As long as you are not calling a glClear() operation at the start of your frames, then the hardware should preserve the colour buffer from your previous frame and upload the preserved colour for each tile before your current frame render begins to update the tile/blend with it. Is this how you have implemented your application, or are you attempting to solve the problem in some other way?
If this is the approach you are taking and you are still getting errors, then you may have encountered a bug. If this is the case, can you please let us know some details about your platform, such as the name of the platform (or emulation, if that’s where you’re finding the problem) and the GPU driver version (you should be able to query this with glGetString()).
Thanks for your response. I discovered the issue which lead me to my incorrect post above. We had a bug in our render buffer creation which meant it was ignoring the format and always creating a 32 bit render buffer when I was specifying 16bit. This resulted in an unexpected squaring of any alpha values in the textures I rendered into the render buffer, multiply by Alpha once when they were rendered into the buffer and again when the render buffer was rendered to screen.
It’s good to know that the hardware does copy the contents of the previous render when no glClear() exists.