iwmlib/lib/pixi/maps/projections/robinson.js

import Projection from './projection'

/*  */

export default class Robinson extends Projection {
    constructor(lng = 0) {
        super()

        this.lng0 = lng
        this.lengthOfParallel = [
            1.0,
            0.9986,
            0.9954,
            0.99,
            0.9822,
            0.973,
            0.96,
            0.9427,
            0.9216,
            0.8962,
            0.8679,
            0.835,
            0.7986,
            0.7597,
            0.7186,
            0.6732,
            0.6213,
            0.5722,
            0.5322
        ]
        this.distancesFromEquator = [
            0.0,
            0.062,
            0.124,
            0.186,
            0.248,
            0.31,
            0.372,
            0.434,
            0.4958,
            0.5571,
            0.6176,
            0.6769,
            0.7346,
            0.7903,
            0.8435,
            0.8936,
            0.9394,
            0.9761,
            1.0
        ]
    }
    forward(coords) {
        let { x: lat, y: lng } = coords

        lng = this._adjustLng(lng)

        // Get the required indices, the remainder in between low and hight as ratio
        // and the sign of the found indices, as the tables are only in positive direction.
        let { low, high, ratio, sign } = this._getInterpolationValues(lat, 90)

        // Values that lie inbetween two indices are interpolated.
        let y = this._interpolate(this.distancesFromEquator[low], this.distancesFromEquator[high], ratio)

        // Reapply the sign to the vertical position.
        y *= sign

        // The center of the projection is in the center of the map. Therefore we shift the
        // center to the top left corner.
        y = 1 - (y + 1) / 2

        // The lengthOfParallel table provides us with the corresponding scaling factor
        // for a specific latitude. Inbetween values are interpolated as before.
        let proportionalLength = this._interpolate(this.lengthOfParallel[low], this.lengthOfParallel[high], ratio)

        //To normalize the value to a range from -1 to 1.
        let x = (proportionalLength * lng) / 180

        x = (x + 1) / 2

        return { x, y }
    }

    backward(position) {
        let { x, y } = position

        y = 1 - 2 * y
        let sign = Math.sign(y)
        y = Math.abs(y)

        let low = 0
        let high = 0
        for (let i = 0; i < this.distancesFromEquator.length - 1 && y > this.distancesFromEquator[i]; i++) {
            low = i
            high = i + 1
        }

        let lowDist = this.distancesFromEquator[low]
        let highDist = this.distancesFromEquator[high]

        let ratio = highDist - lowDist == 0 ? 0 : (y - lowDist) / (highDist - lowDist)

        let lat = low * 5 + ratio * 5

        let parallelLengthMin = this.lengthOfParallel[low]
        let parallelLengthMax = this.lengthOfParallel[high]

        let completeLength = parallelLengthMin + (parallelLengthMax - parallelLengthMin) * ratio

        x = x * 2 - 1
        let normalizedLength = x / completeLength

        let lng = normalizedLength * 180

        return { x: lat * sign, y: this._adjustLng(lng, true) }
    }

    _adjustLng(lng, inv = false) {
        let moved = inv ? lng + this.lng0 : lng - this.lng0
        if (moved < -180) moved += 360
        if (moved > 180) moved -= 360

        return moved
    }

    _interpolate(a, b, ratio) {
        return a * (1 - ratio) + b * ratio
    }
    _getInterpolationValues(value, max) {
        let sign = Math.sign(value)
        value = Math.min(Math.abs(value), max)
        // Note that min and max can be the same. Which is true
        // when lat is dividable by 5. This also covers the edge cases 0 and 90.
        let minIndex = Math.floor(value / 5)
        let maxIndex = Math.ceil(value / 5)
        let ratio = (value % 5) / 5
        // console.log({ value, minIndex, maxIndex, ratio })
        // console.log(this.lengthOfParallel.length)
        return { low: minIndex, high: maxIndex, ratio, sign }
    }

    toString() {
        return
    }
    get name() {
        return 'Robinson Projection'
    }
}