Descriptions of CSP_VERTICAL and CSP_COLOCATED refer to "(0, 0) luma sample", which is not defined

[wantehchang]

What version / commit were you testing with?
(git describe can produce this info.)

AV1 Bitstream & Decoding Process Specification
Last modified: 2019-01-08 11:48 PT
Version 1.0.0 with Errata 1

In Section 6.4.2. Color config semantics, the last table describes the chroma sample position name CSP_VERTICAL as:

Horizontally co-located with (0, 0) luma sample, vertical position in the middle between two luma samples

and describes CSP_COLOCATED as:

co-located with (0, 0) luma sample

The descriptions refer to "(0, 0) luma sample", but that term is not defined in the spec.

[wantehchang]

I suspect the context for these descriptions is four luma samples in two rows and two columns, with coordinates (0, 0), (0, 1), (1, 0), and (1, 1), like so:

  (0, 0)  (0, 1)

  (1, 0)  (1, 1)

But this context is not described in the AV1 spec.

[xzhaao]

One possible change:

CSP_VERTICAL: the position of the corresponding luma sample for a chroma sample is not shifted horizontally, but shifted by 0.5 luma sample vertically toward the bottom of picture.
CSP_COLLOCATED: the position of the corresponding luma sample for a chroma sample is not shifted either horizontally or vertically.

[xzhaao]

In addition, it seems none of CSP_VERTICAL, CSP_COLOCATED, CSP_RESERVED has been referenced in the spec text, maybe we can remove these names for the chroma sample positions.