Chainlink VRF is a service using the VRF(Verifiable random function) to generate verifiable randomness on-chain, a smart contract supplies a seed (which should be unpredictable to the oracles to whom it is provided), and the seed in turn is used to generate a random number that is sent back to the contract.
I read documents and some codes, function
RequestRandomWords in smart contract
VRFCoordinator supplies preseed to event log. Then Oracle node takes the preseed and a blockhash as input to generate a seed for VRF. At last, VRF uses the seed and its private key to generate verifiable randomness.
My question is:
Which blockhash is used to generate seed in VRF? It looks like the next block after the block where VRF request was made but I do not see the details in document.
Thanks in advance.