SDL2源码分析5：更新纹理（SDL

int SDLCALL SDL_UpdateTexture(SDL_Texture * texture,

                                          const SDL_Rect * rect,

                                              const void *pixels, int pitch);

參数的含义例如以下。

texture：目标纹理。

rect：更新像素的矩形区域。设置为NULL的时候更新整个区域。

pixels：像素数据。

pitch：一行像素数据的字节数。

成功的话返回0，失败的话返回-1。

函数调用关系图

SDL_UpdateTexture()关键函数的调用关系能够用下图表示。

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上面的图片不太清晰。更清晰的图片上传到了相冊里面：

http://my.csdn.net/leixiaohua1020/album/detail/1793769

把相冊里面的图片保存下来就能够得到清晰的图片。

源码分析

SDL_UpdateTexture()的源码位于render\SDL_render.c中。例如以下所看到的。

int SDL_UpdateTexture(SDL_Texture * texture, const SDL_Rect * rect,

                  const void *pixels, int pitch)

{

    SDL_Renderer *renderer;

    SDL_Rect full_rect;

    CHECK_TEXTURE_MAGIC(texture, -1);

    if (!pixels) {

        return SDL_InvalidParamError("pixels");

    }

    if (!pitch) {

        return SDL_InvalidParamError("pitch");

    }

    if (!rect) {

        full_rect.x = 0;

        full_rect.y = 0;

        full_rect.w = texture->w;

        full_rect.h = texture->h;

        rect = &full_rect;

    }

    if (texture->yuv) {

        return SDL_UpdateTextureYUV(texture, rect, pixels, pitch);

    } else if (texture->native) {

        return SDL_UpdateTextureNative(texture, rect, pixels, pitch);

    } else {

        renderer = texture->renderer;

        return renderer->UpdateTexture(renderer, texture, rect, pixels, pitch);

    }

}

从源码中能够看出，SDL_UpdateTexture()的大致流程例如以下。

1. 检查输入參数的合理性。比如像素格式是否支持，宽和高是否小于等于0等等。
2. 假设是一些特殊的格式，进行一定的处理：

a) 假设输入的像素数据是YUV格式的，则会调用SDL_UpdateTextureYUV()进行处理。

b) 假设输入的像素数据的像素格式不是渲染器支持的格式，则会调用SDL_UpdateTextureNative()进行处理。

3. 调用SDL_Render的UpdateTexture()方法更新纹理。

这一步是整个函数的核心。
以下我们具体看一下几种不同的渲染器的UpdateTexture ()的方法。

1. Direct3D

Direct3D 渲染器中相应UpdateTexture ()的函数是D3D_UpdateTexture()，它的源码例如以下所看到的（位于render\direct3d\SDL_render_d3d.c）。

static int

D3D_UpdateTexture(SDL_Renderer * renderer, SDL_Texture * texture,

                  const SDL_Rect * rect, const void *pixels, int pitch)

{

    D3D_TextureData *data = (D3D_TextureData *) texture->driverdata;

    SDL_bool full_texture = SDL_FALSE;

#ifdef USE_DYNAMIC_TEXTURE

    if (texture->access == SDL_TEXTUREACCESS_STREAMING &&

        rect->x == 0 && rect->y == 0 &&

        rect->w == texture->w && rect->h == texture->h) {

        full_texture = SDL_TRUE;

    }

#endif

    if (!data) {

        SDL_SetError("Texture is not currently available");

        return -1;

    }

    if (D3D_UpdateTextureInternal(data->texture, texture->format, full_texture, rect->x, rect->y, rect->w, rect->h, pixels, pitch) < 0) {

        return -1;

    }

    if (data->yuv) {

        /* Skip to the correct offset into the next texture */

        pixels = (const void*)((const Uint8*)pixels + rect->h * pitch);

        if (D3D_UpdateTextureInternal(texture->format == SDL_PIXELFORMAT_YV12 ? data->vtexture : data->utexture, texture->format, full_texture, rect->x / 2, rect->y / 2, rect->w / 2, rect->h / 2, pixels, pitch / 2) < 0) {

            return -1;

        }

        /* Skip to the correct offset into the next texture */

        pixels = (const void*)((const Uint8*)pixels + (rect->h * pitch)/4);

        if (D3D_UpdateTextureInternal(texture->format == SDL_PIXELFORMAT_YV12 ?

data->utexture : data->vtexture, texture->format, full_texture, rect->x / 2, rect->y / 2, rect->w / 2, rect->h / 2, pixels, pitch / 2) < 0) {

            return -1;

        }

    }

    return 0;

}

从代码中能够看出，该函数调用了D3D_UpdateTextureInternal()函数。在这里须要注意，假设输入像素格式是YUV。就会使用3个纹理，对于多出的那2个纹理会单独进行处理。调用的函数D3D_UpdateTextureInternal()代码例如以下。

static int D3D_UpdateTextureInternal(IDirect3DTexture9 *texture, Uint32 format, SDL_bool full_texture, int x, int y, int w, int h, const void *pixels, int pitch)

{

    RECT d3drect;

    D3DLOCKED_RECT locked;

    const Uint8 *src;

    Uint8 *dst;

    int row, length;

    HRESULT result;

    if (full_texture) {

        result = IDirect3DTexture9_LockRect(texture, 0, &locked, NULL, D3DLOCK_DISCARD);

