GetDIBits 返回所有值为 0 的数组

Question

我需要在屏幕上找到一些图像。我决定做一个简单的比较循环。

我发现this answer似乎有帮助，写了下一段代码：

void Capt()
{
  HDC hdcSource = GetDC(GetDesktopWindow()); // the source device context
  HDC hdc = CreateCompatibleDC(hdcSource);
  HBITMAP hSource = CreateCompatibleBitmap(hdcSource, xw, yw); // the bitmap selected into the device context

  SelectObject(hdc, hSource);

  BITMAPINFO MyBMInfo = { 0 };
  MyBMInfo.bmiHeader.biSize = sizeof(MyBMInfo.bmiHeader);

  // Get the BITMAPINFO structure from the bitmap
  if (0 == GetDIBits(hdc, hSource, 0, 0, NULL, &MyBMInfo, DIB_RGB_COLORS))
  {
    mb("error1");
    IA(GetLastError());
  }

  // create the pixel buffer
  BYTE* lpPixels = new BYTE[MyBMInfo.bmiHeader.biSizeImage];

  // We'll change the received BITMAPINFOHEADER to request the data in a
  // 32 bit RGB format (and not upside-down) so that we can iterate over
  // the pixels easily. 

  // requesting a 32 bit image means that no stride/padding will be necessary,
  // although it always contains an (possibly unused) alpha channel
  MyBMInfo.bmiHeader.biBitCount = 32;
  MyBMInfo.bmiHeader.biCompression = BI_RGB;  // no compression -> easier to use
  // correct the bottom-up ordering of lines (abs is in cstdblib and stdlib.h)
  MyBMInfo.bmiHeader.biHeight = abs(MyBMInfo.bmiHeader.biHeight);

  // Call GetDIBits a second time, this time to (format and) store the actual
  // bitmap data (the "pixels") in the buffer lpPixels
  if (0 == GetDIBits(hdc, hSource, 0, MyBMInfo.bmiHeader.biHeight,
    lpPixels, &MyBMInfo, DIB_RGB_COLORS))
  {
    mb("error2");
    IA(GetLastError());
  }

  DeleteObject(hSource);
  ReleaseDC(NULL, hdcSource);


  for (int i = 0, j=0; i < MyBMInfo.bmiHeader.biSizeImage&&j<100; i++)
  {
    if (lpPixels[i] != 0)
    {
      char buf[1024] = {};
      _itoa_s(lpPixels[i], buf, 10);
  
      //output
    }
  }
}

两个问题：

1. 我的屏幕分辨率是 1280x800=1 024 000 px，MyBMInfo.bmiHeader.biSizeImage 等于 4 096 000。是 rgba 还是什么？
2. 主要问题：虽然，没有错误，而且我在上一个问题中提到的值不为零，但是在代码底部的循环中，lpPixels的所有值都是零，我没有得到任何输出。为什么？

Answer 1

对于不透明的屏幕数据，可以简单地将第四个字节（Alpha）设置为255，即不透明。

GetDIBits得到的位图数据的颜色格式写在BGR中（lpPixels参数）。如果我们想用 RGB 格式将数据分配给一个字节数组，我们需要进行一些转换。

e.p.

for (size_t i = 0; i < myWidth * myHeight; i++)
    {
        size_t tempIndex = (i * 4);

        // The colors are in BGR-format (windows format)
        // Hence, the order is reversed
        rgbPixels[tempIndex] = bgrPixels[tempIndex + 2]; // r
        rgbPixels[tempIndex + 1] = bgrPixels[tempIndex + 1]; // g
        rgbPixels[tempIndex + 2] = bgrPixels[tempIndex]; // b
        rgbPixels[tempIndex + 3] = 255; // a (always 255)
    }

更新：

HDC hdc, hdcTemp;
RECT rect;
BYTE* bitPointer;
int x, y;
int red, green, blue, alpha;

hdc = GetDC(HWND_DESKTOP);
GetWindowRect(hWND_Desktop, &rect);
int MAX_WIDTH = rect.right;
int MAX_HEIGHT = rect.bottom;

hdcTemp = CreateCompatibleDC(hdc);
BITMAPINFO bitmap;
bitmap.bmiHeader.biSize = sizeof(bitmap.bmiHeader);
bitmap.bmiHeader.biWidth = MAX_WIDTH;
bitmap.bmiHeader.biHeight = MAX_HEIGHT;
bitmap.bmiHeader.biPlanes = 1;
bitmap.bmiHeader.biBitCount = 32;
bitmap.bmiHeader.biCompression = BI_RGB;
bitmap.bmiHeader.biSizeImage = MAX_WIDTH * 4 * MAX_HEIGHT;
bitmap.bmiHeader.biClrUsed = 0;
bitmap.bmiHeader.biClrImportant = 0;
HBITMAP hBitmap2 = CreateDIBSection(hdcTemp, &bitmap, DIB_RGB_COLORS, (void**)(&bitPointer), NULL, NULL);
SelectObject(hdcTemp, hBitmap2);
BitBlt(hdcTemp, 0, 0, MAX_WIDTH, MAX_HEIGHT, hdc, 0, 0, SRCCOPY);

for (int i=0; i<(MAX_WIDTH * 4 * MAX_HEIGHT); i+=4)
{
    red = (int)bitPointer[i];
    green = (int)bitPointer[i+1];
    blue = (int)bitPointer[i+2];
    alpha = (int)bitPointer[i+3];

}