@takram/three-atmosphere/src/helpers/sampleTexture.ts

A Three.js and R3F implementation of Precomputed Atmospheric Scattering

import { HalfFloatType, Vector3, type DataTexture, type Vector2 } from 'three'
import invariant from 'tiny-invariant'

import {
  clamp,
  Float16Array,
  isTypedArray,
  type TypedArray
} from '@takram/three-geospatial'

const vectorScratch1 = /*#__PURE__*/ new Vector3()
const vectorScratch2 = /*#__PURE__*/ new Vector3()
const vectorScratch3 = /*#__PURE__*/ new Vector3()

function samplePixel(
  data: TypedArray,
  index: number,
  result: Vector3
): Vector3 {
  const dataIndex = index * 4 // Assume RGBA
  return result.set(data[dataIndex], data[dataIndex + 1], data[dataIndex + 2])
}

export function sampleTexture(
  texture: DataTexture,
  uv: Vector2,
  result: Vector3
): Vector3 {
  const { width, height } = texture.image
  invariant(isTypedArray(texture.image.data))
  let data = texture.image.data
  if (texture.type === HalfFloatType && data instanceof Uint16Array) {
    // TODO: Cache instance
    data = new Float16Array(data.buffer)
  }

  const x = clamp(uv.x, 0, 1) * (width - 1)
  const y = clamp(uv.y, 0, 1) * (height - 1)
  const xi = Math.floor(x)
  const yi = Math.floor(y)
  const tx = x - xi
  const ty = y - yi
  const sx = tx
  const sy = ty
  const rx0 = xi % width
  const rx1 = (rx0 + 1) % width
  const ry0 = yi % height
  const ry1 = (ry0 + 1) % height
  const v00 = samplePixel(data, ry0 * width + rx0, vectorScratch1)
  const v10 = samplePixel(data, ry0 * width + rx1, vectorScratch2)
  const nx0 = v00.lerp(v10, sx)
  const v01 = samplePixel(data, ry1 * width + rx0, vectorScratch2)
  const v11 = samplePixel(data, ry1 * width + rx1, vectorScratch3)
  const nx1 = v01.lerp(v11, sx)
  return result.copy(nx0.lerp(nx1, sy))
}