Re: [docbook] The semantics of mediaobject

[Dave Pawson <dave.pawson@gmail.com>]

On Thu, 6 Apr 2023 at 01:47, Bob Stayton <bobs@sagehill.net> wrote:
>
> It seems ok to me.  The idea of a mediaobject is to contain markup for one or more versions of the same content.  Presenting more than one of those to the end user would be redundant and annoying.  If you consider the whole rendering system to be the transformation from DocBook to HTML5 *and* the browser rendering of that HTML5, then your use case does not seem to violate the description because the consumer of the content sees only one version, right?


Relies on an odd (rare?) circumstance, despite being logical.

Is there any backwards compatibility reason not to adopt Norms logic?


regards








>
> Bob
>
> On 4/5/2023 9:27 AM, Norm Tovey-Walsh wrote:
>
> Hello world,
>
> The description of mediaobject states:
>
>   This element contains a set of alternative âmedia objects.â Exactly
>   one object will be selected and rendered. â Under no circumstances
>   should more than one object in a mediaobject be used or presented at
>   the same time.
>
> Bearing in mind that this markup was invented 15 or 20 years ago, long
> before any (widely used) web browser offered anything sophisticated in
> the way of fallback, we anticipated that markup like this:
>
>   <mediaobject>
>     <videoobject>â</videoobject>
>     <imageobject>â</imageobject>
>     <textobject>â</textobject>
>   </mediaobject>
>
> would be intepreted along the following lines: ââ a mediaobject might
> contain a video, a high-resolution image, a low-resolution image, a long
> text description, and a short text description. In a âhigh-endâ online
> system, the video is used. For print publishing, the high-resolution
> image is used. For other online systems, either the high- or the
> low-resolution image is used, possibly including the short text
> description as the online alternative. In a text-only environment,
> either the long or the short text description is used.â
>
> <aside>I observe that âpossibly including the short textâ is, by a very
> narrow reading, a violation of the semantics weâve already
> asserted.</aside>
>
> Starting in V5.1 we allow more than one *data element in a *object
> element[1] with the semantics that the downstream processor will pick
> one. So now you can do:
>
>   <mediaobject>
>     <videoobject>
>       <videodata type="video/mp4" â/>
>       <videodata type="video/webm" â/>
>     </videoobject>
>     <imageobject>â</imageobject>
>     <textobject>â</textobject>
>   </mediaobject>
>
> And let the browser decide whether it wants to render MP4 or WebM.
>
> Trouble is, you canât put imagedata in the videoobject wrapper, so
> thereâs no way to fallback to a still image. We could fix this by
> changing videoobject (and audioobject) to allow imageobject, but at that
> point, we might as well do away with the various flavors and just have
> one wrapper. (Thatâs probably what weâd do today, if we were starting
> over.)
>
> Rather than make the markup even more complicated, Iâm tempted to squint
> really hard at the semantics and try to persuade you that selecting
> multiple objects from the same mediaobject is not a violation of the
> semantics if the rendering system will only render one of them.
>
> In other words, I want a license to cheat and say that I can render that
> example above in HTML5 as a video element containing three sources, two
> videos and an image. What say you?
>
>                                         Be seeing you,
>                                           norm
>
> [1] But the content model of imageobject weirdly forbids you from mixing
> SVG and MathML and other formats. What is that about?
>
> --
> Norm Tovey-Walsh <ndw@nwalsh.com>
> https://norm.tovey-walsh.com/
>
> Advancement is powered by the dynamic interaction of theory and
> practice.



-- 
Dave Pawson
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