序言

关于SDL

SDL为方便制作能跨跃Linux、BSD、MacOS、Win32和BeOS平台，使用本地高性能媒体接口，并且让您可以只需使用一份源代码级API而设计。SDL是相当低层的API，但使用它可以让你以极大的灵活性写出完全跨平台的程序。

 SDL:http://www.libsdl.org/

关于SDL文档

SDLdoc项目即要重新编写SDL文档并同步更新。项目组有使用SDL的志愿者组成。

最新版本可在http://sdldoc.sourceforge.net下载

谢启

Sam Lantinga, slouken@libsdl.org

Martin Donlon, akawaka@skynet.ie

Mattias Engdeg?rd

Julian Peterson

Ken Jordan

Maxim Sobolev

Wesley Poole

Michael Vance

Andreas Umbach

Andreas Hofmeister

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

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。

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 );
    }

OpenGL绘图

除了初始化，在SDL程序中使用OpenGL和其他情况基本相同，是同样函数和数据类型。但是如果您使用双缓冲，则必须用SDL_GL_SwapBuffers()来交换前后缓冲，而不是glxSwapBuffers()（GLX）或SwapBuffers()（Windows）。

例2-8 SDL和OpenGL

/*
 * SDL OpenGL Tutorial.
 * (c) Michael Vance, 2000
 * briareos@lokigames.com
 *
 * Distributed under terms of the LGPL. 
 */

#include <SDL/SDL.h>
#include <GL/gl.h>
#include <GL/glu.h>

#include <stdio.h>
#include <stdlib.h>

static GLboolean should_rotate = GL_TRUE;

static void quit_tutorial( int code )
{
    /*
     * Quit SDL so we can release the fullscreen
     * mode and restore the previous video settings,
     * etc.
     */
    SDL_Quit( );

    /* Exit program. */
    exit( code );
}

static void handle_key_down( SDL_keysym* keysym )
{

    /* 
     * We‘re only interested if ‘Esc‘ has
     * been presssed.
     *
     * EXERCISE: 
     * Handle the arrow keys and have that change the
     * viewing position/angle.
     */
    switch( keysym->sym ) {
    case SDLK_ESCAPE:
        quit_tutorial( 0 );
        break;
    case SDLK_SPACE:
        should_rotate = !should_rotate;
        break;
    default:
        break;
    }

}

static void process_events( void )
{
    /* Our SDL event placeholder. */
    SDL_Event event;

    /* Grab all the events off the queue. */
    while( SDL_PollEvent( &event ) ) {

        switch( event.type ) {
        case SDL_KEYDOWN:
            /* Handle key presses. */
            handle_key_down( &event.key.keysym );
            break;
        case SDL_QUIT:
            /* Handle quit requests (like Ctrl-c). */
            quit_tutorial( 0 );
            break;
        }

    }

}

static void draw_screen( void )
{
    /* Our angle of rotation. */
    static float angle = 0.0f;

    /*
     * EXERCISE:
     * Replace this awful mess with vertex
     * arrays and a call to glDrawElements.
     *
     * EXERCISE:
     * After completing the above, change
     * it to use compiled vertex arrays.
     *
     * EXERCISE:
     * Verify my windings are correct here ;).
     */
    static GLfloat v0[] = { -1.0f, -1.0f,  1.0f };
    static GLfloat v1[] = {  1.0f, -1.0f,  1.0f };
    static GLfloat v2[] = {  1.0f,  1.0f,  1.0f };
    static GLfloat v3[] = { -1.0f,  1.0f,  1.0f };
    static GLfloat v4[] = { -1.0f, -1.0f, -1.0f };
    static GLfloat v5[] = {  1.0f, -1.0f, -1.0f };
    static GLfloat v6[] = {  1.0f,  1.0f, -1.0f };
    static GLfloat v7[] = { -1.0f,  1.0f, -1.0f };
    static GLubyte red[]    = { 255,   0,   0, 255 };
    static GLubyte green[]  = {   0, 255,   0, 255 };
    static GLubyte blue[]   = {   0,   0, 255, 255 };
    static GLubyte white[]  = { 255, 255, 255, 255 };
    static GLubyte yellow[] = {   0, 255, 255, 255 };
    static GLubyte black[]  = {   0,   0,   0, 255 };
    static GLubyte orange[] = { 255, 255,   0, 255 };
    static GLubyte purple[] = { 255,   0, 255,   0 };

    /* Clear the color and depth buffers. */
    glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );

    /* We don‘t want to modify the projection matrix. */
    glMatrixMode( GL_MODELVIEW );
    glLoadIdentity( );

