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On hardware as weak as the one you got there is just no way to do any complex task without a massive performance hit. Whether you draw a pixel by rendering the scene or by doing non-trivial scaling will roughly cost the same. The only chance to get a smaller scene image to the screen fast is to use ...

I need some more gpu processing. I now only have an Intel HD 2000. I would even need a little more blur. An optional setting for more or less blur. Is that possible?

It is possible to use lanczos resampling for resolution scale? That would be great.

Maybe you could support multiple versions of opengl just like in Yamagi Quake 2. You have it anyway. You offer opengl and software.