPENGL | SDL_FULLSCREEN;
/*
* Set the video mode
*/
if( SDL_SetVideoMode( width, height, bpp, flags ) == 0 ) {
/*
* This could happen for a variety of reasons,
* including DISPLAY not being set, the specified
* resolution not being available, etc.
*/
fprintf( stderr, "Video mode set failed: %s\n",
SDL_GetError( ) );
quit_tutorial( 1 );
}