fast-png/src/helpers/decodeInterlaceNull.ts

PNG image decoder and encoder written entirely in JavaScript

import type { PngDataArray } from '../types';

import {
  unfilterAverage,
  unfilterNone,
  unfilterPaeth,
  unfilterSub,
  unfilterUp,
} from './unfilter';

const uint16 = new Uint16Array([0x00ff]);
const uint8 = new Uint8Array(uint16.buffer);
const osIsLittleEndian = uint8[0] === 0xff;

const empty = new Uint8Array(0);

interface DecodeInterlaceNullParams {
  data: Uint8Array;
  width: number;
  height: number;
  channels: number;
  depth: number;
}

export function decodeInterlaceNull(
  params: DecodeInterlaceNullParams,
): PngDataArray {
  const { data, width, height, channels, depth } = params;

  const bytesPerPixel = (channels * depth) / 8;
  const bytesPerLine = width * bytesPerPixel;
  const newData = new Uint8Array(height * bytesPerLine);

  let prevLine = empty;
  let offset = 0;
  let currentLine;
  let newLine;

  for (let i = 0; i < height; i++) {
    currentLine = data.subarray(offset + 1, offset + 1 + bytesPerLine);
    newLine = newData.subarray(i * bytesPerLine, (i + 1) * bytesPerLine);
    switch (data[offset]) {
      case 0:
        unfilterNone(currentLine, newLine, bytesPerLine);
        break;
      case 1:
        unfilterSub(currentLine, newLine, bytesPerLine, bytesPerPixel);
        break;
      case 2:
        unfilterUp(currentLine, newLine, prevLine, bytesPerLine);
        break;
      case 3:
        unfilterAverage(
          currentLine,
          newLine,
          prevLine,
          bytesPerLine,
          bytesPerPixel,
        );
        break;
      case 4:
        unfilterPaeth(
          currentLine,
          newLine,
          prevLine,
          bytesPerLine,
          bytesPerPixel,
        );
        break;
      default:
        throw new Error(`Unsupported filter: ${data[offset]}`);
    }
    prevLine = newLine;
    offset += bytesPerLine + 1;
  }

  if (depth === 16) {
    const uint16Data = new Uint16Array(newData.buffer);
    if (osIsLittleEndian) {
      for (let k = 0; k < uint16Data.length; k++) {
        // PNG is always big endian. Swap the bytes.
        uint16Data[k] = swap16(uint16Data[k]);
      }
    }
    return uint16Data;
  } else {
    return newData;
  }
}

function swap16(val: number): number {
  return ((val & 0xff) << 8) | ((val >> 8) & 0xff);
}