Re: Create a mask from transparent PNG

Once you get it into a bitmap, this should explain how to create a mask, and
also how to draw the image.

Also take a look at CDC:TransparentBlt, and if you don't mind using GDI+
CImage::TransparentBlt.

 //this function will draw a bitmap in a dc with all the pixels with
 //cTransparentColor as transparent
void DrawTransparentBitmap(HDC hdc, HBITMAP hBitmap, long xStart,
                           long yStart, long Width, long Height,
                           COLORREF cTransparentColor)
{
BITMAP bm;
COLORREF cColor;
HBITMAP bmAndBack, bmAndObject, bmAndMem, bmSave;
HBITMAP bmBackOld, bmObjectOld, bmMemOld, bmSaveOld;
HDC hdcMem, hdcBack, hdcObject, hdcTemp, hdcSave;
POINT ptSize;

   hdcTemp = CreateCompatibleDC(hdc);
   SelectObject(hdcTemp, hBitmap); // Select the bitmap

   GetObject(hBitmap, sizeof(BITMAP), (LPSTR)&bm);

   ptSize.x = bm.bmWidth; // Get width of bitmap
   ptSize.y = bm.bmHeight; // Get height of bitmap
   DPtoLP(hdcTemp, &ptSize, 1); // Convert from device
                                     // to logical points

   // Create some DCs to hold temporary data.
   hdcBack = CreateCompatibleDC(hdc);
   hdcObject = CreateCompatibleDC(hdc);
   hdcMem = CreateCompatibleDC(hdc);
   hdcSave = CreateCompatibleDC(hdc);

   // Create a bitmap for each DC. DCs are required for a number of
   // GDI functions.

   // Monochrome DC

   bmAndBack = CreateBitmap(ptSize.x, ptSize.y, 1, 1, NULL);

   // Monochrome DC
   bmAndObject = CreateBitmap(ptSize.x, ptSize.y, 1, 1, NULL);

   bmAndMem = CreateCompatibleBitmap(hdc, ptSize.x, ptSize.y);
   bmSave = CreateCompatibleBitmap(hdc, ptSize.x, ptSize.y);

   // Each DC must select a bitmap object to store pixel data.
   bmBackOld = (HBITMAP)SelectObject(hdcBack, bmAndBack);
   bmObjectOld = (HBITMAP)SelectObject(hdcObject, bmAndObject);
   bmMemOld = (HBITMAP)SelectObject(hdcMem, bmAndMem);
   bmSaveOld = (HBITMAP)SelectObject(hdcSave, bmSave);

   // Set proper mapping mode.
   SetMapMode(hdcTemp, GetMapMode(hdc));

   // Save the bitmap sent here, because it will be overwritten.
   BitBlt(hdcSave, 0, 0, ptSize.x, ptSize.y, hdcTemp, 0, 0, SRCCOPY);

   // Set the background color of the source DC to the color.
   // contained in the parts of the bitmap that should be transparent
   cColor = SetBkColor(hdcTemp, cTransparentColor);

   // Create the object mask for the bitmap by performing a BitBlt()
   // from the source bitmap to a monochrome bitmap.
   BitBlt(hdcObject, 0, 0, ptSize.x, ptSize.y, hdcTemp, 0, 0,
          SRCCOPY);

   // Set the background color of the source DC back to the original
   // color.
   SetBkColor(hdcTemp, cColor);

   // Create the inverse of the object mask.
   BitBlt(hdcBack, 0, 0, ptSize.x, ptSize.y, hdcObject, 0, 0,
          NOTSRCCOPY);

   // Copy the background of the main DC to the destination.
   BitBlt(hdcMem, 0, 0, ptSize.x, ptSize.y, hdc, xStart, yStart,
          SRCCOPY);

   // Mask out the places where the bitmap will be placed.
   BitBlt(hdcMem, 0, 0, ptSize.x, ptSize.y, hdcObject, 0, 0, SRCAND);

   // Mask out the transparent colored pixels on the bitmap.
   BitBlt(hdcTemp, 0, 0, ptSize.x, ptSize.y, hdcBack, 0, 0, SRCAND);

   // XOR the bitmap with the background on the destination DC.
   BitBlt(hdcMem, 0, 0, ptSize.x, ptSize.y, hdcTemp, 0, 0, SRCPAINT);

   // Copy the destination to the screen.
   BitBlt(hdc, xStart, yStart, Width, Height, hdcMem, 0, 0,
          SRCCOPY);

   // Place the original bitmap back into the bitmap sent here.
   BitBlt(hdcTemp, 0, 0, ptSize.x, ptSize.y, hdcSave, 0, 0, SRCCOPY);

   // Delete the memory bitmaps.
   DeleteObject(SelectObject(hdcBack, bmBackOld));
   DeleteObject(SelectObject(hdcObject, bmObjectOld));
   DeleteObject(SelectObject(hdcMem, bmMemOld));
   DeleteObject(SelectObject(hdcSave, bmSaveOld));

   // Delete the memory DCs.
   DeleteDC(hdcMem);
   DeleteDC(hdcBack);

   DeleteDC(hdcObject);
   DeleteDC(hdcSave);
   DeleteDC(hdcTemp);
}

AliR.

"mosfet" <john.doe@anonymous.org> wrote in message
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