color-management-v1: Image description types in definition of `wp_image_description_info_v1`

In the description of wp_image_description_info_v1, the protocol describes two types of image descriptions:

      - if the image description is parametric, it must send
        - primaries
        - named_primaries, if applicable
        - at least one of tf_power and tf_named, as applicable
        - luminances
        - target_primaries
        - target_luminance
      - if the image description is parametric, it may send, if applicable,
        - target_max_cll
        - target_max_fall
      - if the image description contains an ICC profile, it must send the
        icc_file event

That is, parametric descriptions and ICC profile descriptions. However, it does not say if these are exactly the descriptions that can be used as the source of a wp_image_description_info_v1 or if there could be other types of descriptions (e.g. windows_scrgb).

Unless the client is using the wp_color_management_surface_feedback_v1::get_preferred_parametric requests, it cannot know, a-priori, what type of image description it is dealing with.

• Is this paragraph normative, i.e. can clients rely on receiving any particular sequence of events?
• Is this list supposed to be exhaustive, i.e. do all compositor-created descriptions fall into one of these two categories?
• If not, are there any restrictions on which types of events the compositor is required/allowed to send?

mentioned in issue #243 (closed)

changed the description

Nothing restricts the usage to specific image description kinds so I don't think there is something wrong here but it might be more clear to either

• mention every kind explicitly
• mention a fallback for every other kind

The main point is, if get_information request is allowed, the compositor must deliver a complete serialization of the image description. An incomplete serialization means that the client cannot know exactly what the image description means, and therefore the image description is useless. The wording is trying to explain what constitutes a complete serialization.

If the wp_image_description_info_v1 interface is inadequate for providing a complete serialization for an image description that would allow get_information, the compositor must gracefully fail the image description. This can happen if wp_image_description_info_v1 gets version bumps, a client creates an old version protocol object, and the compositor has an image description whose serialization needs events from the latest version. This also happens with compositor-created windows_scrgb, because wp_image_description_info_v1 cannot communicate it.

The list of image description types is not exhaustive, compositors can use other types for their output for example. The caveat is that other types cannot be serialized to clients.