Perspective Transformation II

A perspective projection can be precisely specified through four pairs of corresponding points. By dragging the four corners of the grid above, you can adjust the perspective projection interactively and place the grid anywhere you like in the scene.

The points of the grid are transformed using a transformation matrix, which is computed by solving a series of linear equations derived from the four point-pairs using LU decomposition as implemented by numeric.js.

This technique can also be used to compute a CSS matrix3d transform to place a DOM element within a scene.

<!DOCTYPE html>
<meta charset="utf-8">
<style>

body {
  position: relative;
  width: 960px;
  height: 500px;
}

#background {
  width: 960px;
  height: 500px;
  background: url(mosque.jpg);
}

svg {
  position: absolute;
  top: 0;
  left: 0;
}

.line {
  stroke: cyan;
  stroke-width: 1.5px;
  stroke-linecap: square;
}

.handle {
  fill: none;
  pointer-events: all;
  stroke: #fff;
  stroke-width: 2px;
  cursor: move;
}

#buttons {
  position: absolute;
  top: 20px;
  left: 20px;
  z-index: 2;
}

button {
  display: block;
  width: 10em;
}

button:focus {
  outline: none;
}

</style>
<div id="background"></div>
<div id="buttons">
  <button data-targets="[[492,329],[542,330],[569,434],[424,424]]">Floor</button>
  <button data-targets="[[-28,287],[74,288],[72,413],[-31,404]]">Near Wall</button>
  <button data-targets="[[-194,282],[-95,282],[-100,354],[-200,365]]">Far Wall</button>
  <button data-targets="[[0,0],[400,0],[400,400],[0,400]]">Reset</button>
</div>
<script src="//d3js.org/d3.v3.min.js"></script>
<script src="numeric-solve.min.js"></script>
<script>

var margin = {top: 50, right: 280, bottom: 50, left: 280},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

var sourcePoints = [[0, 0], [width, 0], [width, height], [0, height]],
    targetPoints = [[0, 0], [width, 0], [width, height], [0, height]];

var svg = d3.select("body").append("svg")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom)
  .append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

var line = svg.selectAll(".line")
    .data(d3.range(0, width + 1, 40).map(function(x) { return [[x, 0], [x, height]]; })
      .concat(d3.range(0, height + 1, 40).map(function(y) { return [[0, y], [width, y]]; })))
  .enter().append("path")
    .attr("class", "line line--x");

d3.selectAll("button")
    .datum(function(d) { return JSON.parse(this.getAttribute("data-targets")); })
    .on("click", clicked)
    .call(transformed);

function dragged(d) {
  d3.select(this).attr("transform", "translate(" + (d[0] = d3.event.x) + "," + (d[1] = d3.event.y) + ")");
  transformed();
}

function transformed() {
  for (var a = [], b = [], i = 0, n = sourcePoints.length; i < n; ++i) {
    var s = sourcePoints[i], t = targetPoints[i];
    a.push([s[0], s[1], 1, 0, 0, 0, -s[0] * t[0], -s[1] * t[0]]), b.push(t[0]);
    a.push([0, 0, 0, s[0], s[1], 1, -s[0] * t[1], -s[1] * t[1]]), b.push(t[1]);
  }

var X = solve(a, b, true), matrix = [
    X[0], X[3], 0, X[6],
    X[1], X[4], 0, X[7],
       0,    0, 1,    0,
    X[2], X[5], 0,    1
  ].map(function(x) {
    return d3.round(x, 6);
  });

line.attr("d", function(d) {
    return "M" + project(matrix, d[0]) + "L" + project(matrix, d[1]);
  });
}

// Given a 4x4 perspective transformation matrix, and a 2D point (a 2x1 vector),
// applies the transformation matrix by converting the point to homogeneous
// coordinates at z=0, post-multiplying, and then applying a perspective divide.
function project(matrix, point) {
  point = multiply(matrix, [point[0], point[1], 0, 1]);
  return [point[0] / point[3], point[1] / point[3]];
}

// Post-multiply a 4x4 matrix in column-major order by a 4x1 column vector:
// [ m0 m4 m8  m12 ]   [ v0 ]   [ x ]
// [ m1 m5 m9  m13 ] * [ v1 ] = [ y ]
// [ m2 m6 m10 m14 ]   [ v2 ]   [ z ]
// [ m3 m7 m11 m15 ]   [ v3 ]   [ w ]
function multiply(matrix, vector) {
  return [
    matrix[0] * vector[0] + matrix[4] * vector[1] + matrix[8 ] * vector[2] + matrix[12] * vector[3],
    matrix[1] * vector[0] + matrix[5] * vector[1] + matrix[9 ] * vector[2] + matrix[13] * vector[3],
    matrix[2] * vector[0] + matrix[6] * vector[1] + matrix[10] * vector[2] + matrix[14] * vector[3],
    matrix[3] * vector[0] + matrix[7] * vector[1] + matrix[11] * vector[2] + matrix[15] * vector[3]
  ];
}

</script>

https://d3js.org/d3.v3.min.js