Function OverlayImage

libraries/lib-theme/ImageManipulation.cpp:111–170

Takes a background image, foreground image, and mask (i.e. the alpha channel for the foreground), and returns a NEW image where the foreground has been overlaid onto the background using alpha-blending, at location (xoff, yoff).

/// overlaid onto the background using alpha-blending,
/// at location (xoff, yoff).
std::unique_ptr<wxImage> OverlayImage(wxImage * background, wxImage * foreground,
                      wxImage * mask, int xoff, int yoff)
{
   unsigned char *bg = background->GetData();
   unsigned char *fg = foreground->GetData();
   unsigned char *mk = mask->GetData();

   int bgWidth = background->GetWidth();
   int bgHeight = background->GetHeight();
   int fgWidth = foreground->GetWidth();
   int fgHeight = foreground->GetHeight();
   int mkWidth = mask->GetWidth();
   int mkHeight = mask->GetHeight();


   //Now, determine the dimensions of the images to be masked together
   //on top of the background.  This should be equal to the area of the
   //smaller of the foreground and the mask, as long as it is
   //within the area of the background, given the offset.

   //Make sure the foreground size is no bigger than the mask
   int wCutoff = (fgWidth < mkWidth) ? fgWidth : mkWidth;
   int hCutoff = (fgHeight < mkHeight) ? fgHeight : mkHeight;


   // If the masked foreground + offset is bigger than the background, masking
   // should only occur within these bounds of the foreground image
   wCutoff = (bgWidth - xoff > wCutoff) ? wCutoff : bgWidth - xoff;
   hCutoff = (bgHeight - yoff > hCutoff) ? hCutoff : bgHeight - yoff;


   //Make a NEW image the size of the background
   auto dstImage = std::make_unique<wxImage>(bgWidth, bgHeight);
   unsigned char *dst = dstImage->GetData();
   memcpy(dst, bg, bgWidth * bgHeight * 3);


   // Go through the foreground image bit by bit and mask it on to the
   // background, at an offset of xoff,yoff.
   // BUT...Don't go beyond the size of the background image,
   // the foreground image, or the mask
   int x, y;
   for (y = 0; y < hCutoff; y++) {

      unsigned char *bkp = bg + 3 * ((y + yoff) * bgWidth + xoff);
      unsigned char *dstp = dst + 3 * ((y + yoff) * bgWidth + xoff);

      for (x = 0; x < wCutoff; x++) {

         int value = mk[3 * (y * mkWidth + x)];
         int opp = 255 - value;

         for (int c = 0; c < 3; c++)
            dstp[x * 3 + c] =
               ((bkp[x * 3 + c] * opp) +
                (fg[3 * (y * fgWidth + x) + c] * value)) / 255;
      }
   }