    /* Move down the z-axis. */
    glTranslatef( 0.0, 0.0, -5.0 );

    /* Rotate. */
    glRotatef( angle, 0.0, 1.0, 0.0 );

    if( should_rotate ) {

        if( ++angle > 360.0f ) {
            angle = 0.0f;
        }

    }

    /* Send our triangle data to the pipeline. */
    glBegin( GL_TRIANGLES );

    glColor4ubv( red );
    glVertex3fv( v0 );
    glColor4ubv( green );
    glVertex3fv( v1 );
    glColor4ubv( blue );
    glVertex3fv( v2 );

    glColor4ubv( red );
    glVertex3fv( v0 );
    glColor4ubv( blue );
    glVertex3fv( v2 );
    glColor4ubv( white );
    glVertex3fv( v3 );

    glColor4ubv( green );
    glVertex3fv( v1 );
    glColor4ubv( black );
    glVertex3fv( v5 );
    glColor4ubv( orange );
    glVertex3fv( v6 );

    glColor4ubv( green );
    glVertex3fv( v1 );
    glColor4ubv( orange );
    glVertex3fv( v6 );
    glColor4ubv( blue );
    glVertex3fv( v2 );

    glColor4ubv( black );
    glVertex3fv( v5 );
    glColor4ubv( yellow );
    glVertex3fv( v4 );
    glColor4ubv( purple );
    glVertex3fv( v7 );

    glColor4ubv( black );
    glVertex3fv( v5 );
    glColor4ubv( purple );
    glVertex3fv( v7 );
    glColor4ubv( orange );
    glVertex3fv( v6 );

    glColor4ubv( yellow );
    glVertex3fv( v4 );
    glColor4ubv( red );
    glVertex3fv( v0 );
    glColor4ubv( white );
    glVertex3fv( v3 );

    glColor4ubv( yellow );
    glVertex3fv( v4 );
    glColor4ubv( white );
    glVertex3fv( v3 );
    glColor4ubv( purple );
    glVertex3fv( v7 );

    glColor4ubv( white );
    glVertex3fv( v3 );
    glColor4ubv( blue );
    glVertex3fv( v2 );
    glColor4ubv( orange );
    glVertex3fv( v6 );

    glColor4ubv( white );
    glVertex3fv( v3 );
    glColor4ubv( orange );
    glVertex3fv( v6 );
    glColor4ubv( purple );
    glVertex3fv( v7 );

    glColor4ubv( green );
    glVertex3fv( v1 );
    glColor4ubv( red );
    glVertex3fv( v0 );
    glColor4ubv( yellow );
    glVertex3fv( v4 );

    glColor4ubv( green );
    glVertex3fv( v1 );
    glColor4ubv( yellow );
    glVertex3fv( v4 );
    glColor4ubv( black );
    glVertex3fv( v5 );

    glEnd( );

    /*
     * EXERCISE:
     * Draw text telling the user that ‘Spc‘
     * pauses the rotation and ‘Esc‘ quits.
     * Do it using vetors and textured quads.
     */

    /*
     * Swap the buffers. This this tells the driver to
     * render the next frame from the contents of the
     * back-buffer, and to set all rendering operations
     * to occur on what was the front-buffer.
     *
     * Double buffering prevents nasty visual tearing
     * from the application drawing on areas of the
     * screen that are being updated at the same time.
     */
    SDL_GL_SwapBuffers( );
}

static void setup_opengl( int width, int height )
{
    float ratio = (float) width / (float) height;

    /* Our shading model--Gouraud (smooth). */
    glShadeModel( GL_SMOOTH );

    /* Culling. */
    glCullFace( GL_BACK );
    glFrontFace( GL_CCW );
    glEnable( GL_CULL_FACE );

    /* Set the clear color. */
    glClearColor( 0, 0, 0, 0 );

    /* Setup our viewport. */
    glViewport( 0, 0, width, height );

    /*
     * Change to the projection matrix and set
     * our viewing volume.
     */
    glMatrixMode( GL_PROJECTION );
    glLoadIdentity( );
    /*
     * EXERCISE:
     * Replace this with a call to glFrustum.
     */
    gluPerspective( 60.0, ratio, 1.0, 1024.0 );
}

int main( int argc, char* argv[] )
{
    /* 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 );
    }

    /*
     * At this point, we should have a properly setup
     * double-buffered window for use with OpenGL.
     */
    setup_opengl( width, height );

    /*
     * Now we want to begin our normal app process--
     * an event loop with a lot of redrawing.
     */
    while( 1 ) {
        /* Process incoming events. */
        process_events( );
        /* Draw the screen. */
        draw_screen( );
    }

