Question about lossy and lossless

Hi all, I have some troubles of understanding lossy and lossless video. For ex: I encode raw source A1 to B1 through lossy method. Then when decoding B1 I have raw A2. So B1 is lossy of A1 and B1 is lossless of A2, is it right? If yes, then I encode A2 to B2 through lossless method, but It'll never touch the size of B1. I can not understand this. Please help me, thank you guys :D

no it's not right if you encode raw A1 to lossy B1, you'll get a compressed video (lossy). Decoding of B1 does not imply that you get true raw video (without modifications compared to raw A1) since B1 is compressed and the decoder decodes the compressed video. You cannot go from compressed video to raw video which is exactly the same as the original raw input. Note that raw video can mean two things: either it's truly raw without any compression or it's a compressed raw video which is extracted from a container

Thank froggy1. I know that A2 will be different from A1. I just mean decoding B1 we'll have something call A2. So, as you said, A2 is not a raw? Is it still compressed? I have one more question. Does decoding a lossy video with different decoders bring different results? Thank you.

Yes that's right "lossless" always refers to comparing to the uncompressed video state. A compressed video has to be decompressed (decoded) to uncompressed before it's recompressed. So lossless compression will always yield larger filesize , when the source is already lossy compressed.

Speaking of lossless, how lossless is encoding with "-QP 0" really? I have a project that requires several passes, with lossless intermediate steps. Will "-QP 0" do it? Thanks.

qp 0 is mathematically lossless, identical to totally lossless