Re: [MMUSIC] FW: New Version Notification for draft-abhishek-mmusic-superimposition-grouping-01.txt

[Christer Holmberg <[e-mail]>]

Hi,

>* I'm still unsure how transparency works. Does the transparency value apply to all pixels in the associated stream? Or is is also possible that some pixels are transparent
>and others are opaque? (I'm thinking of closed captioning, where the characters themselves are opaque but their own background is opaque.) I would like to see some
>discussion of the role of streams that contain some transparent pixels.

I don't think it is up to us to define how transparency works, but I agree that it would be good with a short description and/or reference to where it is specified.

>* Are you assuming there is time synchronization among all the streams in the superimposition group? If so please say so. If not then you need to say more about how they are to be synchronized.

I agree it would be good to say something about this.

However, if there is no requirement for synchronization, I don't think we shall impose such requirement. However, if needed, we do have the lipsynch (LS) group attribute (RFC5888).

>* You are using both "superimposition" and "superposition". These have somewhat different (but fuzzy to me) distinctions. Unless you need both along with their distinctions I suggest you pick one and use it throughout.

Agree.

>* You also use the term "supim" as the grouping token. I don't think choosing a separate term here is beneficial. I suggest you use the same term (superimposition or superposition) you choose.

This I don't agree with. If you look in the IANA registry, *ALL* registered token are short 2-6 character abbreviations.

>* I can't find anything that says how the layer numbers are to be interpreted. Is zero the background, with higher numbered layers stacking on top of it? Or is zero the foreground, with higher numbered
>layers stacked behind it. (I guess zero is the background, but am not certain.)

Agree.

>* You seem to use "alpha" and "beta" in a way that implies that these terms have a specific meaning in the context of superimposition. But I find nothing in the document that makes this explicit.
>The text of section 5 associates them with numeric values, but the ABNF uses them as rule names without defining them. At the very least you need to define them in ABNF.

In addition, I don't think one should use "alpha" in ABNF to begin with, because people may confuse it with ALPHA.

Regards,

Christer



* In your reply to Christer you mentioned RFC7195 as having a syntax style that you like. If so, you certainly can copy that style. (There is no benefit in actually referencing RFC7195. I don't see anything in there that it would be meaningful to reference directly.) For instance, you might write your syntax as follows:

      superimposition-attribute =
         "superimposition:" super-opt *(SP super-opt)
      super-opt = super-trans / super-layer
      super-trans = "transparency:" super-trans-val
      super-layer = "layer:" super-layer-val
      super-trans-val = signed-integer ; range [-128, 127]
      super-layer-val = signed-integer ; range [0, 255]

      signed-integer =
         <zero-based-integer defined in RFC8866>
         / "-" <integer defined in RFC8866>

      attribute = <attribute defined in RFC4566>
      attribute =/ superimposition-attribute

You can adjust as you see fit. In addition to this formal ABNF you will also need some normative (e.g. MUST) text for a few things:

- *normatively* define the ranges for super-trans-val and
   super-layer-val. (The comment in the ABNF, while helpful,
   isn't normative. E.g.,
   "The value of the 'transparency' option MUST be within the range
   [-128, 127]."

   (Alternatively the ABNF could be written to accept only the
   permitted values. But doing so requires complex and hard to
   read ABNF.)

- Normatively specify what happens when either transparency or layer
   is specified multiple times within the same media description.
   This can occur in more than one way. E.g.,

   m=...
   a=superimposition:transparency:0 transparency:1

   m=...
   a=superimposition:layer:0
   a=superimposition:layer:1

You have options here: you can forbid specifying either of these more than once (via MUST NOT), or you can define what it means if it happens. 
(E.g., use the last specified value.)

	Thanks,
	Paul