Zooming In on a Selection: Step 3

/* Save the starting position in screen pixels (used by GDI functions) */
StartGDIX = point.x;
StartGDIY = point.y;
/* Use the mouse position's percentage offsets in the image rectangle
to determine the pixel offsets in the bitmap.
Using percentages allows for the possibility that the image is zoomed. */
StartPixelX = MulDiv(LeadBitmap.GetWidth(), StartGDIX - rLeadDest.left, DisplayWidth);
StartPixelY = MulDiv(LeadBitmap.GetHeight(), StartGDIY - rLeadDest.top, DisplayHeight);

/* Initialize a variable used when the WM_MOUSEMOVE event outlines the area */
FirstDraw = TRUE;