Receivers

A KausticsReceiver is a rectangle that caustics land on: a table top, a wall, a pool floor. Each receiver gets its own irradiance texture covering exactly its rectangle, so you can use it in your own shaders. A renderer draws at most eight receivers, and KausticsRenderer.maxReceivers says so.

Swift

.floor(size: [2, 2])
.wall(center: [0, 1, -1], facing: [0, 0, 1], size: [2, 2])

#Making one

Made with	What it is
`.floor(center:size:resolution:)`	A horizontal rectangle facing up, its width along x.
`.wall(center:facing:size:resolution:)`	A vertical rectangle facing `facing`, its height up the world’s y.
`KausticsReceiver(center:normal:uAxis:size:resolution:)`	Any rectangle. `uAxis` is made square to the normal if it is not.

#Properties

Property	Meaning
`center`	The rectangle’s middle, in world space.
`size`	Its width, along `uAxis`, and its height, along `vAxis`, in world units.
`normal`	The unit normal, which is the side light lands on. Light only lands on that side.
`uAxis`	The unit axis along its width, and texture x.
`vAxis`	The unit axis along its height, and texture y. It is always `cross(normal, uAxis)`.
`resolution`	Texels along its longer side, rounded to a multiple of 32. `nil` uses the renderer’s `resolution`, 1024 by default.

#Texture coordinates

Texture x runs along uAxis and texture y along vAxis. Two methods move between the world and the texture:

Swift

let uv = receiver.uv(of: worldPoint)    // 0 to 1 across the rectangle
let point = receiver.point(at: uv)      // the world point at that uv

In a shader, you sample the irradiance texture at that same UV. See Your own Metal pipeline.

#Where receivers meet

Each receiver takes every triangle against its own plane. That is seamless where receivers meet at an edge and face into the same space, like a floor and its walls, or the inside of a pool.

Receivers that hide one another from the light, such as a box standing on a floor, both get the light.