Livery Spec

**Livery Spec**

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*Feature 14 / Liveries*

Action 0 props

* ID
* Text ID
* Introduction date
* End of life date

*Feat 14 things I don't know about*

Do we need an abstraction from Action 0 ID for the livery?
* in vehicle props / varact 2, just using livery action 0 ID could be fine, but
    * it's not very author-friendly
    * creates savegame compatibility issues if livery ID is changed
* nml authors can use an nml identifier
    * but nml identifiers won't be stable across savegames if the author adds/removes/changes order of liveries
    * nml authors could provide a numeric ID explicitly
* ideas
    * label - similar to cargo and railtype labels?
    * some other abstraction (table)?
    * no abstraction, just use ID?
* I don't see any benefit to sharing abstractions across grfs ('common livery label scheme' etc), I can only forsee complications from that

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*Feature 00 / 01 / 02 / 03 Vehicles*

Action 0

* prop 0x?? - list of liveries this vehicle supports
* prop 0x?? - default livery (or rely on first available in list?)

Vehicle Varact 2

* var 0x?? - 'current livery', available in relevant action 2 chains
    * do any callbacks need it restricted?
    * does this have any desync risk?

Vehicle callbacks

* extend CB 36 to support 
    * list of liveries
    * default livery
* CB 23 purchase menu text 
    * method to resolve the string(s) for the list of supported liveries, so author can optionally present them in buy menu details?

Purchase sprite

* var 'current livery' uses the default livery in purchase list?
* the actual choice of sprite to show will be determined in the action 2 chain by author
    * if they want to do more fancy things there, they can 

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*Livery refit UI*

UI window

* similar to cargo subtypes UI
* for trains only
    * how to build the list of liveries when more than one vehicle is selected?
    * some AND or OR of the supported liveries for the selected vehicles?
* how to open the UI?  If only we had fewer button in vehicle window..like the eye icon? :P

Preview sprite

* livery menu will be more useful if displays a sprite showing how the livery will look 
* use the purchase sprite?
    * when cycling the livery options to draw the UI, provide the cycled livery ID as extra callback info 
* what about when multiple vehicles are selected (trains only)?

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*Autoreplace*

* Replace type A-> B
* Either
    * choose explicit livery for B from the list of supported liveries
    * or choose 'keep livery if supported'
        * persists the livery value of each A vehicle if vehicle B supports it
        * fall back to B's default livery if A's livery not supported by B
* No magic to support rules on 'only type A with livery Foo'
    * make groups for this case

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*Autorenew*

* preserve the livery if available
* otherwise fall back
* it's possible that some authors will want to be able to control this to provide specific repaints when vehicles are auto-renewed
    * could have a CB, 'replacing vehicle with livery X, return livery'
    * TMWFTLB imho
    * nothing to stop this being provided later, let's not try to boil the ocean?

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*Vehicle list windows*

* sort by livery?
* group by livery?

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*Sorting / filtering of purchase menu*

* sorting by livery?
* filtering by livery?
* I am -1 to purchase menu sort or filter by livery
    * would add to window clutter, purchase menu has a lot of stuff already
    * adds complications - would 'current livery' var change to match filter?  What if author doesn't handle that in a way that matches the filter? 

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*Use of liveries to modify vehicle stats*

* 'red trains are faster'
* this would be possible, but is an author choice
* do nothing to specifically prevent or enable it
* probably unwise, would be better using vehicles on different IDs
    * but authors gonna author :)

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*AI*

* does the AI need 
    * the list of liveries supported for a vehicle?
    * a method to set the livery on a vehicle?

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*GS* 

* does GS need 
    * a list of liveries per grf?
    * a method to modify the availability of a livery?
* use case would be 'complete goals to unlock this special livery'
    * as per every every 'level-up' grinding-based casual game ever ever