modeling/src/maths/mat4/rightMultiplyVec2.js

  1. const fromValues = require('../vec2/fromValues')
  3. /**
  4.  * Multiply a 2D vector by a matrix (interpreted as 2 row, 1 column).
  5.  *
  6.  * Calculation: result = v*M, where the fourth element is set to 1.
  7.  * @param {vec2} vector - input vector
  8.  * @param {mat4} matrix - input matrix
  9.  * @returns {vec2} a new vector
  10.  * @alias module:modeling/maths/mat4.rightMultiplyVec2
  11.  */
  12. const rightMultiplyVec2 = (vector, matrix) => {
  13.   const [v0, v1] = vector
  14.   const v2 = 0
  15.   const v3 = 1
  16.   let x = v0 * matrix[0] + v1 * matrix[1] + v2 * matrix[2] + v3 * matrix[3]
  17.   let y = v0 * matrix[4] + v1 * matrix[5] + v2 * matrix[6] + v3 * matrix[7]
  18.   const w = v0 * matrix[12] + v1 * matrix[13] + v2 * matrix[14] + v3 * matrix[15]
  20.   // scale such that fourth element becomes 1:
  21.   if (w !== 1) {
  22.     const invw = 1.0 / w
  23.     x *= invw
  24.     y *= invw
  25.   }
  26.   return fromValues(x, y)
  27. }
  29. module.exports = rightMultiplyVec2