fertig-classic-games/tools/lib/png.js

// A minimal PNG reader, built on node:zlib.
//
// This repository has no dependencies and is not about to grow one for a
// one-off job, so decoding is done here: signature, chunks, inflate, unfilter.
// Enough of the format to read the NES track maps that tools/readExcitebikeMaps.js
// transcribes — greyscale, truecolour, palette and alpha variants at bit
// depths 1 through 16, non-interlaced.

import { inflateSync } from 'node:zlib';

const SIGNATURE = Buffer.from([0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a]);

const CHANNELS = { 0: 1, 2: 3, 3: 1, 4: 2, 6: 4 };

function paeth(a, b, c) {
  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;
  if (pb <= pc) return b;
  return c;
}

/** Undo the per-scanline filters PNG applies before compression. */
function unfilter(raw, width, height, bpp, bytesPerRow) {
  const out = Buffer.alloc(height * bytesPerRow);
  let pos = 0;
  for (let y = 0; y < height; y += 1) {
    const filter = raw[pos];
    pos += 1;
    const row = y * bytesPerRow;
    const prev = row - bytesPerRow;
    for (let i = 0; i < bytesPerRow; i += 1) {
      const x = raw[pos + i];
      const a = i >= bpp ? out[row + i - bpp] : 0;
      const b = y > 0 ? out[prev + i] : 0;
      const c = i >= bpp && y > 0 ? out[prev + i - bpp] : 0;
      let value;
      switch (filter) {
        case 0: value = x; break;
        case 1: value = x + a; break;
        case 2: value = x + b; break;
        case 3: value = x + ((a + b) >> 1); break;
        case 4: value = x + paeth(a, b, c); break;
        default: throw new Error(`unknown PNG filter ${filter} on row ${y}`);
      }
      out[row + i] = value & 0xff;
    }
    pos += bytesPerRow;
  }
  return out;
}

/** Pull `bitDepth`-wide samples out of a packed scanline. */
function sampleAt(row, index, bitDepth) {
  if (bitDepth === 8) return row[index];
  if (bitDepth === 16) return row[index * 2];
  const perByte = 8 / bitDepth;
  const byte = row[Math.floor(index / perByte)];
  const shift = 8 - bitDepth * ((index % perByte) + 1);
  return (byte >> shift) & ((1 << bitDepth) - 1);
}

/**
 * Decode a PNG buffer to `{ width, height, data }`, where `data` is flat RGBA.
 * Interlaced files are rejected rather than silently mis-read.
 */
export function decodePng(buffer) {
  if (!buffer.subarray(0, 8).equals(SIGNATURE)) throw new Error('not a PNG');

  let width = 0;
  let height = 0;
  let bitDepth = 8;
  let colorType = 6;
  let palette = null;
  let transparency = null;
  const idat = [];

  let offset = 8;
  while (offset < buffer.length) {
    const length = buffer.readUInt32BE(offset);
    const type = buffer.toString('ascii', offset + 4, offset + 8);
    const body = buffer.subarray(offset + 8, offset + 8 + length);
    offset += 12 + length;

    if (type === 'IHDR') {
      width = body.readUInt32BE(0);
      height = body.readUInt32BE(4);
      bitDepth = body[8];
      colorType = body[9];
      if (body[12] !== 0) throw new Error('interlaced PNGs are not supported');
    } else if (type === 'PLTE') {
      palette = body;
    } else if (type === 'tRNS') {
      transparency = body;
    } else if (type === 'IDAT') {
      idat.push(body);
    } else if (type === 'IEND') {
      break;
    }
  }

  const channels = CHANNELS[colorType];
  if (!channels) throw new Error(`unsupported PNG colour type ${colorType}`);

  const bitsPerPixel = channels * bitDepth;
  const bytesPerRow = Math.ceil((width * bitsPerPixel) / 8);
  const bpp = Math.max(1, Math.ceil(bitsPerPixel / 8));
  const raw = inflateSync(Buffer.concat(idat));
  const rows = unfilter(raw, width, height, bpp, bytesPerRow);

  const data = new Uint8ClampedArray(width * height * 4);
  const max = (1 << bitDepth) - 1;

  for (let y = 0; y < height; y += 1) {
    const row = rows.subarray(y * bytesPerRow, (y + 1) * bytesPerRow);
    for (let x = 0; x < width; x += 1) {
      const o = (y * width + x) * 4;
      let r; let g; let b; let a = 255;

      if (colorType === 3) {
        const idx = sampleAt(row, x, bitDepth);
        r = palette[idx * 3];
        g = palette[idx * 3 + 1];
        b = palette[idx * 3 + 2];
        if (transparency && idx < transparency.length) a = transparency[idx];
      } else if (colorType === 0 || colorType === 4) {
        const v = sampleAt(row, x * channels, bitDepth);
        r = bitDepth === 8 || bitDepth === 16 ? v : Math.round((v / max) * 255);
        g = r; b = r;
        if (colorType === 4) a = sampleAt(row, x * channels + 1, bitDepth);
      } else {
        r = sampleAt(row, x * channels, bitDepth);
        g = sampleAt(row, x * channels + 1, bitDepth);
        b = sampleAt(row, x * channels + 2, bitDepth);
        if (colorType === 6) a = sampleAt(row, x * channels + 3, bitDepth);
      }

      data[o] = r; data[o + 1] = g; data[o + 2] = b; data[o + 3] = a;
    }
  }

  return { width, height, data };
}

/** 0xRRGGBB at (x, y), ignoring alpha. */
export function pixelAt(img, x, y) {
  const o = (y * img.width + x) * 4;
  return (img.data[o] << 16) | (img.data[o + 1] << 8) | img.data[o + 2];
}