Progressive transition into WebP formats

[mitogh]

Feature Description

Based on:

• When uploading, generate both WebP and jpeg format images by default #142
• Update the_content with the correct image format #149

At this point, we can assume that every image would have at least 2 formats when the WebP module is enabled in this case the well-known and supported JPEG and the WebP format, (eventually leading into other formats as those are supported in WP). So this issue is mostly to define the strategy behind this image generation and decide into the approach into the transition into a modern format and still have a fallback for older browsers or users using this plugin but not necessarily interested in test/using this section of the performance plugin.

Strategy 1 - Default JPEG and WebP as an enhancement

This approach suggests keeping using JPEG in the sizes definition of the metadata of each image, creating the WebP version after all the JPEG versions were created and WebP images are created only as an enchantment for the existing images.

Benefits

• WebP would be used as an enchantment for JPEG replacing existing JPEG images with WebP versions
• Backwards compatible this approach allows for a transition into WebP formats incrementally, meaning older attachments where WebP version was not created would keep working as they are until a WebP version is created for that attachment see Regenerate existing images #24 and then WebP version would be used instead.

Problems

• Requires content modification like Update the_content with the correct image format #149 in order to render the new image format
• New image format is not used by default and requires filtering in core functions in order to ensure a more modern image format is used appropriately.

Strategy 2 - WebP by default and JPEG as a fallback

This strategy suggests instead a logic similar to the current behavior of the plugin which is for new image uploads only create the WebP version and create the JPEG version of the images as fallback, and WebP format is used as the default image format for all new image uploads. JPEG versions for the images are created as a fallback approach.

Benefits

• Core functions like wp_get_attachment_image_src and the_conent would render automatically the modern image format, in this case WebP
• Benefits from modern images out of the box due this image is already rendered by default for more core functions.

Problems

• If a user of the plugin opts out of not using WebP version a filter should be used in order to adjust the content to render the JPEG version for each image instead.
• When moving or deactivating the plugin WebP versions would remain as the main image format as the logic to render JPEG won't be available at this point.
• Requires an additional logic in place to migrate JPEG versions as the main images for each image size instead of WebP

Reasoning

The main reason to discuss this approach is to have a standard way of doing things, due if we have WebP versions for some attachments as the main images and JPEG for others things start to become more complex due we need to deal with multiple combinations, in some cases JPEG version is used an in others WebP. So knowing before approaching #149 or other places where we need to replace or update references to images we know which mime type is the main image type and which one is the one used as an enchantment.

Note in all scenarios JPEG and WebP versions would be present in the WordPress installations and no file would be removed, unless the main attachment image is removed.

[adamsilverstein]

I'm feeling more and more we should stick with Strategy 1 here for the backwards compatibility reason alone.

We can still achieve the goal of displaying WebP by default on the output side.

In addition, the API/filter(s) should support an approach that enables only outputting WebP sub-sizes (with the exception of the full size -scaled image) - maybe not as the default, but as one possible option. This may turn out to be the best option for most users, especially over time.

[felixarntz]

I agree with @adamsilverstein's feedback above, and I would extend it to say that we should probably always use the original format of the image uploaded as the default. In other words, it won't always be JPEG, it will only be JPEG if the original image is JPEG. If the original image is PNG, it would be PNG, and if someone directly uploads WebP, it would be WebP.

I think that would preserve backward compatibility, and it would also keep the original format the primary one which makes it IMO more clear to understand. It also ties in with your draft PR #154, where you're exploring to store the "other" formats in a separate meta key. That makes me think even more that the default format should be the originally uploaded one.

[mitogh]

Closing this one as we have merged:

• When uploading, generate both WebP and jpeg format images by default #142
• Update the_content with the correct image format #149

And we have #186 to customize the behavior for rendering the correct image.