Question

My program uses blending via glBlendFunc(GL_SRC_ALPHA, GL_ONE) to make a pleasent effect when the background is set to black with clearColor(0,0,0,1). When
the background is set to white with clearColor(1,1,1,1) the screen is blank white. I don’t know how to generate the same trasparency effect on the white background. Suggestions appreciated.

Answer 1

glBlendFunc(GL_SRC_ALPHA, GL_ONE) roughly translates into the following equation (the clamping happens at another place, technically)

New_Framebuffer_Value :=
    New_Red   := min(1, Incoming_Red   * Incoming_Alpha + Previous_Framebuffer_Red   )
    New_Green := min(1, Incoming_Green * Incoming_Alpha + Previous_Framebuffer_Green )
    New_Blue  := min(1, Incoming_Blue  * Incoming_Alpha + Previous_Framebuffer_Blue  )

However if you cleared to while, the Previous_Framebuffer_{Red,Green,Blue} are already at 1.0 and so all following will be clamped.

So you need to modulate not only the incoming fragments, but also the previous ones:

New_Framebuffer_Value :=
    New_Red   := min(1, Incoming_Red   * Incoming_Alpha + Previous_Framebuffer_Red   * ( 1 - Incoming_Alpha ) )
    New_Green := min(1, Incoming_Green * Incoming_Alpha + Previous_Framebuffer_Green * ( 1 - Incoming_Alpha ) )
    New_Blue  := min(1, Incoming_Blue  * Incoming_Alpha + Previous_Framebuffer_Blue  * ( 1 - Incoming_Alpha ) )

which is done by glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)