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hgt.js
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hgt.js
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const { readFile } = require('fs');
const { promisify } = require('util');
const asyncReadFile = promisify(readFile);
const INT_MAX = 32767
function avg(v1, v2, f) {
// Case 1: One pixel is INT_MAX --> return other pixel. Do not interpolate.
if (v1 === INT_MAX && v2 !== INT_MAX) return v2
if (v2 === INT_MAX && v1 !== INT_MAX) return v1
// Case 2: Both of the pixels are INT_MAX --> return INT_MAX
// Case 3: Normal interpolation
return v1 + (v2 - v1) * f;
}
function bufferStream(stream) {
return new Promise((resolve, reject) => {
const bufs = [];
stream.on('data', (d) => {
bufs.push(d);
});
stream.on('end', () => {
console.log("end")
resolve(Buffer.concat(bufs));
});
stream.on('error', (err) => {
console.log("error")
reject(err);
});
});
}
class HGT {
constructor(buffer, swLatLng, options) {
this._buffer = buffer;
this._swLatLng = swLatLng;
this.options = Object.assign(
{},
{
interpolation: HGT.bilinear,
},
options,
);
if (buffer.length === 12967201 * 2) {
this._resolution = 1;
this._size = 3601;
} else if (buffer.length === 1442401 * 2) {
this._resolution = 3;
this._size = 1201;
} else {
throw new Error('Unknown tile format (1 arcsecond and 3 arcsecond supported).');
}
}
static async loadFile(path, swLatLng, options) {
const buffer = await asyncReadFile(path);
return new HGT(buffer, swLatLng, options);
}
static async loadStream(stream, swLatLng, options) {
const buffer = await bufferStream(stream);
return new HGT(buffer, swLatLng, options);
}
static nearestNeighbour(row, col) {
return this._rowCol(Math.round(row), Math.round(col));
}
static pixel_ring(x, y, radius) {
// initial position (top-left)
const sx = x-radius
const sy = y-radius
const ptr = [sx, sy]
// width of square to center on x,y with radius
const limit = 2*radius + 1
const offsets = []
for (let j = 0; j < limit; j++) {
let row = []
// produce x-dimension (row)
for (let i = 0; i < limit; i++) {
row.push([...ptr])
ptr[0]++
}
offsets.push([...row])
//console.error(row.map(pair => pair.join(',')).join("\t"))
// shift to next row
ptr[0] = sx
ptr[1]++
}
return offsets.flat(1)
}
// Low-resolution interpolation, worst case solution...
// Average the valid neighbors (e.g. 8 points).
// Visualization: '*' is invalid; so, use neighbor points 'o'
// o o o
// o * o
// o o o
static avg_neighbor(row, col, radius=1) {
// Closest integer point
const x = Math.round(row);
const y = Math.round(col);
console.error(`Warning: falling back to avg_neighbor(); Rounded: (${x}, ${y})`)
// Get elevation for each position
const coords = HGT.pixel_ring(x, y, radius)
const values = coords.map(coord => this._rowCol(...coord))
//console.error(values)
// Average valid neighbors
const valid = values.flat().filter(x => x !== INT_MAX)
const avg = valid.reduce((a, b) => a + b, 0) / valid.length
console.error({avg, valid})
console.error("")
// Success
if (!isNaN(avg)) return avg
// Failed at radius=2. Aborting.
if (radius == 2) throw Error('avg is NaN at radius=2')
// Failed at radius=1. Try radius=2
return HGT.avg_neighbor.call(this, row, col, 2)
}
static bilinear(row, col) {
const rowLow = Math.floor(row);
const rowHi = rowLow + 1;
const rowFrac = row - rowLow;
const colLow = Math.floor(col);
const colHi = colLow + 1;
const colFrac = col - colLow;
const v00 = this._rowCol(rowLow, colLow);
const v10 = this._rowCol(rowLow, colHi);
const v11 = this._rowCol(rowHi, colHi);
const v01 = this._rowCol(rowHi, colLow);
const v1 = avg(v00, v10, colFrac);
const v2 = avg(v01, v11, colFrac);
const vfinal = avg(v1, v2, rowFrac);
/*
console.log("\n")
console.log('row = ' + row);
console.log('col = ' + col);
console.log('rowLow = ' + rowLow);
console.log('rowHi = ' + rowHi);
console.log('rowFrac = ' + rowFrac);
console.log('colLow = ' + colLow);
console.log('colHi = ' + colHi);
console.log('colFrac = ' + colFrac);
console.log('v00 = ' + v00);
console.log('v10 = ' + v10);
console.log('v11 = ' + v11);
console.log('v01 = ' + v01);
console.log('v1 = ' + v1);
console.log('v2 = ' + v2);
console.log('--> vfinal = ' + vfinal);
*/
if (vfinal !== 32767) return vfinal
// Bilinear interpolation failed, so we fallback to avg_neighbor()
console.error(`Lossy bilinear failed. No valid pixels in quadrant. (${row},${col})`)
return HGT.avg_neighbor.call(this, row, col)
}
getElevation(latLng) {
const size = this._size - 1;
const ll = latLng;
const row = (ll[0] - this._swLatLng[0]) * size;
const col = (ll[1] - this._swLatLng[1]) * size;
if (row < 0 || col < 0 || row > size || col > size) {
throw new Error(
'Latitude/longitude is outside tile bounds (row=' + row + ', col=' + col + '; size=' + size,
);
}
return this.options.interpolation.call(this, row, col);
}
_rowCol(row, col) {
const size = this._size;
const offset = ((size - row - 1) * size + col) * 2;
return this._buffer.readInt16BE(offset);
}
}
module.exports = HGT;