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[mediaqueries] definition of video width and height #5043
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I'm not sure what you mean by "video plane" here @frivoal ? I'm not sure there's a good W3C resource that defines the terms properly, which is awkward because there's a crucial difference between the decoded video size and the viewport size. (This has a big impact on subtitle/caption positioning, by the way.) Encoded video size != Decoded video size != viewport size. Let's ignore encoded video for the time being, because we're probably not really interested in it. Instead, think about decoded video, and call that the "video" as opposed to the rectangular area displaying that video, let's call that the "viewport". For example, the video might have an aspect ratio of 4:3 and be shown in a viewport with aspect ratio 16:9. There are any number of ways to arrange for this, but looking at just three approaches that are used:
How does this affect captions/subtitles?(not sure how on-topic this is for this issue but it seems likely to be relevant) I mentioned this really affects subtitle and caption positioning. The typical reason why captions are positioned relative to video, for accessibility, is to avoid having them obscure some part of the video that is important for understanding/experiencing the video. There is a general problem with specs like WebVTT that position captions only relative to the viewport, because the caption author doesn't know how big the viewport is, they only have access to the video. If we can arrange it, caption positions should be authored relative to the video, in terms of proportional size and position, and then if there's some way to fix up a mismatch between the video aspect ratio against which they were authored, and the presented video aspect ratio, then we have some hope of player code doing something sensible. By the way, the IMSC |
@nigelmegitt commented
That is crucial to understanding why we have two parallel sets of properties. In dome devices, such as 4k and 8k televisions, there is not one framebuffer but two and they have different properties:
There is some sort of hardware compositing for the output of the two framebuffers They cover the same screen area, so the graphics plane is lower resolution |
The definition of the
video-width
media feature (same thing forvideo-height
) includes this:That text comes from the definition of width, but I don't think it is relevant here: I don't think there currently exists such a thing as a paginated media with a dedicated video plane (or a paginated video plane being itself paginated).
On the other hand, we should add a mention that this is about the size of the entire video plane, not the layout-dependent portion of it that is being clipped and composited with the rest of the page.
Therefore, I suggest deleting the sentence quoted above, and instead, inserting the following one:
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