本篇文章紧张讲解如何学习SDL以及SDL主要的根本知识，希望能够帮到大家。
1.SDL简述
SDL（Simple DirectMedia Layer）是一套开放源代码的跨平台多媒体开拓库，利用C措辞写成。SDL供应了数种掌握图像、声音、输出的函数，让开发者只要用相同或是相似的代码就可以开拓出跨多个平台（Linux、Windows、Mac OS X等）的运用软件，对付跨平台比较好。目前SDL多用于开拓游戏、仿照器、媒体播放器等多媒体运用领域。SDL（Simple DirectMedia Layer）被广泛的用于许多著名的游戏。最著名的游戏是赢得Linux组游戏开拓大奖的 文明：权利的召唤（Civilization: Call To Power）。SDL的作者是Sam Lantinga, Loki Entertainment Software的主力程序员（Lead Programmer）。
1.1 SDL的紧张框架
2.SDL的紧张功能
(1)视频(SDL_INIT_VIDEO)
设置8bpp或更高的任意色彩深度的视频模式。如果某个模式硬件不支持，可以选择转化为另一模式。直接写入线性的图像帧缓冲（framebuffer）。用颜色键值（colorkey）或者alpha稠浊属性创建surface。Surface的blit能自动的转化为目标格式。blit是优化过的，并能利用硬件加速。x86平台上有针对MMX优化过的blit。硬件加速的blit和fill（添补）操作，如果硬件支持的话。这个功能在音视频显示，开拓领域显得很主要。
(2)事宜(SDL_INIT_EVENTS)
供应以下事宜：
运用程序的visibility发生改变
键盘输入
鼠标输入
用户哀求的退出
每种事宜都能通过SDL_EventState()关闭或者打开。
事宜经由用户指定的过滤函数再被加入到内部的事宜行列步队。
线程安全的事宜行列步队。
这个功能在音视频显示，开拓领域显得很主要。
(3)音频(SDL_INIT_AUDIO)
设置8位和16位的音频，单声道或者立体声，如果格式硬件不支持，可以选择转换。
由独立的线程实行音频部分，并供应用户回调（callback）机制。
设计上考虑到了客户定制的软混音器，但实际上在例程中就包含了一个完全的音频/音乐输出库。
这个功能在音视频显示，开拓领域显得很主要。
(4)CD音频
完全的CD音频掌握API
(5)线程
大略的线程创建API
用于同步的大略的二进制旗子暗记量（semaphores）
(6)定时器(SDL_INIT_AUDIO_TIMER)
读取已流逝的毫秒数。
等待指定的毫秒数。
设置一个10毫秒精度的周期性定时器。
这个功能在音视频显示，开拓领域显得很主要。
(7)字节序无关
侦测当前系统的字节序
快速转换数据的函数
读写指定字节序的数据
(8)摇杆(SDL_INIT_JOYSTICK)
(9)触摸屏(SDL_INIT_HAPTIC)
(10)游戏掌握器(SDL_INIT_GAMECONTROLLER)
(11)包含上述所有选项的事宜(SDL_INIT_EVERYTHING)
3.SDL支持哪些系统平台
(1)Linux
视频显示利用X11，利用XFree86 DGA扩展的上风，全屏显示利用新的w MTRR加速。声音利用OSS API。利用clone()系统调用和SysV IPC，或者glibc-2.1的pthreads实现线程。
(2)Win32
有两个版本，一个是适宜所有基于Win32的系统的安全版本，另一个是基于DirectX的高性能版本。安全版本的视频显示采取GDI。高性能版本采取DirectDraw，并支持硬件加速。安全版本的音频回放采取waveOut API。高性能版本采取DirectSound。
(3)BeOS
视频显示采取BWindow。音频回放采取BSoundPlayer API。非正式的移植版本，进展中Solaris, IRIX, FreeBSD MacOS
4.SDL的其它描述
SDL内置了调用OpenGL的函数。通过利用SDL_image、SDL_ttf、SDL_mixer、SDL_net等外部扩展库，可以轻松实现JPG、PNG、TIFF图像的加载利用，TrueType字体的利用，MP3文件的利用、网络干系的利用等。
SDL也有其他措辞的包装，可以在这里查看 SDL用C措辞写成，但是可以很随意马虎在C++下面事情，并且SDL绑定了许多其它的措辞，这个中就包括Ada, C#, Eiffel, Erlang, Euphoria, Guile, Haskell, Java, Lisp, Lua, ML, Objective C, Pascal, Perl, PHP, Pike, Pliant, Python, Ruby, and Smalltalk。包装得比较好的是python措辞的pygame。不过仍旧建议你熟习c/c++环境下的SDL后再利用，会得心应手许多。末了，我们来理解一下SDL的版权问题，SDL在GNU LGPL 2（一个国际上的开源组织）下发布，这个版本许可你将SDL以动态链接库（dynamic link library）的形式免费地用于商业游戏软件的开拓。
5.SDL下载和编译
5.1 下载SDL的win平台库
(1)SDL官网地址：
https://www.libsdl.org/
界面如下:
(2)SDL官网文档链接：
http://wiki.libsdl.org/Introduction
(3)下载SDL的dll和lib，链接地址如下:
https://www.libsdl.org/download-2.0.php
界面如下:
也可以下载源码下来，自己编译。比如在linux环境下，自己去编译。win平台一样平常是直策应用，已经下载编译好的库。
(4)下载的库，内容如下:
(5)doc目录如下:
(6)includle目录如下:
(7)lib目录如下:
(8)x86的库
(9)x64的库
SDL只是音视频用来显示和输出，以是用来关心这部分接口即可，知道怎么去用，如果项目中，有利用其它功能，可以再去研究文档和实例代码。