    /*
     * EXERCISE:
     * Record timings using SDL_GetTicks() and
     * and print out frames per second at program
     * end.
     */

    /* Never reached. */
    return 0;
}

第三章上 游戏杆输入处理

初始化

使用游戏杆的第一步是初始化游戏杆子系统。即在SDL_Init时使用参数SDL_INIT_JOYSTICK。

例3-1 初始化SDL并带游戏杆支持

    if ( ! SDL_Init( SDL_INIT_VIDEO | SDL_INIT_JOYSTICK ) )
    {
        fprintf(stderr, "Couldn‘t initialize SDL: %s/n", SDL_GetError());
        exit(1);
    }

此例启动SDL并带视频和游戏杆支持。

查询

至此，我们可以假定初始化以完成。但我们还需指导有没有、有几个游戏杆。即使您指导有一个游戏杆可用，也最好经常检查，因为这样可以帮助检测游戏杆被拔掉的情况。

检查游戏杆的函数是SDL_NumJoysticks()。此函数简单的返回系统中游戏杆的数量。下一步是决定用户想用哪一个。当然，如果只有一个就不用决定了。SDL_JoystickName取得系统赋给游戏杆的名字。游戏杆用序号指定，第一个为0，最后一个为SDL_NumJoysticks - 1。

例3-2 打印所有游戏杆的名字

    printf("%i joysticks were found./n/n", SDL_NumJoysticks() );
    printf("The names of the joysticks are:/n");
		
    for( i=0; i < SDL_NumJoysticks(); i++ ) 
    {
        printf("    %s/n", SDL_JoystickName(i));
    }

启动游戏杆取得事件

SDL使用事件架构，游戏杆可以触发四种事件。

• SDL_JoyAxisEvent 轴改变
• SDL_JoyBallEvent 轨迹球坐标改变
• SDL_JoyHatEvent hat（sorry，在下不解何为hat）方向改变
• SDL_JoyButtonEvent 按钮按下或释放

所有启动的游戏杆都会触发事件。为了收到事件，首先要用SDL_ENABLE调用SDL_JoystickEventState，以允许事件。其次要启动指定的游戏杆，用SDL_JoystickOpen()。

例3-3 启动第一个游戏杆

    SDL_Joystick *joystick;

    SDL_JoystickEventState(SDL_ENABLE);
    joystick = SDL_JoystickOpen(0);

游戏杆对象的指针只在查询和关闭游戏杆时有用。

现在为了响应事件，我们需要一个消息循环。所有的SDL程序至少要接受系统退出消息。设想我们的消息循环象这样：

    SDL_Event event;
    /* Other initializtion code goes here */   

    /* Start main game loop here */

    while(SDL_PollEvent(&event))
    {  
        switch(event.type)
        {  
            case SDL_KEYDOWN:
            /* handle keyboard stuff here */				
            break;

            case SDL_QUIT:
            /* Set whatever flags are necessary to */
            /* end the main game loop here */
            break;
        }
    }

    /* End loop here */

要响应游戏杆事件，只需添加一个case。轴检测得有些技巧，因为大部分事件是垃圾。摇杆动一点就会有事件。所以必须设定一个阈值，并且忽略未达到阈值的事件。一般10%是个比较好的阈值。

例3-4 游戏杆轴事件

    case SDL_JOYAXISMOTION:  /* Handle Joystick Motion */
    if ( ( event.jaxis.value < -3200 ) || (event.jaxis.value > 3200 ) ) 
    {
      /* code goes here */
    }
    break;

另一个技巧是上下和左右是两组不同的运动。最重要的轴是轴0（左右）和轴1（上下）。按如下方法作不同处理：

例3-5

    case SDL_JOYAXISMOTION:  /* Handle Joystick Motion */
    if ( ( event.jaxis.value < -3200 ) || (event.jaxis.value > 3200 ) ) 
    {
        if( event.jaxis.axis == 0) 
        {
            /* Left-right movement code goes here */
        }

        if( event.jaxis.axis == 1) 
        {
            /* Up-Down movement code goes here */
        }
    }
    break;

理想情况下，应该用event.jaxis.value来调整一些值。例如你在用游戏杆控制飞船，将摇杆推一点则慢速前进，推很多则快速前进。这样设计会使用户的体验更好。如果你的游戏杆有更多的轴，可用于其他控制，用法完全一样，event.jaxis.axis会有不同的值。

例3-6 游戏杆按钮事件

    case SDL_JOYBUTTONDOWN:  /* Handle Joystick Button Presses */
    if ( event.jbutton.button == 0 ) 
    {
        /* code goes here */
    }
    break;

