Locking a GDI+ Bitmap in Native C++?

Question

I can find many examples on how to do this in managed c++ but none for unmanaged.

I want to get all the pixel data as efficiently as possible, but some of the scan0 stuff I would need more info about so I can properly iterate through the pixel data and get each rgba value from it.

right now I have this:

  Bitmap *b = new Bitmap(filename);
  if(b == NULL)
  {
   return 0;
  }

  UINT w,h;
  w = b->GetWidth();
  h = b->GetHeight();
  Rect *r = new Rect(0,0,w,h);
  BitmapData *lockdat;
 b->LockBits(r,ImageLockModeRead,PixelFormatDontCare,lockdat);

  delete(r);
  if(w == 0 && h == 0)
  {
   return 0;
  }

  Color c;

  std::vector<GLubyte> pdata(w * h * 4,0.0);

  for (unsigned int i = 0; i < h; i++) {
   for (unsigned int j = 0; j < w; j++) {
    b->GetPixel(j,i,&c);

    pdata[i * 4 * w + j * 4 + 0] = (GLubyte) c.GetR();
    pdata[i * 4 * w + j * 4 + 1] = (GLubyte) c.GetG();
    pdata[i * 4 * w + j * 4 + 2] = (GLubyte) c.GetB();
    pdata[i * 4 * w + j * 4 + 3] = (GLubyte) c.GetA();
   }
  }
  delete(b);

  return CreateTexture(pdata,w,h);

How do I use lockdat to do the equivalent of getpixel? Thanks

Answer 1

lockdat->Scan0 is a pointer to the pixel data of the bitmap. Note that you really do care what pixel format you ask for, PixelFormatDontCare won't do. Because how you use the pointer is affected by the pixel format. PixelFormat32bppARGB is the easiest, one pixel will be the size of an int, 4 bytes representing alpha, red, green and blue. And the stride will be equal to the width of the bitmap. Making it likely that a simple memcpy() will get the job done. Beware the bitmaps are stored upside-down.

Answer 2

Bitmap *m_image = new Bitmap(...) // a 24-bit RGB bitmap

BitmapData bmData;
Rect rect(0, 0, m_image->GetWidth(), m_image->GetHeight());
m_image->LockBits(&rect , ImageLockModeRead , PixelFormat24bppRGB,&bmData  );
memcpy(your_bytes_buffer, bmData.Scan0, min(bmData.Height * bmData.Stride, your_buffer_size));
m_image->UnlockBits(&bmData);