Understanding RectangleEx

I've been playing with ImageLibEx's AlphaBlend function to merge images together. I get an error message which I can't explain when rectangeEx is of a certain dimension. Load attached file and press the draw btn several times to see error.

You are setting RectangleEx X and Y, by successively calling Offset, so that the area covered by RectangleEx is outside the area of bitMapEx2. There is no valid data to alpha blend into bitMapEx1 from here so an error occurs.

What I'd like to do in the attached example is to merge 2 images, i.e. the areas that will be visible on the screen.

rectangleEx is declared to be the size of the screen and stays fixed. There is a thick brown diagonal line which extends out of the blue box when drawn (line 31). However the line gets truncated insn't drawn outside the box when it should.

On a slightly different note, is it possible to declare an empty bitmap and draw some lines on it, then draw the bitmap directly onto an image such that now the image shows with the lines from the bitmap as well. Reason I ask is I think this may be faster then AlphaBlend (i.e executing every sec).