Author: Tomas Rutkauskas 

I have 3 Bitmaps. I copy BM2 on BM1. Then the problem: Copy a defined polygon from 
BM3 (or BM2) into BM1. I only want to map the defined polygon into BM1 without 
destroying any pixels outside the poly (within the rectangle).

Answer:

Here is one way you can try. It defines a polygon shaped clip region for the 
destination bitmap, then copies the origin bitmap:

1   { ... }
2   var
3     pts: array of TPoint;
4     rgn: HRgn;
5   begin
6     SetLength(pts, 4);
7     pts[0] := Point(0, 0);
8     pts[1] := Point(50, 20);
9     pts[2] := Point(20, 50);
10    pts[3] := pts[0];
11    rgn := CreatePolygonRgn(pts[0], 4, Winding);
12    SelectClipRgn(bm1.Canvas.Handle, rgn);
13    bm1.Canvas.Copyrect(rect(0, 0, bm2.width, bm2.height),
14      bm2.canvas, rect(0, 0, bm2.width, bm2.height);
15      DeleteObject(rgn);
16  end;
17  { ... }