6.在linux环境下搭建SDL的开拓环境
(1)下载SDL源码库，如SDL2-2.0.10.tar.gz
(2)解压，然后依次实行如下命令
./configuremakesudo make install
(3)如果涌现Could not initialize SDL - No available video device(Did you set the DISPLAY variable?)缺点提示，就解释系统中没有安装x11的库文件，因此编译出来的SDL库实际上不能用。须要安装如下库，命令如下:
sudo apt-get install libx11-devsudo apt-get install xorg-dev
7.SDL的win显示
天生的SDL窗口。如下:
(1)初始化SDL window
(2)创建SDL Window，可以设置利用opengl做加速。
把稳：SDL_Init()和SDL_Quit()这两个函数都是必须利用。
8.SDL的渲染器和纹理
SDL_Window: 代表了一个“窗口”。
SDL_Renderer ：代表了一个“渲染器”。
SDL_Texture：代表了一个“纹理”。
SDL_Rect：矩形构造
把稳：存储RGB和存储纹理的差异如下：
如一个从左到右由赤色渐变到蓝色的矩形，用存储RGB的话就须要把矩形中每个点的详细颜色值存储下来；而纹理只是一些描述信息，比如记录了矩形的大小、起始颜色、终止颜色等信息，显卡可以通过这些信息推算出矩形块的详细信息。以是相对付存储RGB而已，存储纹理占用的内存要少的多。
SDL绘制图形效果:
(1)创建渲染器SDL_Renderer
一样平常一个窗口，对应一个渲染器render，也可以对应多个渲染器，每个渲染器可以设置不同的纹理。
基于上面所说的window去创建渲染器，源代码接口如下:
/ \brief Set a texture as the current rendering target. \param renderer The renderer. \param texture The targeted texture, which must be created with the SDL_TEXTUREACCESS_TARGET flag, or NULL for the default render target \return 0 on success, or -1 on error \sa SDL_GetRenderTarget() /extern DECLSPEC int SDLCALL SDL_SetRenderTarget(SDL_Renderer renderer, SDL_Texture texture);
(2)创建纹理SDL_Texture
创建纹理，也是基于上面的渲染器render去创建。源代码接口如下:
/ \brief Copy a portion of the texture to the current rendering target. \param renderer The renderer which should copy parts of a texture. \param texture The source texture. \param srcrect A pointer to the source rectangle, or NULL for the entire texture. \param dstrect A pointer to the destination rectangle, or NULL for the entire rendering target. \return 0 on success, or -1 on error /extern DECLSPEC int SDLCALL SDL_RenderCopy(SDL_Renderer renderer, SDL_Texture texture, const SDL_Rect srcrect, const SDL_Rect dstrect);
(3)设置渲染器目标
以下操作都是在一个循环里面去实行。
设置渲染目标为纹理，如下所示:
SDL_SetRenderTarget(renderer, texture);
源码接口如下:
/ \brief Set a texture as the current rendering target. \param renderer The renderer. \param texture The targeted texture, which must be created with the SDL_TEXTUREACCESS_TARGET flag, or NULL for the default render target \return 0 on success, or -1 on error \sa SDL_GetRenderTarget() /extern DECLSPEC int SDLCALL SDL_SetRenderTarget(SDL_Renderer renderer, SDL_Texture texture);
(4)设置纹理背景的颜色
SDL_SetRenderDrawColor(renderer, 255, 0, 0, 255)
源码接口如下:
/ \brief Set the color used for drawing operations (Rect, Line and Clear). \param renderer The renderer for which drawing color should be set. \param r The red value used to draw on the rendering target. \param g The green value used to draw on the rendering target. \param b The blue value used to draw on the rendering target. \param a The alpha value used to draw on the rendering target, usually ::SDL_ALPHA_OPAQUE (255). \return 0 on success, or -1 on error /extern DECLSPEC int SDLCALL SDL_SetRenderDrawColor(SDL_Renderer renderer, Uint8 r, Uint8 g, Uint8 b, Uint8 a);
