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SDL源代码分析系列文章列表：
SDL2源代码分析5：更新纹理（SDL_UpdateTexture()）
SDL2源代码分析6：复制到渲染器（SDL_RenderCopy()）
SDL2源代码分析7：显示（SDL_RenderPresent()）
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上一篇文章分析了SDL中创建渲染器的函数SDL_CreateRenderer()。这篇文章继续分析SDL的源代码。本文分析SDL的纹理（SDL_Texture）。
SDL播放视频的代码流程如下所示。
初始化:
SDL_Init(): 初始化SDL。循环渲染数据:
SDL_CreateWindow(): 创建窗口（Window）。
SDL_CreateRenderer(): 基于窗口创建渲染器（Render）。
SDL_CreateTexture(): 创建纹理（Texture）。
SDL_UpdateTexture(): 设置纹理的数据。上篇文章分析了该流程中的第3个函数SDL_CreateRenderer()。本文继续分析该流程中的第4个函数SDL_CreateTexture()。
SDL_RenderCopy(): 纹理复制给渲染器。
SDL_RenderPresent(): 显示。
SDL_Texture
SDL_Texture结构定义了一个SDL中的纹理。如果直接使用SDL2编译好的SDK的话，是看不到SDL_Texture的内部结构的。有关它的定义在头文件中只有一行代码，如下所示。
/** * \brief An efficient driver-specific representation of pixel data */struct SDL_Texture;typedef struct SDL_Texture SDL_Texture;在源代码工程中可以看到SDL_Texture的定义，位于render\SDL_sysrender.h文件中。它的定义如下。
/* Define the SDL texture structure */struct SDL_Texture{ const void *magic; Uint32 format; /**< The pixel format of the texture */ int access; /**< SDL_TextureAccess */ int w; /**< The width of the texture */ int h; /**< The height of the texture */ int modMode; /**< The texture modulation mode */ SDL_BlendMode blendMode; /**< The texture blend mode */ Uint8 r, g, b, a; /**< Texture modulation values */ SDL_Renderer *renderer; /* Support for formats not supported directly by the renderer */ SDL_Texture *native; SDL_SW_YUVTexture *yuv; void *pixels; int pitch; SDL_Rect locked_rect; void *driverdata; /**< Driver specific texture representation */ SDL_Texture *prev; SDL_Texture *next;};可以看出其中包含了一个“纹理”所具备的各种属性。下面来看看如何创建这个SDL_Texture。
SDL_CreateTexture()
函数简介
使用SDL_CreateTexture()基于渲染器创建一个纹理。SDL_CreateTexture()的原型如下。SDL_Texture * SDLCALL SDL_CreateTexture(SDL_Renderer * renderer, Uint32 format, int access, int w, int h);参数的含义如下。
renderer：目标渲染器。
format ：纹理的格式。后面会详述。access ：可以取以下值（定义位于SDL_TextureAccess中）
SDL_TEXTUREACCESS_STATIC ：变化极少SDL_TEXTUREACCESS_STREAMING ：变化频繁
SDL_TEXTUREACCESS_TARGET ：暂时没有理解
w ：纹理的宽
h ：纹理的高
创建成功则返回纹理的ID，失败返回0。
函数调用关系图
SDL_ CreateTexture ()关键函数的调用关系可以用下图表示。
上面的图片不太清晰，更清晰的图片上传到了相册里面：
http://my.csdn.net/leixiaohua1020/album/detail/1793543
把相册里面的图片保存下来就可以得到清晰的图片了。
源代码分析
SDL_CreateTexture()的源代码位于render\SDL_render.c中。如下所示。
SDL_Texture * SDL_CreateTexture(SDL_Renderer * renderer, Uint32 format, int access, int w, int h){ SDL_Texture *texture; CHECK_RENDERER_MAGIC(renderer, NULL); if (!format) { format = renderer->info.texture_formats[0]; } if (SDL_ISPIXELFORMAT_INDEXED(format)) { SDL_SetError("Palettized textures are not supported"); return NULL; } if (w <= 0 || h <= 0) { SDL_SetError("Texture dimensions can't be 0"); return NULL; } if ((renderer->info.max_texture_width && w > renderer->info.max_texture_width) || (renderer->info.max_texture_height && h > renderer->info.max_texture_height)) { SDL_SetError("Texture dimensions are limited to %dx%d", renderer->info.max_texture_width, renderer->info.max_texture_height); return NULL; } texture = (SDL_Texture *) SDL_calloc(1, sizeof(*texture)); if (!texture) { SDL_OutOfMemory(); return NULL; } texture->magic = &texture_magic; texture->format = format; texture->access = access; texture->w = w; texture->h = h; texture->r = 255; texture->g = 255; texture->b = 255; texture->a = 255; texture->renderer = renderer; texture->next = renderer->textures; if (renderer->textures) { renderer->textures->prev = texture; } renderer->textures = texture; if (IsSupportedFormat(renderer, format)) { if (renderer->CreateTexture(renderer, texture) < 0) { SDL_DestroyTexture(texture); return 0; } } else { texture->native = SDL_CreateTexture(renderer, GetClosestSupportedFormat(renderer, format), access, w, h); if (!texture->native) { SDL_DestroyTexture(texture); return NULL; } /* Swap textures to have texture before texture->native in the list */ texture->native->next = texture->next; if (texture->native->next) { texture->native->next->prev = texture->native; } texture->prev = texture->native->prev; if (texture->prev) { texture->prev->next = texture; } texture->native->prev = texture; texture->next = texture->native; renderer->textures = texture; if (SDL_ISPIXELFORMAT_FOURCC(texture->format)) { texture->yuv = SDL_SW_CreateYUVTexture(format, w, h); if (!texture->yuv) { SDL_DestroyTexture(texture); return NULL; } } else if (access == SDL_TEXTUREACCESS_STREAMING) { /* The pitch is 4 byte aligned */ texture->pitch = (((w * SDL_BYTESPERPIXEL(format)) + 3) & ~3); texture->pixels = SDL_calloc(1, texture->pitch * h); if (!texture->pixels) { SDL_DestroyTexture(texture); return NULL; } } } return texture;}从源代码中可以看出，SDL_CreateTexture()的大致流程如下。
1. 检查输入参数的合理性。例如像素格式是否支持，宽和高是否小于等于0等等。
2. 新建一个SDL_Texture。调用SDL_calloc()（实际上就是calloc()）为新建的SDL_Texture分配内存。
3. 调用SDL_Render的CreateTexture()方法创建纹理。这一步是整个函数的核心。
下面我们详细看一下几种不同的渲染器的CreateTexture()的方法。
1. Direct3D
Direct3D 渲染器中对应CreateTexture()的函数是D3D_CreateTexture()，它的源代码如下所示（位于render\direct3d\SDL_render_d3d.c）。static int D3D_CreateTexture(SDL_Renderer * renderer, SDL_Texture * texture){ D3D_RenderData *renderdata = (D3D_RenderData *) renderer->driverdata; D3D_TextureData *data; D3DPOOL pool; DWORD usage; HRESULT result; data = (D3D_TextureData *) SDL_calloc(1, sizeof(*data)); if (!data) { return SDL_OutOfMemory(); } data->scaleMode = GetScaleQuality(); texture->driverdata = data;#ifdef USE_DYNAMIC_TEXTURE if (texture->access == SDL_TEXTUREACCESS_STREAMING) { pool = D3DPOOL_DEFAULT; usage = D3DUSAGE_DYNAMIC; } else#endif if (texture->access == SDL_TEXTUREACCESS_TARGET) { /* D3DPOOL_MANAGED does not work with D3DUSAGE_RENDERTARGET */ pool = D3DPOOL_DEFAULT; usage = D3DUSAGE_RENDERTARGET; } else { pool = D3DPOOL_MANAGED; usage = 0; } result = IDirect3DDevice9_CreateTexture(renderdata->device, texture->w, texture->h, 1, usage, PixelFormatToD3DFMT(texture->format), pool, &data->texture, NULL); if (FAILED(result)) { return D3D_SetError("CreateTexture()", result); } if (texture->format == SDL_PIXELFORMAT_YV12 || texture->format == SDL_PIXELFORMAT_IYUV) { data->yuv = SDL_TRUE; result = IDirect3DDevice9_CreateTexture(renderdata->device, texture->w / 2, texture->h / 2, 1, usage, PixelFormatToD3DFMT(texture->format), pool, &data->utexture, NULL); if (FAILED(result)) { return D3D_SetError("CreateTexture()", result); } result = IDirect3DDevice9_CreateTexture(renderdata->device, texture->w / 2, texture->h / 2, 1, usage, PixelFormatToD3DFMT(texture->format), pool, &data->vtexture, NULL); if (FAILED(result)) { return D3D_SetError("CreateTexture()", result); } } return 0;}从代码中可以看出，该函数调用了Direct3D的API函数IDirect3DDevice9_CreateTexture()创建了一个纹理。