按钮检测很简单，因为只有按下和放开两个状态。按下时SDL_JOYBUTTONDOWN触发，放开时SDL_JOYBUTTONUP触发。event.jbutton.button指示是哪个按钮。

最后，程序结束时用SDL_JoystickClose()关闭游戏杆。例如关闭前面启动的0号游戏杆：

SDL_JoystickClose(joystick);

高级游戏杆函数

轨迹球消息包含X和Y方向的改变量。

例3-7 轨迹球事件

    case SDL_JOYBALLMOTION:  /* Handle Joyball Motion */
    if( event.jball.ball == 0 )
    {
      /* ball handling */
    }
    break;

此例检测第一个轨迹球。坐标改变量在event.jball.xrel和event.jball.yrel中。

最后是hat事件。hat只报告方向。我们通过位掩码检测：

SDL_HAT_CENTERED 
SDL_HAT_UP 
SDL_HAT_RIGHT 
SDL_HAT_DOWN 
SDL_HAT_LEFT 

预定义的组合：

SDL_HAT_RIGHTUP 
SDL_HAT_RIGHTDOWN 
SDL_HAT_LEFTUP 
SDL_HAT_LEFTDOWN 

例3-8 游戏杆hat事件

    case SDL_JOYHATMOTION:  /* Handle Hat Motion */
    if ( event.jhat.hat | SDL_HAT_UP )
    {
        /* Do up stuff here */
    }

    if ( event.jhat.hat | SDL_HAT_LEFT )
    {
        /* Do left stuff here */
    }

    if ( event.jhat.hat | SDL_HAT_RIGHTDOWN )
    {
        /* Do right and down together stuff here */
    }
    break;

除了游戏杆的数量，还可查询：

• SDL_JoystickNumAxes 轴数量
• SDL_JoystickNumButtons 按钮数量
• SDL_JoystickNumBalls 轨迹球数量
• SDL_JoystickNumHats hat数量

只需将启动游戏杆时得到的指针传给这些函数即可。

例3-9 查询游戏杆特性

int number_of_buttons;
    SDL_Joystick *joystick;

    joystick = SDL_JoystickOpen(0);
    number_of_buttons = SDL_JoystickNumButtons(joystick);

第三章下 键盘输入

键盘相关数据结构

• SDLKey 枚举类型，每一个符号代表一个键，如SDLK_a、SDLK_SPACE，在SDL_keysym.h中定义。
• SDLMod 枚举类型，类似SDLKey，但用于修饰键，如Control、Alt、Shift，可以组合。
• SDL_keysym

  typedef struct{
    Uint8 scancode;
    SDLKey sym;
    SDLMod mod;
    Uint16 unicode;
  } SDL_keysym;
  

  用于按键和放开消息。scancode是硬件相关键码。除非你有特殊用途，否则忽略。sym指示键，mod为修饰键集合，例如KMOD_NUM | KMOD_CAPS | KMOD_LSHIFT 为Numlock、Capslock和左Shift组合。最后unicode为对应字符。注意，仅当是按键消息时unicode才有效，放开消息时无效。Unicode变换必须用SDL_EnableUNICODE()打开。
• SDL_KeyboardEvent

  typedef struct{
    Uint8 type;
    Uint8 state;
    SDL_keysym keysym;
  } SDL_KeyboardEvent;
  

  描述一个键盘消息。type指示按键（SDL_KEYDOWN）或放开（SDL_KEYUP）。state是冗余的，和含义type相同只是符号不同（SDL_PRESSED，SDL_RELEASED）。

读取键盘消息

使用消息循环用SDL_PollEvent()从消息队列里读取，并用switch-case检测SDL_KEYUP和SDL_KEYDOWN。下面是个基本的例子。

例3-10 读键盘消息

  SDL_Event event;
  .
  .
  /* Poll for events. SDL_PollEvent() returns 0 when there are no  */
  /* more events on the event queue, our while loop will exit when */
  /* that occurs.                                                  */
  while( SDL_PollEvent( &event ) ){
    /* We are only worried about SDL_KEYDOWN and SDL_KEYUP events */
    switch( event.type ){
      case SDL_KEYDOWN:
        printf( "Key press detected/n" );
        break;

      case SDL_KEYUP:
        printf( "Key release detected/n" );
        break;

      default:
        break;
    }
  }

更详细的内容

我们已经知道要用SDL_Init和SDL_SetVideoMode初始化，但我们还得用另外两个函数取得必要信息。要调用SDL_EnableUNICODE(1)以允许unicode变换，还要用SDL_GetKeyName将SDLKey转换成可打印字符。注意：小于0x80的unicode字符直接映射到ASCII码。下例用到这一点。

