squoosh/src/codecs/rotate/processor.ts

import { RotateOptions } from './processor-meta';

const bpp = 4;

export function rotate(data: ImageData, opts: RotateOptions): ImageData {
  const { rotate } = opts;

  // Early exit if there's no transform.
  if (rotate === 0) return data;

  const flipDimensions = rotate % 180 !== 0;
  const { width: inputWidth, height: inputHeight } = data;
  const outputWidth = flipDimensions ? inputHeight : inputWidth;
  const outputHeight = flipDimensions ? inputWidth : inputHeight;
  const out = new ImageData(outputWidth, outputHeight);
  let i = 0;

  // In the straight-copy case, d1 is x, d2 is y.
  // x starts at 0 and increases.
  // y starts at 0 and increases.
  let d1Start = 0;
  let d1Limit = inputWidth;
  let d1Advance = 1;
  let d1Multiplier = 1;
  let d2Start = 0;
  let d2Limit = inputHeight;
  let d2Advance = 1;
  let d2Multiplier = inputWidth;

  if (rotate === 90) {
    // d1 is y, d2 is x.
    // y starts at its max value and decreases.
    // x starts at 0 and increases.
    d1Start = inputHeight - 1;
    d1Limit = inputHeight;
    d1Advance = -1;
    d1Multiplier = inputWidth;
    d2Start = 0;
    d2Limit = inputWidth;
    d2Advance = 1;
    d2Multiplier = 1;
  } else if (rotate === 180) {
    // d1 is x, d2 is y.
    // x starts at its max and decreases.
    // y starts at its max and decreases.
    d1Start = inputWidth - 1;
    d1Limit = inputWidth;
    d1Advance = -1;
    d1Multiplier = 1;
    d2Start = inputHeight - 1;
    d2Limit = inputHeight;
    d2Advance = -1;
    d2Multiplier = inputWidth;
  } else if (rotate === 270) {
    // d1 is y, d2 is x.
    // y starts at 0 and increases.
    // x starts at its max and decreases.
    d1Start = 0;
    d1Limit = inputHeight;
    d1Advance = 1;
    d1Multiplier = inputWidth;
    d2Start = inputWidth - 1;
    d2Limit = inputWidth;
    d2Advance = -1;
    d2Multiplier = 1;
  }

  for (let d2 = d2Start; d2 >= 0 && d2 < d2Limit; d2 += d2Advance) {
    for (let d1 = d1Start; d1 >= 0 && d1 < d1Limit; d1 += d1Advance) {
      // Iterate over channels:
      const start = ((d1 * d1Multiplier) + (d2 * d2Multiplier)) * bpp;
      for (let j = 0; j < bpp; j += 1) {
        out.data[i] = data.data[start + j];
        i += 1;
      }
    }
  }

  return out;
}