[MPlayer-dev-eng] Recommend error for bad dimensions

[Zoltan Hidvegi]

> I really don't understand the objection some people have to scaling.
> The quality loss from scaling to a nearby size is almost nothing,

Yes, I'm not totally against scaling, actually sometimes I like to
scale things up a little bit to blur and reduce the required bitrate
without much quality loss.  Often cropping and scaling to fill the
original area would do just that.  But I would like to understand
exatly what I lose by not using multiple of 16.  It is really hard to
compare scalers, especially after encoding, since the scaler best for
direct viewing is often not the best for encoding.

> I would understand trying to keep things 'identical' to the DVD if the
> DVD were a good original, but it's not. It's a sloppily encoded
> low-res copy of the actual film, and so IMO the goal should be to make
> your reencoding look as good as possible, not as close to the DVD as
> possible.

Sure.  Some DVDs are good, others are horrible.

> And as for v4mv, I don't think it will help at all. You still have to
> encode full macroblocks (16x16). Aligning at multiple-of-8 is better
> than no alignment at all, though.

I thought that the DCT coding is done for 8x8 blocks.  So if your
dimensions are a multiple of 8, then the luma coding will be fine,
only the chroma part can have a partial block.

But when motion vectors originate from outside of the picture, a
portion of the edge blocks will not have any prediction information
even if the dimensions are multiple of 16.  Do B-frames have
predictors from both time directions, so they can be better at the
edges?

> And don't even THINK about using odd dimensions or odd crop offsets.
> This will ruin chroma alignment!

No, of course I never thought about that.

Zoli