(5)绘制一个方形。如下所示:
SDL_RenderDrawRect(renderer, &rect)
源码接口如下:
/ \brief Draw a rectangle on the current rendering target. \param renderer The renderer which should draw a rectangle. \param rect A pointer to the destination rectangle, or NULL to outline the entire rendering target. \return 0 on success, or -1 on error /extern DECLSPEC int SDLCALL SDL_RenderDrawRect(SDL_Renderer renderer, const SDL_Rect rect);
(6)给上面绘制的方形，添补颜色，如下所示:
SDL_SetRenderDrawColor(renderer, 0, 255, 255, 255);SDL_RenderFillRect(renderer, &rect);
源码接口如下:
/ \brief Set the color used for drawing operations (Rect, Line and Clear). \param renderer The renderer for which drawing color should be set. \param r The red value used to draw on the rendering target. \param g The green value used to draw on the rendering target. \param b The blue value used to draw on the rendering target. \param a The alpha value used to draw on the rendering target, usually ::SDL_ALPHA_OPAQUE (255). \return 0 on success, or -1 on error /extern DECLSPEC int SDLCALL SDL_SetRenderDrawColor(SDL_Renderer renderer, Uint8 r, Uint8 g, Uint8 b, Uint8 a);
/ \brief Fill a rectangle on the current rendering target with the drawing color. \param renderer The renderer which should fill a rectangle. \param rect A pointer to the destination rectangle, or NULL for the entire rendering target. \return 0 on success, or -1 on error /extern DECLSPEC int SDLCALL SDL_RenderFillRect(SDL_Renderer renderer, const SDL_Rect rect);
(7)设置上面的纹理texture为渲染器目标，如下所示:
SDL_SetRenderTarget(renderer, NULL);
源码接口如下:
/ \brief Set a texture as the current rendering target. \param renderer The renderer. \param texture The targeted texture, which must be created with the SDL_TEXTUREACCESS_TARGET flag, or NULL for the default render target \return 0 on success, or -1 on error \sa SDL_GetRenderTarget() /extern DECLSPEC int SDLCALL SDL_SetRenderTarget(SDL_Renderer renderer, SDL_Texture texture);
(8)拷贝纹理texture到渲染器上。如下所示:
SDL_RenderCopy(renderer, texture, NULL, NULL);
源码接口如下:
/ \brief Update the screen with rendering performed. /extern DECLSPEC void SDLCALL SDL_RenderPresent(SDL_Renderer renderer);
(9)显示
终极是更新窗口上的纹理，如下所示:
SDL_RenderPresent(renderer);
源码接口如下:
/ \brief Update the screen with rendering performed. /extern DECLSPEC void SDLCALL SDL_RenderPresent(SDL_Renderer renderer);