例3-11 解释按键消息

    #include "SDL.h"

    /* Function Prototypes */
    void PrintKeyInfo( SDL_KeyboardEvent *key );
    void PrintModifiers( SDLMod mod );

    /* main */
    int main( int argc, char *argv[] ){
        
        SDL_Event event;
        int quit = 0;
        
        /* Initialise SDL */
        if( SDL_Init( SDL_INIT_VIDEO ) ){
            fprintf( stderr, "Could not initialise SDL: %s/n", SDL_GetError() );
            exit( -1 );
        }

        /* Set a video mode */
        if( !SDL_SetVideoMode( 320, 200, 0, 0 ) ){
            fprintf( stderr, "Could not set video mode: %s/n", SDL_GetError() );
            SDL_Quit();
            exit( -1 );
        }

        /* Enable Unicode translation */
        SDL_EnableUNICODE( 1 );

        /* Loop until an SDL_QUIT event is found */
        while( !quit ){

            /* Poll for events */
            while( SDL_PollEvent( &event ) ){
                
                switch( event.type ){
                    /* Keyboard event */
                    /* Pass the event data onto PrintKeyInfo() */
                    case SDL_KEYDOWN:
                    case SDL_KEYUP:
                        PrintKeyInfo( &event.key );
                        break;

                    /* SDL_QUIT event (window close) */
                    case SDL_QUIT:
                        quit = 1;
                        break;

                    default:
                        break;
                }

            }

        }

        /* Clean up */
        SDL_Quit();
        exit( 0 );
    }

    /* Print all information about a key event */
    void PrintKeyInfo( SDL_KeyboardEvent *key ){
        /* Is it a release or a press? */
        if( key->type == SDL_KEYUP )
            printf( "Release:- " );
        else
            printf( "Press:- " );

        /* Print the hardware scancode first */
        printf( "Scancode: 0x%02X", key->keysym.scancode );
        /* Print the name of the key */
        printf( ", Name: %s", SDL_GetKeyName( key->keysym.sym ) );
        /* We want to print the unicode info, but we need to make */
        /* sure its a press event first (remember, release events */
        /* don‘t have unicode info                                */
        if( key->type == SDL_KEYDOWN ){
            /* If the Unicode value is less than 0x80 then the    */
            /* unicode value can be used to get a printable       */
            /* representation of the key, using (char)unicode.    */
            printf(", Unicode: " );
            if( key->keysym.unicode < 0x80 && key->keysym.unicode > 0 ){
                printf( "%c (0x%04X)", (char)key->keysym.unicode,
                        key->keysym.unicode );
            }
            else{
                printf( "? (0x%04X)", key->keysym.unicode );
            }
        }
        printf( "/n" );
        /* Print modifier info */
        PrintModifiers( key->keysym.mod );
    }

    /* Print modifier info */
    void PrintModifiers( SDLMod mod ){
        printf( "Modifers: " );

        /* If there are none then say so and return */
        if( mod == KMOD_NONE ){
            printf( "None/n" );
            return;
        }

        /* Check for the presence of each SDLMod value */
        /* This looks messy, but there really isn‘t    */
        /* a clearer way.                              */
        if( mod & KMOD_NUM ) printf( "NUMLOCK " );
        if( mod & KMOD_CAPS ) printf( "CAPSLOCK " );
        if( mod & KMOD_LCTRL ) printf( "LCTRL " );
        if( mod & KMOD_RCTRL ) printf( "RCTRL " );
        if( mod & KMOD_RSHIFT ) printf( "RSHIFT " );
        if( mod & KMOD_LSHIFT ) printf( "LSHIFT " );
        if( mod & KMOD_RALT ) printf( "RALT " );
        if( mod & KMOD_LALT ) printf( "LALT " );
        if( mod & KMOD_CTRL ) printf( "CTRL " );
        if( mod & KMOD_SHIFT ) printf( "SHIFT " );
        if( mod & KMOD_ALT ) printf( "ALT " );
        printf( "/n" );
    }

