Add viewing rules to the ACES configs

[doug-walker]

For applications that deal with a mix of both scene-referred and display-referred images, it is helpful to have the viewing rules in the config so that the appropriate default view can vary as a function of the color space of the image. For example, it would prevent an ACES view transform being applied to an image that is already display-referred.

This would be useful in OIIO, as mentioned in this issue.

[remia]

As mentioned in #155, we used a very slightly modified ACES studio config for our xStudio open source build. I believe the ViewingRules are taken from Doug's OCIO v2 demo config.

viewing_rules:
  - !<Rule> {name: scene-linear, encodings: scene-linear}
  - !<Rule> {name: data, encodings: data}
  - !<Rule> {name: log-or-linear, encodings: [ log, scene-linear ]}
  - !<Rule> {name: video, encodings: [ sdr-video, hdr-video ]}

In practice the last 2 rules are used to apply an ACES viewing transform on scene-referred content, while display-referred media are just converted for the display colorimetry using the Un-tone-mapped view. As mentioned in the other ticket, this is not used internally in our studio so we haven't got much feedback about this yet.

[doug-walker]

The rules we need now for the ACES config are the following, should we start with just these two for now? (Or we could add an additional view for mattes that uses the data rule?)

I'll propose a slight renaming since these may be visible in the UI of applications (such as Flame):

viewing_rules:
  - !<Rule> {name: Any Scene-linear or Log, encodings: [log, scene-linear]}
  - !<Rule> {name: Any Video, encodings: [sdr-video, hdr-video]}

In order to make these useful, we need to add an additional view that would be used for video content. I will propose naming this view: "Video (colorimetric)". The "(colorimetric)" part is intended to indicate the type of video view. The wording choice was influenced by the colorimetric rendering intent in ICC Profiles. In the future, we may want to add additional video views that do things such as HDR-to-SDR remapping. But to start, the proposed view would just take the colorimetry from the reference space and put it on the display, as is. Anything outside the gamut is simply clipped. The implementation in the config looks like this:

  - !<ViewTransform>
    name: Video (colorimetric)
    from_display_reference: !<MatrixTransform> {}

And here is some sample code to update an existing ACES config:

def add_viewing_rules(config):

    rules = ocio.ViewingRules()
    rules.insertRule(0, 'Any Scene-linear or Log')
    rules.addEncoding(0, 'log')
    rules.addEncoding(0, 'scene-linear')
    rules.insertRule(1, 'Any Video')
    rules.addEncoding(1, 'sdr-video')
    rules.addEncoding(1, 'hdr-video')
    config.setViewingRules(rules)

def add_rules_to_views(config):

    # Add rule to all shared views.

    views_list = list(config.getSharedViews())  # need list() to work around issue
    for view_name in views_list:
        vt = config.getDisplayViewTransformName(None, view_name)

        # There is no way to edit a shared view, need to delete and make a new one.
        config.removeSharedView(view_name)

        # Add shared view, with rule.
        config.addSharedView(view=view_name, viewTransformName=vt,
            colorSpaceName="<USE_DISPLAY_NAME>", ruleName='Any Scene-linear or Log',
            looks="", description="")

    # Note: Raw is a non-shared view (though it could be), but there is no need to add
    # a rule since it is appropriate for use with all color spaces.

def add_video_view(config):

    # Add view transform.
    name = "Video (colorimetric)"
    tr = ocio.MatrixTransform.Identity()
    vt = ocio.ViewTransform(referenceSpace=ocio.REFERENCE_SPACE_DISPLAY, 
            name=name, family="", description="", 
            toReference=None, fromReference=tr, categories=[])
    config.addViewTransform(vt)

    # Add shared view.
    config.addSharedView(view=name, viewTransformName=name,
        colorSpaceName="<USE_DISPLAY_NAME>", ruleName="Any Video",
        looks="", description="")

    # Add shared view to all displays.
    for dis in config.getDisplays():
        config.addDisplaySharedView(dis, name)

    # Add active view. Needs to go before Raw.
    active_views = config.getActiveViews()
    active_list = active_views.split(', ')
    out_str = active_list[0]
    for i in range(1, len(active_list) - 1):
        out_str = out_str + ', ' + active_list[i]
    out_str = out_str + ', ' + name + ', Raw'
    config.setActiveViews(out_str)

[KelSolaar]

Feels rather inelegant to have to do this as post cleanup, the shared views are created here and the only thing missing is the rule value:

OpenColorIO-Config-ACES/opencolorio_config_aces/config/reference/generate/config.py

Line 1219 in 6c06e59

shared_view = {

We should look if we can pass this via a new column of the spreadsheet or "smart" heuristics.

[doug-walker]

Implemented in PR #160.