OpenGL/JOGL: Can you render two quads with transparency at the same point?

Question

I'm learning about how to use JOGL and OpenGL to render texture-mapped quads. I have a test program and a test quad, and I figured out how to enable GL_BLEND so that I can specify the alpha value of a vertex to make a quad with a sort of gradient... but now I want this to show through to another textured quad at the same position.

Drawing two quads with the same vertex locations didn't work, it only renders the first quad. Is this possible then, or will I need to basically construct a custom texture on-the-fly based on what I want and then draw one quad with this texture? I was really hoping to take advantage of blending in this case...

Answer 1

Its difficult to make out of the question what exactly you're trying to achieve but here's a try

For transparency to work correctly in OpenGL you need to draw the polygons from the furthest to the nearest to the camera. If you're scene is static this is definitely something you can do. But if it's rotating and moving then this is usually not feasible since you'll have to sort the polygons for each and every frame.

More on this can be found in this FAQ page:
http://www.opengl.org/resources/faq/technical/transparency.htm

Answer 2

For alpha blending, the renderer blends all colors behind the current transparent object (from the camera's point of view) at the time the transparent object is rendered. If the transparent object is rendered first, there is nothing behind it to blend with. If it's rendered second, it will have something to blend it with.

Try rendering your opaque quad first, then rendering your transparent quad second. Plus, make sure your opaque quad is slightly behind your transparent quad (relative to the camera) so you don't get z-buffer striping.

Answer 3

Have a look at which glDepthFunc you're using, perhaps you're using GL_LESS/GL_GREATER and it could work if you're using GL_LEQUAL/GL_GEQUAL.