游戏式键盘输入

键盘消息仅在键的状态在按下和放开间变化时才触发。

设想你用光标键控制飞船运动以改变眼前看到的太空景象，当你按左键并希望镜头向左转时。看看下面的代码，并注意它为什么是错的。

    /* Alien screen coordinates */
    int alien_x=0, alien_y=0;
    .
    .
    /* Initialise SDL and video modes and all that */
    .
    /* Main game loop */
    /* Check for events */
    while( SDL_PollEvent( &event ) ){
        switch( event.type ){
            /* Look for a keypress */
            case SDL_KEYDOWN:
                /* Check the SDLKey values and move change the coords */
                switch( event.key.keysym.sym ){
                    case SDLK_LEFT:
                        alien_x -= 1;
                        break;
                    case SDLK_RIGHT:
                        alien_x += 1;
                        break;
                    case SDLK_UP:
                        alien_y -= 1;
                        break;
                    case SDLK_DOWN:
                        alien_y += 1;
                        break;
                    default:
                        break;
                }
            }
        }
    }

问题在于你必须按100次左以便得到100次键盘消息，才能向左转100像素。正确的方法是收到事件时设定标志，根据标志来移动。

例3-12 正确的运动控制

    /* Alien screen coordinates */
    int alien_x=0, alien_y=0;
    int alien_xvel=0, alien_yvel=0;
    .
    .
    /* Initialise SDL and video modes and all that */
    .
    /* Main game loop */
    /* Check for events */
    while( SDL_PollEvent( &event ) ){
        switch( event.type ){
            /* Look for a keypress */
            case SDL_KEYDOWN:
                /* Check the SDLKey values and move change the coords */
                switch( event.key.keysym.sym ){
                    case SDLK_LEFT:
                        alien_xvel = -1;
                        break;
                    case SDLK_RIGHT:
                        alien_xvel =  1;
                        break;
                    case SDLK_UP:
                        alien_yvel = -1;
                        break;
                    case SDLK_DOWN:
                        alien_yvel =  1;
                        break;
                    default:
                        break;
                }
                break;
            /* We must also use the SDL_KEYUP events to zero the x */
            /* and y velocity variables. But we must also be       */
            /* careful not to zero the velocities when we shouldn‘t*/
            case SDL_KEYUP:
                switch( event.key.keysym.sym ){
                    case SDLK_LEFT:
                        /* We check to make sure the alien is moving */
                        /* to the left. If it is then we zero the    */
                        /* velocity. If the alien is moving to the   */
                        /* right then the right key is still press   */
                        /* so we don‘t tocuh the velocity            */
                        if( alien_xvel < 0 )
                            alien_xvel = 0;
                        break;
                    case SDLK_RIGHT:
                        if( alien_xvel > 0 )
                            alien_xvel = 0;
                        break;
                    case SDLK_UP:
                        if( alien_yvel < 0 )
                            alien_yvel = 0;
                        break;
                    case SDLK_DOWN:
                        if( alien_yvel > 0 )
                            alien_yvel = 0;
                        break;
                    default:
                        break;
                }
                break;
            
            default:
                break;
        }
    }
    .
    .
    /* Update the alien position */
    alien_x += alien_xvel;
    alien_y += alien_yvel;

如您所见，我们用了两个变量alien_xvel和alien_yvel来表示飞船的运动，并在响应键盘消息时更新它们。

第四章 样例

注：重复的例子没有列出

最快的图像平面块传送

将图像画到屏幕上有三种方式：1.创建一个图像平面并用SDL_BlitSurface传送到屏幕；2.在系统内存创建视频平面并调用SDL_UpdateRect；3.在显存创建视频平面并调用SDL_LockSurface。最好的方法是混合方式：

#include <stdio.h>
#include <stdlib.h>
#include "SDL.h"
#include "SDL_timer.h"

void ComplainAndExit(void)
{
    fprintf(stderr, "Problem: %s/n", SDL_GetError());
    exit(1);
}

int main(int argc, char *argv[])
{
    SDL_PixelFormat fmt;
    SDL_Surface *screen, *locked;
    SDL_Surface *imagebmp, *image;
    SDL_Rect dstrect;
    int i;
    Uint8 *buffer;

    /* Initialize SDL */
    if ( SDL_Init(SDL_INIT_VIDEO) < 0 ) {
        ComplainAndExit();
    }
    atexit(SDL_Quit);

    /* Load a BMP image into a surface */
    imagebmp = SDL_LoadBMP("image.bmp");
    if ( imagebmp == NULL ) {
        ComplainAndExit();
    }

    /* Set the video mode (640x480 at native depth) */
    screen = SDL_SetVideoMode(640, 480, 0, SDL_HWSURFACE|SDL_FULLSCREEN);
    if ( screen == NULL ) {
        ComplainAndExit();
    }

    /* Set the video colormap */
    if ( imagebmp->format->palette != NULL ) {
        SDL_SetColors(screen,
                      imagebmp->format->palette->colors, 0,
                      imagebmp->format->palette->ncolors);
    }

