opengl - blending with previous contents of framebuffer

Question

I am rendering to a texture through a framebuffer object, and when I draw transparent primitives, the primitives are blended properly with other primitives drawn in that sin

Answer 1

As Ben Supnik said, the best way to do this is rendering the entire scene with separate blend functions for color and alpha. If you are using the classic non premultiplied blend function try glBlendFuncSeparateOES(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ONE) to render your scene to FBO. and glBlendFuncSeparateOES(GL_ONE, GL_ONE_MINUS_SRC_ALPHA) to render the FBO to screen.

It is not 100% accurate, but in most of the cases that will create no unexpected transparency.

Keep in mind that old Hardware and some mobile devices (mostly OpenGL ES 1.x devices, like the original iPhone and 3G) does not support separated blend functions. :(