2. OpenGL
OpenGL渲染器中对应CreateTexture()的函数是GL_CreateTexture ()，它的源代码如下所示（位于render\opengl\SDL_render_gl.c）。
static int GL_CreateTexture(SDL_Renderer * renderer, SDL_Texture * texture){ GL_RenderData *renderdata = (GL_RenderData *) renderer->driverdata; GL_TextureData *data; GLint internalFormat; GLenum format, type; int texture_w, texture_h; GLenum scaleMode; GL_ActivateRenderer(renderer); if (!convert_format(renderdata, texture->format, &internalFormat, &format, &type)) { return SDL_SetError("Texture format %s not supported by OpenGL", SDL_GetPixelFormatName(texture->format)); } data = (GL_TextureData *) SDL_calloc(1, sizeof(*data)); if (!data) { return SDL_OutOfMemory(); } if (texture->access == SDL_TEXTUREACCESS_STREAMING) { size_t size; data->pitch = texture->w * SDL_BYTESPERPIXEL(texture->format); size = texture->h * data->pitch; if (texture->format == SDL_PIXELFORMAT_YV12 || texture->format == SDL_PIXELFORMAT_IYUV) { /* Need to add size for the U and V planes */ size += (2 * (texture->h * data->pitch) / 4); } data->pixels = SDL_calloc(1, size); if (!data->pixels) { SDL_free(data); return SDL_OutOfMemory(); } } if (texture->access == SDL_TEXTUREACCESS_TARGET) { data->fbo = GL_GetFBO(renderdata, texture->w, texture->h); } else { data->fbo = NULL; } GL_CheckError("", renderer); renderdata->glGenTextures(1, &data->texture); if (GL_CheckError("glGenTexures()", renderer) < 0) { SDL_free(data); return -1; } texture->driverdata = data; if ((renderdata->GL_ARB_texture_rectangle_supported) /* && texture->access != SDL_TEXTUREACCESS_TARGET */){ data->type = GL_TEXTURE_RECTANGLE_ARB; texture_w = texture->w; texture_h = texture->h; data->texw = (GLfloat) texture_w; data->texh = (GLfloat) texture_h; } else { data->type = GL_TEXTURE_2D; texture_w = power_of_2(texture->w); texture_h = power_of_2(texture->h); data->texw = (GLfloat) (texture->w) / texture_w; data->texh = (GLfloat) texture->h / texture_h; } data->format = format; data->formattype = type; scaleMode = GetScaleQuality(); renderdata->glEnable(data->type); renderdata->glBindTexture(data->type, data->texture); renderdata->glTexParameteri(data->type, GL_TEXTURE_MIN_FILTER, scaleMode); renderdata->glTexParameteri(data->type, GL_TEXTURE_MAG_FILTER, scaleMode); /* According to the spec, CLAMP_TO_EDGE is the default for TEXTURE_RECTANGLE and setting it causes an INVALID_ENUM error in the latest NVidia drivers. */ if (data->type != GL_TEXTURE_RECTANGLE_ARB) { renderdata->glTexParameteri(data->type, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE); renderdata->glTexParameteri(data->type, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE); }#ifdef __MACOSX__#ifndef GL_TEXTURE_STORAGE_HINT_APPLE#define GL_TEXTURE_STORAGE_HINT_APPLE 0x85BC#endif#ifndef STORAGE_CACHED_APPLE#define STORAGE_CACHED_APPLE 0x85BE#endif#ifndef STORAGE_SHARED_APPLE#define STORAGE_SHARED_APPLE 0x85BF#endif if (texture->access == SDL_TEXTUREACCESS_STREAMING) { renderdata->glTexParameteri(data->type, GL_TEXTURE_STORAGE_HINT_APPLE, GL_STORAGE_SHARED_APPLE); } else { renderdata->glTexParameteri(data->type, GL_TEXTURE_STORAGE_HINT_APPLE, GL_STORAGE_CACHED_APPLE); } if (texture->access == SDL_TEXTUREACCESS_STREAMING && texture->format == SDL_PIXELFORMAT_ARGB8888 && (texture->w % 8) == 0) { renderdata->glPixelStorei(GL_UNPACK_CLIENT_STORAGE_APPLE, GL_TRUE); renderdata->glPixelStorei(GL_UNPACK_ALIGNMENT, 1); renderdata->glPixelStorei(GL_UNPACK_ROW_LENGTH, (data->pitch / SDL_BYTESPERPIXEL(texture->format))); renderdata->glTexImage2D(data->type, 0, internalFormat, texture_w, texture_h, 0, format, type, data->pixels); renderdata->glPixelStorei(GL_UNPACK_CLIENT_STORAGE_APPLE, GL_FALSE); } else#endif { renderdata->glTexImage2D(data->type, 0, internalFormat, texture_w, texture_h, 0, format, type, NULL); } renderdata->glDisable(data->type); if (GL_CheckError("glTexImage2D()", renderer) < 0) { return -1; } if (texture->format == SDL_PIXELFORMAT_YV12 || texture->format == SDL_PIXELFORMAT_IYUV) { data->yuv = SDL_TRUE; renderdata->glGenTextures(1, &data->utexture); renderdata->glGenTextures(1, &data->vtexture); renderdata->glEnable(data->type); renderdata->glBindTexture(data->type, data->utexture); renderdata->glTexParameteri(data->type, GL_TEXTURE_MIN_FILTER, scaleMode); renderdata->glTexParameteri(data->type, GL_TEXTURE_MAG_FILTER, scaleMode); renderdata->glTexParameteri(data->type, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE); renderdata->glTexParameteri(data->type, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE); renderdata->glTexImage2D(data->type, 0, internalFormat, texture_w/2, texture_h/2, 0, format, type, NULL); renderdata->glBindTexture(data->type, data->vtexture); renderdata->glTexParameteri(data->type, GL_TEXTURE_MIN_FILTER, scaleMode); renderdata->glTexParameteri(data->type, GL_TEXTURE_MAG_FILTER, scaleMode); renderdata->glTexParameteri(data->type, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE); renderdata->glTexParameteri(data->type, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE); renderdata->glTexImage2D(data->type, 0, internalFormat, texture_w/2, texture_h/2, 0, format, type, NULL); renderdata->glDisable(data->type); } return GL_CheckError("", renderer);}从代码中可以看出，该函数调用了OpenGL的API函数glGenTextures()，glBindTexture()创建了一个纹理。并且使用glTexParameteri()设置了有关的一些参数。
在这里有一点需要注意，在OpenGL渲染器中，如果输入像素格式是YV12或者IYUV，就会使用3个纹理。
3. Software
Software渲染器中对应CreateTexture()的函数是SW_CreateTexture ()，它的源代码如下所示（位于render\software\SDL_render_sw.c）。
static int SW_CreateTexture(SDL_Renderer * renderer, SDL_Texture * texture){ int bpp; Uint32 Rmask, Gmask, Bmask, Amask; if (!SDL_PixelFormatEnumToMasks (texture->format, &bpp, &Rmask, &Gmask, &Bmask, &Amask)) { return SDL_SetError("Unknown texture format"); } texture->driverdata = SDL_CreateRGBSurface(0, texture->w, texture->h, bpp, Rmask, Gmask, Bmask, Amask); SDL_SetSurfaceColorMod(texture->driverdata, texture->r, texture->g, texture->b); SDL_SetSurfaceAlphaMod(texture->driverdata, texture->a); SDL_SetSurfaceBlendMode(texture->driverdata, texture->blendMode); if (texture->access == SDL_TEXTUREACCESS_STATIC) { SDL_SetSurfaceRLE(texture->driverdata, 1); } if (!texture->driverdata) { return -1; } return 0;}该函数的源代码还没有详细分析。可以看出其中调用了SDL_CreateRGBSurface()创建了“Surface”。
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