I am facing a problem on image blend. I blended 2 images with 50% transparency, the 2 images cross with 90 degree. but when I rotate the this object, a part of image where behind the front image is invisible also with 50% transparency, why?
Unfortunately (at least up to opengl 1.2), opengl simply cannot do transparency like that. So it uses blending, along with the depth buffer that only remember the depth value.
You should draw your objects in a given order: from the farest translucent one to the nearest one.
Disable depth test when drawing transparent geometry.
With 50% transparency, drawing order may not matter. Nor with additive transparency.
In most other cases, you would need to sort yourself from back to front the transparent polygons.