    /* Convert the image to the video format (maps colors) */
    image = SDL_DisplayFormat(imagebmp);
    SDL_FreeSurface(imagebmp);
    if ( image == NULL ) {
        ComplainAndExit();
    }

    /* Draw bands of color on the raw surface */
    if ( SDL_MUSTLOCK(screen) ) {
        if ( SDL_LockSurface(screen) < 0 )
            ComplainAndExit();
    }
    buffer=(Uint8 *)screen->pixels;
    for ( i=0; ih; ++i ) {
        memset(buffer,(i*255)/screen->h,
               screen->w*screen->format->BytesPerPixel);
               buffer += screen->pitch;
    }
    if ( SDL_MUSTLOCK(screen) ) {
        SDL_UnlockSurface(screen);
    }

    /* Blit the image to the center of the screen */
    dstrect.x = (screen->w-image->w)/2;
    dstrect.y = (screen->h-image->h)/2;
    dstrect.w = image->w;
    dstrect.h = image->h;
    if ( SDL_BlitSurface(image, NULL, screen, &dstrect) < 0 ) {
        SDL_FreeSurface(image);
        ComplainAndExit();
    }
    SDL_FreeSurface(image);

    /* Update the screen */
    SDL_UpdateRects(screen, 1, &dstrect);

    SDL_Delay(5000);        /* Wait 5 seconds */
    exit(0);
}

过滤和处理事件

#include <stdio.h>
#include <stdlib.h>

#include "SDL.h"

/* This function may run in a separate event thread */
int FilterEvents(const SDL_Event *event) {
    static int boycott = 1;

    /* This quit event signals the closing of the window */
    if ( (event->type == SDL_QUIT) && boycott ) {
        printf("Quit event filtered out -- try again./n");
        boycott = 0;
        return(0);
    }
    if ( event->type == SDL_MOUSEMOTION ) {
        printf("Mouse moved to (%d,%d)/n",
                event->motion.x, event->motion.y);
        return(0);    /* Drop it, we‘ve handled it */
    }
    return(1);
}

int main(int argc, char *argv[])
{
    SDL_Event event;

    /* Initialize the SDL library (starts the event loop) */
    if ( SDL_Init(SDL_INIT_VIDEO) < 0 ) {
        fprintf(stderr,
                "Couldn‘t initialize SDL: %s/n", SDL_GetError());
        exit(1);
    }

    /* Clean up on exit, exit on window close and interrupt */
    atexit(SDL_Quit);

    /* Ignore key events */
    SDL_EventState(SDL_KEYDOWN, SDL_IGNORE);
    SDL_EventState(SDL_KEYUP, SDL_IGNORE);

    /* Filter quit and mouse motion events */
    SDL_SetEventFilter(FilterEvents);

    /* The mouse isn‘t much use unless we have a display for reference */
    if ( SDL_SetVideoMode(640, 480, 8, 0) == NULL ) {
        fprintf(stderr, "Couldn‘t set 640x480x8 video mode: %s/n",
                        SDL_GetError());
        exit(1);
    }

    /* Loop waiting for ESC+Mouse_Button */
    while ( SDL_WaitEvent(&event) >= 0 ) {
        switch (event.type) {
            case SDL_ACTIVEEVENT: {
                if ( event.active.state & SDL_APPACTIVE ) {
                    if ( event.active.gain ) {
                        printf("App activated/n");
                    } else {
                        printf("App iconified/n");
                    }
                }
            }
            break;
                    
            case SDL_MOUSEBUTTONDOWN: {
                Uint8 *keys;

                keys = SDL_GetKeyState(NULL);
                if ( keys[SDLK_ESCAPE] == SDL_PRESSED ) {
                    printf("Bye bye.../n");
                    exit(0);
                }
                printf("Mouse button pressed/n");
            }
            break;

            case SDL_QUIT: {
                printf("Quit requested, quitting./n");
                exit(0);
            }
            break;
        }
    }
    /* This should never happen */
    printf("SDL_WaitEvent error: %s/n", SDL_GetError());
    exit(1);
}

打开音频设备

    SDL_AudioSpec wanted;
    extern void fill_audio(void *udata, Uint8 *stream, int len);

    /* Set the audio format */
    wanted.freq = 22050;
    wanted.format = AUDIO_S16;
    wanted.channels = 2;    /* 1 = mono, 2 = stereo */
    wanted.samples = 1024;  /* Good low-latency value for callback */
    wanted.callback = fill_audio;
    wanted.userdata = http://www.mamicode.com/NULL;"highlight_tag">SDL_OpenAudio(&wanted, NULL) < 0 ) {
        fprintf(stderr, "Couldn‘t open audio: %s/n", SDL_GetError());
        return(-1);
    }
    return(0);

