Add 2k and 4k scale sizes

[davisagli]

This is needed to avoid a reduction in image quality from plone/volto#7655

I didn't add an upgrade step yet but we probably should (careful to only add the new scales, and not change others)

[mister-roboto]

@davisagli thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

[davisagli]

@jenkins-plone-org please run jobs

[sneridagh]

LGTM

[wesleybl]

@davisagli A curiosity. If the image isn't 2k, these new scales won't be generated, correct?

[davisagli]

@wesleybl In Plone 6 scales are generated on demand the first time they are downloaded.

We generate the metadata which lists the scales in 2 different places:

• Scales in the catalog are generated using https://github.com/plone/plone.namedfile/blob/main/src/plone/namedfile/adapters.py#L39 which only includes sizes with a width less than the original
• The plone.restapi image field serializer https://github.com/plone/plone.restapi/blob/main/src/plone/restapi/imaging.py#L15 which includes all configured scale sizes. I'd like to update this one to use the IImageScalesFieldAdapter and also only include sizes smaller than the original -- but it would be a backwards incompatible change, so I didn't do it so far. Avoiding the possibility of bots accidentally triggering the creation of large scales sounds like a good reason to go ahead and make this change. We have to make sure there is no code in Volto which refers to specific named scales without checking to make sure they are there.

[wesleybl]

@davisagli Are the larger scales generated by the RESTAPI returned to the srcset attribute? If so, is there a risk that the browser might select them, resulting in distorted images?

[davisagli]

@sneridagh I added an upgrade step.

Are the larger scales generated by the RESTAPI returned to the srcset attribute? If so, is there a risk that the browser might select them, resulting in distorted images?

Not distorted images, but when the original image is very large, we start giving the browser more options for serving a smaller one. We also have to set sizes on the img tag so that the browser can pick the appropriate scale from the srcset. That's part of what plone/volto#7655 is for.

[mauritsvanrees]

@jenkins-plone-org please run jobs