Texture problem

I am trying to draw a 2D texture with Blending using the method in NeHe Toturial #20.


The first texture is a mask to lock part of current frame drawings. The frame area covered by black color in the mask picture is blocked. The frame area covered by white in mask picture is transparent.

It seems work except, the frame drawing looks a little bit dirty in the transparent area. If I resize the drawing, the dirty pattern changes.

I am hitting a dead wall. Tried all different ways couldn’t find a solution.

Your help is highly appreciated.