(10)SDL_UpdateTexture()：设置纹理的数据
(11)SDL_Delay()：工具函数，用于延时。
(12)SDL_Quit()：退出SDL系统
上述显示的核心代码，如下：
SDL_Init(SDL_INIT_VIDEO);//初始化函数,可以确定希望激活的子系统 window = SDL_CreateWindow("2 Window", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, 640, 480, SDL_WINDOW_OPENGL | SDL_WINDOW_RESIZABLE);// 创建窗口 if (!window) { return -1; } renderer = SDL_CreateRenderer(window, -1, 0);//基于窗口创建渲染器 if (!renderer) { return -1; } texture = SDL_CreateTexture(renderer, SDL_PIXELFORMAT_RGBA8888, SDL_TEXTUREACCESS_TARGET, 640, 480); //创建纹理 if (!texture) { return -1; } int show_count = 0; while (run) { rect.x = rand() % 600; rect.y = rand() % 400; SDL_SetRenderTarget(renderer, texture); // 设置渲染目标为纹理 SDL_SetRenderDrawColor(renderer, 255, 0, 0, 255); // 纹理背景为玄色 SDL_RenderClear(renderer); //清屏 SDL_RenderDrawRect(renderer, &rect); //绘制一个长方形 SDL_SetRenderDrawColor(renderer, 0, 255, 255, 255); //长方形的颜色可以改变 SDL_RenderFillRect(renderer, &rect); SDL_SetRenderTarget(renderer, NULL); //规复默认，渲染目标为窗口 SDL_RenderCopy(renderer, texture, NULL, NULL); //拷贝纹理到渲染器 SDL_RenderPresent(renderer); //输出到目标窗口上 SDL_Delay(100); if(show_count++ > 60) { run = 0; // 不跑了 } } SDL_DestroyTexture(texture); SDL_DestroyRenderer(renderer); SDL_DestroyWindow(window); //销毁窗口 SDL_Quit();
9.SDL事宜相应
比较主要的两个事宜接口:
(1)等待一个事宜。
SDL_WaitEvent(&event);
(2)发送一个事宜。
SDL_PushEvent(&event_q);
(3)存储事宜
将硬件设备产生的事宜放入事宜行列步队，用于读取事宜，在调用该函数之前，必须调用SDL_PumpEvents搜集键盘等事宜。
SDL_PumpEvents()；
(4)从事宜行列步队提取一个事宜
SDL_PeepEvents()；
(5)代表一个事宜
SDL_Event
效果图如下：
可以看到，能够检测到鼠标和键盘的相应事宜。
(6)源码接口这里，支持很多事宜，如鼠标，键盘等，如下图所示:
typedef enum{ SDL_FIRSTEVENT = 0, /< Unused (do not remove) / / Application events / SDL_QUIT = 0x100, /< User-requested quit / / These application events have special meaning on iOS, see README-ios.md for details / SDL_APP_TERMINATING, /< The application is being terminated by the OS Called on iOS in applicationWillTerminate() Called on Android in onDestroy() / SDL_APP_LOWMEMORY, /< The application is low on memory, free memory if possible. Called on iOS in applicationDidReceiveMemoryWarning() Called on Android in onLowMemory() / SDL_APP_WILLENTERBACKGROUND, /< The application is about to enter the background Called on iOS in applicationWillResignActive() Called on Android in onPause() / SDL_APP_DIDENTERBACKGROUND, /< The application did enter the background and may not get CPU for some time Called on iOS in applicationDidEnterBackground() Called on Android in onPause() / SDL_APP_WILLENTERFOREGROUND, /< The application is about to enter the foreground Called on iOS in applicationWillEnterForeground() Called on Android in