Ok. let me explain. I started with programmaticality generating height map of my scene. Its straight forward. I am rendering small height sprites to the fbo. glBlendFunc(GL_ONE, GL_ONE). With additive function I get fairly nice heightmap. In order to use these heightmap. I am converting it to normalmap. programmability using a shader.
But this operation is hell of expensive and normal where not smooth. Now I want eliminate the hieghtmap and directly trying to generate normal map. (Using normal sprite?). However It seems that normals does not add up. Feels like I should drop the idea..