ADR-0019: An Album Can Have a Look of Its Own, Named on the Music Side and Drawn on the Light Side

Context

Every scene so far is made the same way: the cover’s colors, picked by palette, turned into a scene by lightd. That is right for almost every record and wrong for a few. The Dark Side of the Moon is a black sleeve with a thin spectrum across it. The colors a fair extraction finds on it are not what anyone would light a room with for that album, and what they would light it with is obvious: the spectrum.

That is a fact about an album, not about an image. And the two halves of the system were built not to share such facts: lightd sees image bytes and knows nothing of tracks or albums (ADR-0009), and nowplayd knows what is playing and nothing of scenes, colors or LEDs.

Options considered

lightd reads the current track and has a list of albums. One place to change. It is exactly what ADR-0009 kept out: lightd would know what an album is, and every future special case would be argued there.

Recognize the cover image. lightd stays image-only. But the same album arrives as different bytes from every library, edition and catalog, so it means fuzzy image matching to answer a question the track’s own tags answer outright.

A third program that publishes scenes for special albums. It and lightd would both publish to the stand’s scene topic for the same track change, and which won would be a race.

The music side names a look; the light side draws it. nowplayd knows the album and says, in one word, that this cover has a look. lightd knows what that word looks like. Neither learns the other’s subject.

Decision

A look is a name. nowplayd holds a short list of albums that have one, matched on the artist and album it already knows. When the current track’s album is on it, nowplayd publishes the look’s name on nowplaying/current/look, retained, beside the cover.

A look belongs to a cover, by its hash. The message carries the SHA-256 of the cover it is for. lightd applies a look only to the cover with that hash, so the two messages can arrive in either order and a look can never land on the album after it. An album with no look has a message that says so for its cover.

lightd owns what a look is. A look is a scene preset: an effect and a palette, in lightd, beside the other decisions about color and hardware. A cover with a look gets the preset instead of its own colors. A look lightd does not know is ignored, and the cover is lit from its own colors, as the stand shows an effect it does not know as solid.

The first look is spectrum: six colors, red to violet, in equal shares. It is drawn with an effect the stand already has. Which one is a setting: sweep moves the whole spectrum around the strip, morph shows each color in turn.

No look reaches a stand as anything but a scene. The stand and the scene contract are unchanged.

Consequences

• Dark Side of the Moon lights the room as a spectrum, from any library and any edition whose tags say what it is.
• lightd still knows nothing of tracks or albums. It knows covers, and now that a cover may come with a word.
• The list of special albums is in nowplayd’s code. Adding one is a change and a deployment, and there should not be many: this is for records whose look is as well known as their name.
• Matching is on tags. A copy tagged Dark Side Of The Moon (2011 Remaster) matches; one tagged DSOTM does not, and is lit from its cover like any other.
• A look is not applied to a source that is reported without its cover (ADR-0016). No cover is published for it, so there is nothing for a look to belong to. Spotify playing this album changes nothing in the room.
• At start-up, when both retained messages arrive at once and the cover is first, the stand may show the cover’s own colors for a moment before the look.
• Two more grants: nowplayd publishes the look topic and lightd reads it.
• The colors of a look are fixed, not measured from anything. They are still lightd’s to choose, and are sent to the stand before gamma like any others.