onResume() / SDL_APP_DIDENTERFOREGROUND, /< The application is now interactive Called on iOS in applicationDidBecomeActive() Called on Android in onResume() / / Display events / SDL_DISPLAYEVENT = 0x150, /< Display state change / / Window events / SDL_WINDOWEVENT = 0x200, /< Window state change / SDL_SYSWMEVENT, /< System specific event / / Keyboard events / SDL_KEYDOWN = 0x300, /< Key pressed / SDL_KEYUP, /< Key released / SDL_TEXTEDITING, /< Keyboard text editing (composition) / SDL_TEXTINPUT, /< Keyboard text input / SDL_KEYMAPCHANGED, /< Keymap changed due to a system event such as an input language or keyboard layout change. / / Mouse events / SDL_MOUSEMOTION = 0x400, /< Mouse moved / SDL_MOUSEBUTTONDOWN, /< Mouse button pressed / SDL_MOUSEBUTTONUP, /< Mouse button released / SDL_MOUSEWHEEL, /< Mouse wheel motion / / Joystick events / SDL_JOYAXISMOTION = 0x600, /< Joystick axis motion / SDL_JOYBALLMOTION, /< Joystick trackball motion / SDL_JOYHATMOTION, /< Joystick hat position change / SDL_JOYBUTTONDOWN, /< Joystick button pressed / SDL_JOYBUTTONUP, /< Joystick button released / SDL_JOYDEVICEADDED, /< A new joystick has been inserted into the system / SDL_JOYDEVICEREMOVED, /< An opened joystick has been removed / / Game controller events / SDL_CONTROLLERAXISMOTION = 0x650, /< Game controller axis motion / SDL_CONTROLLERBUTTONDOWN, /< Game controller button pressed / SDL_CONTROLLERBUTTONUP, /< Game controller button released / SDL_CONTROLLERDEVICEADDED, /< A new Game controller has been inserted into the system / SDL_CONTROLLERDEVICEREMOVED, /< An opened Game controller has been removed / SDL_CONTROLLERDEVICEREMAPPED, /< The controller mapping was updated / / Touch events / SDL_FINGERDOWN = 0x700, SDL_FINGERUP, SDL_FINGERMOTION, / Gesture events / SDL_DOLLARGESTURE = 0x800, SDL_DOLLARRECORD, SDL_MULTIGESTURE, / Clipboard events / SDL_CLIPBOARDUPDATE = 0x900, /< The clipboard changed / / Drag and drop events / SDL_DROPFILE = 0x1000, /< The system requests a file open / SDL_DROPTEXT, /< text/plain drag-and-drop event / SDL_DROPBEGIN, /< A new set of drops is beginning (NULL filename) / SDL_DROPCOMPLETE, /< Current set of drops is now complete (NULL filename) / / Audio hotplug events / SDL_AUDIODEVICEADDED = 0x1100, /< A new audio device is available / SDL_AUDIODEVICEREMOVED, /< An audio device has been removed. / / Sensor events / SDL_SENSORUPDATE = 0x1200, /< A sensor was updated / / Render events / SDL_RENDER_TARGETS_RESET = 0x2000, /< The render targets have been reset and their contents need to be updated / SDL_RENDER_DEVICE_RESET, /< The device has been reset and all textures need to be recreated / / Events ::SDL_USEREVENT through ::SDL_LASTEVENT are for your use, and should be allocated with SDL_RegisterEvents() / SDL_USEREVENT = 0x8000, / This last event is only for bounding internal arrays / SDL_LASTEVENT = 0xFFFF} SDL_EventType;
