[Bf-cycles] How to render an arbitrary subregion of the render window?

[Yansi Yao]

Hi all,
How to render an arbitrary subregion of the render window in Cycles.
For example, the resolution of my render window is 800x600, but I want to render the subregion from (10,100) to (300,500).
My question is:
1.Is this feature available in Cycles now? If it is, could you tell me how to do this in Cycles. And which function responsible for the subregion-rendering in Cycles source code?
2.If the subregion-rendering is not available in Cycles, when will this feature be added into Cycles?

Regards
yaoyansi
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