# Underlay Definition
Ground underlays: the base terrain colour that overlays are drawn on top of, and the weight its
hue carries when neighbouring tiles are blended together.


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## UnderlayDefinition.Create
```pascal
function TUnderlayDefinition.Create(id: Integer; const data: TByteArray): TUnderlayDefinition; static;
```
Decodes a ground underlay - the base terrain colour an overlay is drawn on top of.

Beyond the HSL of the colour itself this works out a hue multiplier, which is how heavily this
underlay's hue counts when neighbouring tiles are blended together: a washed out colour has
little hue worth spreading, so it is weighted down, while a saturated mid-tone is weighted up.
`Hue` is pre-multiplied by it so that blending is a plain weighted sum.


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## TUnderlayDefinitionArray.IndexOf
```pascal
function TUnderlayDefinitionArray.IndexOf(id: Integer): Integer;
```
The position of `id` in the array, or `-1` if there is no such underlay.

Underlay definitions are stored one per id with no gaps, so an id is its own index; this exists to
keep that assumption in one place.