(7)添补event_q.type，可以支持用户自定义事宜。如下所示：
event_q.type = FF_QUIT_EVENT;
这些事宜，紧张是用来做播放掌握。
上述事宜的核心代码如下:
SDL_Event event; int b_exit = 0; for (;;) { SDL_WaitEvent(&event); switch (event.type) { case SDL_KEYDOWN:/ 键盘事宜 / switch (event.key.keysym.sym) { case SDLK_a: printf("key down a\n"); break; case SDLK_s: printf("key down s\n"); break; case SDLK_d: printf("key down d\n"); break; case SDLK_q: printf("key down q and push quit event\n"); SDL_Event event_q; event_q.type = FF_QUIT_EVENT; SDL_PushEvent(&event_q); break; default: printf("key down 0x%x\n", event.key.keysym.sym); break; } break; case SDL_MOUSEBUTTONDOWN:/ 鼠标按下事宜 / if (event.button.button == SDL_BUTTON_LEFT) { printf("mouse down left\n"); } else if(event.button.button == SDL_BUTTON_RIGHT) { printf("mouse down right\n"); } else { printf("mouse down %d\n", event.button.button); } break; case SDL_MOUSEMOTION:/ 鼠标移动事宜 / printf("mouse movie (%d,%d)\n", event.button.x, event.button.y); break; case FF_QUIT_EVENT: printf("receive quit event\n"); b_exit = 1; break; } if(b_exit) break; } //destory renderer if (renderer) SDL_DestroyRenderer(renderer); // Close and destroy the window if (window) SDL_DestroyWindow(window); // Clean up SDL_Quit(); return 0;
10.SDL多线程
ffplay便是利用的SDL的多线程。这样做的目的便是为了跨平台。
(1)SDL线程创建：SDL_CreateThread。如下示例:
SDL_Thread t = SDL_CreateThread(thread_work,"thread_work",NULL);
(2)SDL等待线程退出：SDL_WaitThread。如下示例:
SDL_WaitThread(t, NULL);
(3)SDL条件变量(旗子暗记量)：SDL_CreateCond/SDL_DestroyCond。如下示例:
s_cond = SDL_CreateCond();SDL_DestroyCond(s_cond);
旗子暗记量是为了让各个线程之间，能够相互互助。能够起到一个关照的浸染。
(4)SDL互斥锁创建与销毁：SDL_CreateMutex/SDL_DestroyMutex。如下示例:
s_lock = SDL_CreateMutex();SDL_DestroyMutex(s_lock);
(5)SDL锁定互斥：SDL_LockMutex/SDL_UnlockMutex。如下示例:
SDL_LockMutex(s_lock);SDL_UnlockMutex(s_lock);
(6)SDL条件变量(旗子暗记量)等待/关照：SDL_CondWait/SDL_CondSignal。如下示例:
SDL_CondWait(s_cond, s_lock);SDL_CondSignal(s_cond);
如下示例代码:
主线程:
子线程:
实行结果:
创建子线程，获取到锁，先休眠10s。再等待条件变量。这时候主线程实行，运行4s后，获取锁，此时子线程还拿着锁(此时还是一个condwait状态)，此时主线程就要等待子线程开释锁。主线程获取锁后，主线程就会实行它自己的事情，实行完后，就会发送旗子暗记，唤醒等待的子线程，紧接着主线程开释锁，这两个动作都是很主要，否则会涌现问题。
把稳:如果主线程不发送旗子暗记，子线程是不会被唤醒。以是发送旗子暗记和开释lock，是两部必须的动作。
11.总结
本文紧张讲解了SDL的事理，框架，用示例代码解释了SDL的显示，绘制图形，事宜，线程，锁等主要知识，这些都是在音视频开拓有主要的利用了，但是远不止这些功能，如果要更学习其它功能，就须要根据官网供应的文档和wiki连续学习。欢迎关注，收藏，转发，分享。
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