fast-png/src/helpers/unfilter.ts

PNG image decoder and encoder written entirely in JavaScript

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

export function unfilterNone(
  currentLine: PngDataArray,
  newLine: PngDataArray,
  bytesPerLine: number,
): void {
  for (let i = 0; i < bytesPerLine; i++) {
    newLine[i] = currentLine[i];
  }
}

export function unfilterSub(
  currentLine: PngDataArray,
  newLine: PngDataArray,
  bytesPerLine: number,
  bytesPerPixel: number,
): void {
  let i = 0;
  for (; i < bytesPerPixel; i++) {
    // just copy first bytes
    newLine[i] = currentLine[i];
  }
  for (; i < bytesPerLine; i++) {
    newLine[i] = (currentLine[i] + newLine[i - bytesPerPixel]) & 0xff;
  }
}

export function unfilterUp(
  currentLine: PngDataArray,
  newLine: PngDataArray,
  prevLine: PngDataArray,
  bytesPerLine: number,
): void {
  let i = 0;
  if (prevLine.length === 0) {
    // just copy bytes for first line
    for (; i < bytesPerLine; i++) {
      newLine[i] = currentLine[i];
    }
  } else {
    for (; i < bytesPerLine; i++) {
      newLine[i] = (currentLine[i] + prevLine[i]) & 0xff;
    }
  }
}

export function unfilterAverage(
  currentLine: PngDataArray,
  newLine: PngDataArray,
  prevLine: PngDataArray,
  bytesPerLine: number,
  bytesPerPixel: number,
): void {
  let i = 0;
  if (prevLine.length === 0) {
    for (; i < bytesPerPixel; i++) {
      newLine[i] = currentLine[i];
    }
    for (; i < bytesPerLine; i++) {
      newLine[i] = (currentLine[i] + (newLine[i - bytesPerPixel] >> 1)) & 0xff;
    }
  } else {
    for (; i < bytesPerPixel; i++) {
      newLine[i] = (currentLine[i] + (prevLine[i] >> 1)) & 0xff;
    }
    for (; i < bytesPerLine; i++) {
      newLine[i] =
        (currentLine[i] + ((newLine[i - bytesPerPixel] + prevLine[i]) >> 1)) &
        0xff;
    }
  }
}

export function unfilterPaeth(
  currentLine: PngDataArray,
  newLine: PngDataArray,
  prevLine: PngDataArray,
  bytesPerLine: number,
  bytesPerPixel: number,
): void {
  let i = 0;
  if (prevLine.length === 0) {
    for (; i < bytesPerPixel; i++) {
      newLine[i] = currentLine[i];
    }
    for (; i < bytesPerLine; i++) {
      newLine[i] = (currentLine[i] + newLine[i - bytesPerPixel]) & 0xff;
    }
  } else {
    for (; i < bytesPerPixel; i++) {
      newLine[i] = (currentLine[i] + prevLine[i]) & 0xff;
    }
    for (; i < bytesPerLine; i++) {
      newLine[i] =
        (currentLine[i] +
          paethPredictor(
            newLine[i - bytesPerPixel],
            prevLine[i],
            prevLine[i - bytesPerPixel],
          )) &
        0xff;
    }
  }
}

function paethPredictor(a: number, b: number, c: number): number {
  const p = a + b - c;
  const pa = Math.abs(p - a);
  const pb = Math.abs(p - b);
  const pc = Math.abs(p - c);
  if (pa <= pb && pa <= pc) return a;
  else if (pb <= pc) return b;
  else return c;
}