获取 SDL_Texture 中单个像素的 SDL_Color

Question

我在寻找有关如何检索 SDL_Texture 上像素的特定颜色的解决方案时遇到了一些问题... 更具体地说：我正在尝试计算给定纹理中使用的平均颜色量。稍后我想将红色像素的数量除以像素的总数。对于这个任务，我需要一个方法，它可以让我获得每个像素的颜色......

我试图搜索一些功能，但不幸的是我无法弄清楚.. 我看到了 SDL_RenderReadPixels 和 SDL_GetPixelFormatName 之类的方法，但这些方法都没有帮助我...

你有解决办法吗？

Answer 1

要访问 SDL_Texture 的像素，您必须使用 SDL_CreateTexture() 创建一个空白纹理并为访问参数传入 SDL_TEXTUREACCESS_STREAMING，然后将表面的像素复制到其中。完成后，您可以使用 SDL_LockTexture() 函数检索指向像素数据的指针，然后可以访问和修改该像素数据。要保存您的更改，您需要调用 SDL_UnlockTexture()。试试这样的：

SDL_Texture *t;

int main()
{
    // Init SDL 

    SDL_Surface * img = IMG_Load("path/to/file");
    SDL_CreateTexture(renderer, SDL_PIXELFORMAT_RGBA8888, SDL_TEXTUREACCESS_STREAMING, img->w, img->h);

    void * pixels;

    SDL_LockTexture(t, &img->clip_rect, &pixels, img->pitch);

    memcpy(pixels, img->pixels, img->w * img->h);

    Uint32 * upixels = (Uint32 *) pixels;

    // get or modify pixels

    SDL_UnlockTexture(t);

    return 0;
}

Uint32 get_pixel_at(Uint32 * pixels, int x, int y, int w)
{
    return pixels[y * w + x];
}

您可以像这样从像素中获取颜色：

Uint32 pixel = get_pixel_at(pixels, x, y, img->w);
Uint8 * colors = (Uint8 *) pixel;

// colors[0] is red, 1 is green, 2 is blue, 3 is alpha (assuming you've set the blend mode on the texture to SDL_BLENDMODE_BLEND

如果您想了解更多信息，请查看以下 SDL 2.0 教程：http://lazyfoo.net/tutorials/SDL/index.php。教程 40 专门处理这个问题。

如果您有任何问题或不清楚的地方，请告诉我。

祝你好运！

Answer 2

对于 SDL 2，您可以使用它从表面复制像素。获得像素后，您可以从中提取颜色（参见下面的示例）。

/*
Copies the pixels from a SDL2 surface.
You should free() the returned pixels when you're done with it.
*/
Uint8* copySurfacePixels(
  SDL_Surface* surface,  // surface to take pixels from
  Uint32 pixelFormat,    // usually SDL_GetWindowPixelFormat(window)
  SDL_Renderer* renderer,// main SDL2 renderer
  int* width,            // stores result width
  int* height,           // stores result height
  int* pitch)            // stores result pitch
{
  Uint8* pixels = 0;
  SDL_Surface* tmpSurface = 0;
  SDL_Texture* texture = 0;
  int sizeInBytes = 0;
  void* tmpPixels = 0;
  int tmpPitch = 0;

  tmpSurface = SDL_ConvertSurfaceFormat(surface, pixelFormat, 0);
  if (tmpSurface) {
    texture = SDL_CreateTexture( renderer, pixelFormat,
                                 SDL_TEXTUREACCESS_STREAMING,
                                 tmpSurface->w, tmpSurface->h );
  }

  if (texture) {
    if (width) {
      *width = tmpSurface->w;
    }
    if (height) {
      *height = tmpSurface->h;
    }
    if (pitch) {
      *pitch = tmpSurface->pitch;
    }
    sizeInBytes = tmpSurface->pitch * tmpSurface->h;
    pixels = (Uint8*)malloc( sizeInBytes );
    SDL_LockTexture( texture, 0, &tmpPixels, &tmpPitch );
    memcpy( pixels, tmpSurface->pixels, sizeInBytes);
    SDL_UnlockTexture( texture );
  }

  // Cleanup
  if (texture) {
    SDL_DestroyTexture(texture);
  }
  if (tmpSurface) {
    SDL_FreeSurface(tmpSurface);
  }

  return pixels;
}

示例用法。加载位图并在 (1,1) 处打印 RGB 颜色

  SDL_Surface* surface = SDL_LoadBMP("..\\res\\test.bmp");
  Uint32 pf = SDL_GetWindowPixelFormat(window);
  int w=0, h=0, p=0;
  Uint8* pixels = copySurfacePixels(surface, pf, renderer, &w, &h, &p);
  if (pixels)
  {
    printf("width=%d, height=%d, pitch=%d\n", w, h, p);
    
    // Print color at (1,1)
    int x=1, y=1;

    // Assuming BGRA format
    int b = pixels[4 * (y * w + x) + 0]; // Blue
    int g = pixels[4 * (y * w + x) + 1]; // Green
    int r = pixels[4 * (y * w + x) + 2]; // Red
    int a = pixels[4 * (y * w + x) + 3]; // Alpha
    printf("Pixel at (%d,%d) has RGBA color (%d,%d,%d,%d)\n", x, y, r, g, b, a);
  }
  free(pixels);