    } else {

        d3drect.left = x;

        d3drect.right = x + w;

        d3drect.top = y;

        d3drect.bottom = y + h;

        result = IDirect3DTexture9_LockRect(texture, 0, &locked, &d3drect, 0);

    }

    if (FAILED(result)) {

        return D3D_SetError("LockRect()", result);

    }

    src = (const Uint8 *)pixels;

    dst = locked.pBits;

    length = w * SDL_BYTESPERPIXEL(format);

    if (length == pitch && length == locked.Pitch) {

        SDL_memcpy(dst, src, length*h);

    } else {

        if (length > pitch) {

            length = pitch;

        }

        if (length > locked.Pitch) {

            length = locked.Pitch;

        }

        for (row = 0; row < h; ++row) {

            SDL_memcpy(dst, src, length);

            src += pitch;

            dst += locked.Pitch;

        }

    }

    IDirect3DTexture9_UnlockRect(texture, 0);

    return 0;

}

从代码中能够看出，该函数首先调用IDirect3DTexture9_LockRect()锁定纹理。然后使用SDL_memcpy()将新的像素数据拷贝至纹理（SDL_memcpy()实际上就是memcpy()），最后使用IDirect3DTexture9_UnlockRect()解锁纹理。

2. OpenGL

OpenGL渲染器中相应UpdateTexture()的函数是GL_UpdateTexture()。它的源码例如以下所看到的（位于render\opengl\SDL_render_gl.c）。

static int GL_UpdateTexture(SDL_Renderer * renderer, SDL_Texture * texture,

                 const SDL_Rect * rect, const void *pixels, int pitch)

{

    GL_RenderData *renderdata = (GL_RenderData *) renderer->driverdata;

    GL_TextureData *data = (GL_TextureData *) texture->driverdata;

    GL_ActivateRenderer(renderer);

    renderdata->glEnable(data->type);

    renderdata->glBindTexture(data->type, data->texture);

    renderdata->glPixelStorei(GL_UNPACK_ALIGNMENT, 1);

    renderdata->glPixelStorei(GL_UNPACK_ROW_LENGTH,

                              (pitch / SDL_BYTESPERPIXEL(texture->format)));

    renderdata->glTexSubImage2D(data->type, 0, rect->x, rect->y, rect->w,

                                rect->h, data->format, data->formattype,

                                pixels);

    if (data->yuv) {

        renderdata->glPixelStorei(GL_UNPACK_ROW_LENGTH, (pitch / 2));

        /* Skip to the correct offset into the next texture */

        pixels = (const void*)((const Uint8*)pixels + rect->h * pitch);

        if (texture->format == SDL_PIXELFORMAT_YV12) {

            renderdata->glBindTexture(data->type, data->vtexture);

        } else {

            renderdata->glBindTexture(data->type, data->utexture);

        }

        renderdata->glTexSubImage2D(data->type, 0, rect->x/2, rect->y/2,

                                    rect->w/2, rect->h/2,

                                    data->format, data->formattype, pixels);

        /* Skip to the correct offset into the next texture */

        pixels = (const void*)((const Uint8*)pixels + (rect->h * pitch)/4);

        if (texture->format == SDL_PIXELFORMAT_YV12) {

            renderdata->glBindTexture(data->type, data->utexture);

        } else {

            renderdata->glBindTexture(data->type, data->vtexture);

        }

        renderdata->glTexSubImage2D(data->type, 0, rect->x/2, rect->y/2,

                                    rect->w/2, rect->h/2,

                                    data->format, data->formattype, pixels);

    }

    renderdata->glDisable(data->type);

    return GL_CheckError("glTexSubImage2D()", renderer);

}

从代码中能够看出。该函数调用了OpenGL的API函数glBindTexture ()，glTexSubImage2D()等更新了一个纹理。
在这里有一点须要注意，假设输入像素格式是YUV，就会使用3个纹理，对于多出的那2个纹理会单独进行处理。

3. Software

Software渲染器中相应UpdateTexture()的函数是SW_UpdateTexture()。它的源码例如以下所看到的（位于render\software\SDL_render_sw.c）。

static int SW_UpdateTexture(SDL_Renderer * renderer, SDL_Texture * texture,

                 const SDL_Rect * rect, const void *pixels, int pitch)

{

    SDL_Surface *surface = (SDL_Surface *) texture->driverdata;

    Uint8 *src, *dst;

    int row;

    size_t length;

    if(SDL_MUSTLOCK(surface))

        SDL_LockSurface(surface);

    src = (Uint8 *) pixels;

    dst = (Uint8 *) surface->pixels +

                        rect->y * surface->pitch +

                        rect->x * surface->format->BytesPerPixel;

    length = rect->w * surface->format->BytesPerPixel;

    for (row = 0; row < rect->h; ++row) {

        SDL_memcpy(dst, src, length);

        src += pitch;

        dst += surface->pitch;

    }

    if(SDL_MUSTLOCK(surface))

        SDL_UnlockSurface(surface);

    return 0;

}

该函数的源码还没有具体分析。当中最关键的函数要数SDL_memcpy()了，正是这个函数更新了纹理的像素数据。可是Software渲染器纹理改动的时候是否须要Lock()和Unlock()呢？这一点一直也没太搞清。