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ampproject / amp-toolbox

A collection of AMP tools making it easier to publish and host AMP pages.
Apache License 2.0
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Add transformer for adding poster image placeholders to video embeds (e.g. YouTube) #1187

Open westonruter opened 3 years ago

westonruter commented 3 years ago

Something that we do in the WordPress plugin when converting a YouTube iframe into amp-youtube is automatically supply the poster image placeholder, which can greatly reduce LCP since YouTube videos are heavy and amp-youtube doesn't implement a facade.

So instead of:

<amp-youtube data-videoid="gYJ03GyrSrM" layout="responsive" width="16" height="9" title="Introducing How to AMP"></amp-youtube>

We generate:

<amp-youtube data-videoid="gYJ03GyrSrM" layout="responsive" width="16" height="9" title="Introducing How to AMP">
    <a placeholder href="https://www.youtube.com/watch?v=gYJ03GyrSrM">
        <amp-img src="https://i.ytimg.com/vi/gYJ03GyrSrM/hqdefault.jpg" layout="fill" object-fit="cover" alt="Introducing How to AMP" class="amp-wp-enforced-sizes">
            <noscript><img src="https://i.ytimg.com/vi/gYJ03GyrSrM/hqdefault.jpg" alt="Introducing How to AMP"></noscript>
        </amp-img>
    </a>
</amp-youtube>

With Optimized AMP and hero image prerendering, this is further optimized as the poster becomes a hero image:

<amp-youtube data-videoid="gYJ03GyrSrM" layout="responsive" width="16" height="9" title="Introducing How to AMP" class="i-amphtml-layout-responsive i-amphtml-layout-size-defined" i-amphtml-layout="responsive">
    <i-amphtml-sizer style="display:block;padding-top:56.25%"></i-amphtml-sizer>
    <a placeholder href="https://www.youtube.com/watch?v=gYJ03GyrSrM">
        <amp-img src="https://i.ytimg.com/vi/gYJ03GyrSrM/hqdefault.jpg" layout="fill" object-fit="cover" alt="Introducing How to AMP" class="amp-wp-enforced-sizes" i-amphtml-ssr data-hero>
            <img class="i-amphtml-fill-content i-amphtml-replaced-content" decoding="async" alt="Introducing How to AMP" src="https://i.ytimg.com/vi/gYJ03GyrSrM/hqdefault.jpg" style="object-fit:cover">
        </amp-img>
    </a>
</amp-youtube>

(Caveat: It seems currently only the PHP implementation of PreloadHeroImage is identifying such placeholders as hero image.)

It seems generally useful to have an optimizer transformer that does this for all amp-youtube components that lack a placeholder, and to do the same for other popular components which have an easily-determined poster image.

sebastianbenz commented 3 years ago

Thanks for filing. This looks very useful! Is the placeholder image URL scheme an official API?

westonruter commented 3 years ago

De facto official, I believe. See https://stackoverflow.com/a/2068371/93579.

westonruter commented 3 years ago

If the amp-youtube amp-img[data-hero] is identified as a hero image, then the PreloadHeroImage transformer should also add a link[rel=preconnect] for https://i.ytimg.com. This will also improve the performance of loading the YouTube iframe.

This further relates to https://github.com/ampproject/amp-toolbox-php/issues/10: if we identify that amp-youtube is in hero position, shouldn't we also add a link[rel=preconnect] for https://www.youtube.com?

westonruter commented 3 years ago

The poster image URLs are also utilized in the lite-youtube-embed facade.

sebastianbenz commented 3 years ago

Did you test whether this causes a double download of the poster image because of origin keyed caching?

westonruter commented 3 years ago

That's a great point. No, I didn't test that.

sebastianbenz commented 3 years ago

I looked into this. It doesn't matter in this case as the AMP runtime also injects the poster image (which causes a double download anyway).

westonruter commented 3 years ago

Great! (?)