TransparentBitmap.cpp

//==========================================================================
//
//  File: TransparentBitmap.cpp
//
//==========================================================================

#include <windows.h>

void
DrawTransparentBitmap(HDC hdc, HBITMAP hBitmap, short xStart,
					  short yStart, COLORREF cTransparentColor)
{
	BITMAP     bm;
	COLORREF   cColor;
	HBITMAP    bmAndBack, bmAndObject, bmAndMem, bmSave;
	HGDIOBJ    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   = SelectObject(hdcBack, bmAndBack);
	bmObjectOld = SelectObject(hdcObject, bmAndObject);
	bmMemOld    = SelectObject(hdcMem, bmAndMem);
	bmSaveOld   = 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, ptSize.x, ptSize.y, 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);
}