Make page-spread-center an alias for spread-none

[mattgarrish]

The only concern I have now that I'm opening the PR is whether we should formally deprecate page-spread-center or if we should "discourage" its use. I don't know if anyone has used it that we'd affect by this change.

Otherwise, the PR itself removes all traces of page-spread-center except to deprecate and make it an alias and adds the centering prose to the rendition:spread none processing requirements.

• Reading Systems preview
• Reading Systems diff

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[shiestyle]

I understand this change but most EPUB 3 files in Japan use 'page-spread-center' for 1st page as cover and we should avoid that EPUBCheck will recognize it as error and warning.

So alias is acceptable but deprecated is not good label.

[mattgarrish]

most EPUB 3 files in Japan use 'page-spread-center' for 1st page as cover

That's what I was worried about. I'm perfectly fine with only making it an alias and not deprecating.

[mattgarrish]

Last few commits make the following changes:

• removes the deprecated label from page-spread-center
• adds an explanation of what the property does to its definition - I also moved up the explanations of -left and -right into their definitions as it was odd to only say they are aliases and then in a following paragraph explain what they do
• revises the note about the aliases to remove the repeated statement of what -left and -right are aliases for. I also added a new paragraph explaining why -center exists as an alias (to simplify thinking about two-page and single layouts, but check this makes sense)
• restores the example of using page-spread-center but without the confusing text about not needing to apply spread-none since its role as an alias is now clear

[shiestyle]

Thanks, Matt. That's fine for me.

[iherman]

The issue was discussed in a meeting on 2021-12-09

• no resolutions were taken

View the transcript

2. Is page-spread-center a synthetic spread or not? (issue epub-specs#1929)

See github issue epub-specs#1929.

See github pull request epub-specs#1950.

Wendy Reid: i just got through writing the tests for all the FXL rendition properties, which lead to a question about page-spread-center.
… this is a spine property that is not meant to override spread behaviour, but that the page should be centered in the viewport.
… but this doesn't seem to be what is happening in practice.
… rather, the observed behaviour seems to be identical to spread none.
… we've discussed in the issue, and mgarrish came up with the idea of making page-spread-center an alias of spread none.
… and not to deprecate page-spread-center.

Brady Duga: not sure why page-spread-center is confusing. Imagine you are doing a synthetic spread, but you want one specific page centered in the middle of the screen. That's how you would tell the RS to do that..
… spread none sounds like you've turned off spreads completely.

Wendy Reid: an additional level of oddness was finding out that you can put page spread properties on spine level items as well.

Brady Duga: spread none and spread center mean different things, because spread none doesn't tell you where to put the page.
… and yes, it is confusing that you can put this on a spine item (you can also alternate portrait and landscape on spine level items as another weird way of interacting with spreads).

Dave Cramer: is there a concern that this could affect the relative size of pages as you go from spread, to non-spread, and back to spread?.

Brady Duga: i can't see people really doing this.
… one potential use case is if one of your content documents is already a spread, so you want to disable synthetic spreads for that one only.

Shinya Takami (高見真也): in Japan we use spread-center for the cover pages in many many cases, so making spread-center a depreciated feature would not be acceptable.
… aliasing it to spread none is okay.

Wendy Reid: right, so we won't deprecate page-spread-center.
… this might be the sort of thing we need to test out.
… when the page appears in the center of the viewport, i think there is a change in the page size.

Dave Cramer: i'm not comfortable deciding this until we have more info on whether we need to better define how a RS should position the viewport when you use page-spread-center or spread none as an override of a synthetic spread epub.

Wendy Reid: agree that the FXL properties section could use some visual aides.
… we can put this off for tonight.

Brady Duga: if we end up keeping page spread then we need to fix this definition.

Wendy Reid: yeah, definitions in that section are not very good.

Dave Cramer: it was a long time ago that we wrote those, we were balancing against implementations that already existed at the time (amazon, apple).