播放音频

    static Uint8 *audio_chunk;
    static Uint32 audio_len;
    static Uint8 *audio_pos;

    /* The audio function callback takes the following parameters:
       stream:  A pointer to the audio buffer to be filled
       len:     The length (in bytes) of the audio buffer
    */
    void fill_audio(void *udata, Uint8 *stream, int len)
    {
        /* Only play if we have data left */
        if ( audio_len == 0 )
            return;

        /* Mix as much data as possible */
        len = ( len > audio_len ? audio_len : len );
        SDL_MixAudio(stream, audio_pos, len, SDL_MIX_MAXVOLUME)
        audio_pos += len;
        audio_len -= len;
    }

    /* Load the audio data ... */

    ;;;;;

    audio_pos = audio_chunk;

    /* Let the callback function play the audio chunk */
    SDL_PauseAudio(0);

    /* Do some processing */

    ;;;;;

    /* Wait for sound to complete */
    while ( audio_len > 0 ) {
        SDL_Delay(100);         /* Sleep 1/10 second */
    }
    SDL_CloseAudio();

列出所有CDROM

    #include "SDL.h"

    /* Initialize SDL first */
    if ( SDL_Init(SDL_INIT_CDROM) < 0 ) {
        fprintf(stderr, "Couldn‘t initialize SDL: %s/n",SDL_GetError());
        exit(1);
    }
    atexit(SDL_Quit);

    /* Find out how many CD-ROM drives are connected to the system */
    printf("Drives available: %d/n", SDL_CDNumDrives());
    for ( i=0; i<SDL_CDNumDrives(); ++i ) {
        printf("Drive %d:  /"%s/"/n", i, SDL_CDName(i));
    }

打开缺省CDROM驱动器

    SDL_CD *cdrom;
    CDstatus status;
    char *status_str;

    cdrom = SDL_CDOpen(0);
    if ( cdrom == NULL ) {
        fprintf(stderr, "Couldn‘t open default CD-ROM drive: %s/n",
                        SDL_GetError());
        exit(2);
    }

    status = SDL_CDStatus(cdrom);
    switch (status) {
        case CD_TRAYEMPTY:
            status_str = "tray empty";
            break;
        case CD_STOPPED:
            status_str = "stopped";
            break;
        case CD_PLAYING:
            status_str = "playing";
            break;
        case CD_PAUSED:
            status_str = "paused";
            break;
        case CD_ERROR:
            status_str = "error state";
            break;
    }
    printf("Drive status: %s/n", status_str);
    if ( status >= CD_PLAYING ) {
        int m, s, f;
        FRAMES_TO_MSF(cdrom->cur_frame, &m, &s, &f);
        printf("Currently playing track %d, %d:%2.2d/n",
        cdrom->track[cdrom->cur_track].id, m, s);
    }

列出CD上所有音轨

    SDL_CD *cdrom;          /* Assuming this has already been set.. */
    int i;
    int m, s, f;

    SDL_CDStatus(cdrom);
    printf("Drive tracks: %d/n", cdrom->numtracks);
    for ( i=0; i<cdrom->numtracks; ++i ) {
        FRAMES_TO_MSF(cdrom->track[i].length, &m, &s, &f);
        if ( f > 0 )
            ++s;
        printf("/tTrack (index %d) %d: %d:%2.2d/n", i,
        cdrom->track[i].id, m, s);
    }

播放CD

    SDL_CD *cdrom;          /* Assuming this has already been set.. */

    // Play entire CD:
    if ( CD_INDRIVE(SDL_CDStatus(cdrom)) )
        SDL_CDPlayTracks(cdrom, 0, 0, 0, 0);

        // Play last track:
        if ( CD_INDRIVE(SDL_CDStatus(cdrom)) ) {
            SDL_CDPlayTracks(cdrom, cdrom->numtracks-1, 0, 0, 0);
        }

        // Play first and second track and 10 seconds of third track:
        if ( CD_INDRIVE(SDL_CDStatus(cdrom)) )
            SDL_CDPlayTracks(cdrom, 0, 0, 2, 10);

基于时间的游戏主循环

#define TICK_INTERVAL    30

Uint32 TimeLeft(void)
{
    static Uint32 next_time = 0;
    Uint32 now;

    now = SDL_GetTicks();
    if ( next_time <= now ) {
        next_time = now+TICK_INTERVAL;
        return(0);
    }
    return(next_time-now);
}


/* main game loop

    while ( game_running ) {
        UpdateGameState();
        SDL_Delay(TimeLeft());
    }