EpicNext-Cms/src/lib/services/imager/png-decode.ts

import { inflateSync } from 'node:zlib'

export interface DecodedPng {
  width: number
  height: number
  /** RGBA8888, row-major, length = width * height * 4 */
  rgba: Buffer
}

function paeth(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
  if (pb <= pc) return b
  return c
}

function unfilter(filter: number, cur: Buffer, prev: Buffer, bpp: number): void {
  const n = cur.length
  switch (filter) {
    case 0:
      break
    case 1:
      for (let i = 0; i < n; i++) cur[i] = (cur[i] + (i >= bpp ? cur[i - bpp] : 0)) & 0xff
      break
    case 2:
      for (let i = 0; i < n; i++) cur[i] = (cur[i] + prev[i]) & 0xff
      break
    case 3:
      for (let i = 0; i < n; i++) {
        const a = i >= bpp ? cur[i - bpp] : 0
        cur[i] = (cur[i] + ((a + prev[i]) >> 1)) & 0xff
      }
      break
    case 4:
      for (let i = 0; i < n; i++) {
        const a = i >= bpp ? cur[i - bpp] : 0
        const c = i >= bpp ? prev[i - bpp] : 0
        cur[i] = (cur[i] + paeth(a, prev[i], c)) & 0xff
      }
      break
    default:
      throw new Error(`unknown PNG filter ${filter}`)
  }
}

/**
 * Minimal PNG decoder for 8-bit non-interlaced images (color type 6/RGBA,
 * 2/RGB, 0/grayscale). Enough to read Nitro spritesheet PNGs (RGBA8888).
 * Returns RGBA pixels. Throws on unsupported formats.
 */
export function decodePng(buf: Buffer): DecodedPng {
  if (buf.length < 8 || buf[0] !== 0x89 || buf[1] !== 0x50 || buf[2] !== 0x4e || buf[3] !== 0x47) {
    throw new Error('not a PNG')
  }
  let off = 8
  let width = 0
  let height = 0
  let bitDepth = 0
  let colorType = 0
  let interlace = 0
  const idat: Buffer[] = []
  while (off + 8 <= buf.length) {
    const len = buf.readUInt32BE(off)
    const type = buf.toString('ascii', off + 4, off + 8)
    const data = buf.subarray(off + 8, off + 8 + len)
    off += 12 + len // length(4) + type(4) + data + crc(4)
    if (type === 'IHDR') {
      width = data.readUInt32BE(0)
      height = data.readUInt32BE(4)
      bitDepth = data[8]
      colorType = data[9]
      interlace = data[12]
    } else if (type === 'IDAT') {
      idat.push(data)
    } else if (type === 'IEND') {
      break
    }
  }
  if (bitDepth !== 8) throw new Error(`unsupported PNG bit depth ${bitDepth}`)
  if (interlace !== 0) throw new Error('interlaced PNG not supported')
  const channels = colorType === 6 ? 4 : colorType === 2 ? 3 : colorType === 0 ? 1 : -1
  if (channels === -1) throw new Error(`unsupported PNG color type ${colorType}`)

  const raw = inflateSync(Buffer.concat(idat))
  const stride = width * channels
  const planar = Buffer.alloc(height * stride)
  let prev = Buffer.alloc(stride)
  let rp = 0
  for (let y = 0; y < height; y++) {
    const filter = raw[rp++]
    const cur = planar.subarray(y * stride, y * stride + stride)
    raw.copy(cur, 0, rp, rp + stride)
    rp += stride
    unfilter(filter, cur, prev, channels)
    prev = cur
  }

  if (channels === 4) return { width, height, rgba: planar }

  const rgba = Buffer.alloc(width * height * 4)
  for (let i = 0, j = 0; i < width * height; i++) {
    if (channels === 3) {
      rgba[j++] = planar[i * 3]
      rgba[j++] = planar[i * 3 + 1]
      rgba[j++] = planar[i * 3 + 2]
    } else {
      const g = planar[i]
      rgba[j++] = g
      rgba[j++] = g
      rgba[j++] = g
    }
    rgba[j++] = 255
  }
  return { width, height, rgba }
}

/** Crop a w×h RGBA region at (x,y) out of a sheetW-wide RGBA buffer. */
export function cropRgba(
  rgba: Buffer,
  sheetW: number,
  x: number,
  y: number,
  w: number,
  h: number,
): Buffer {
  const out = Buffer.alloc(w * h * 4)
  for (let row = 0; row < h; row++) {
    const src = ((y + row) * sheetW + x) * 4
    rgba.copy(out, row * w * 4, src, src + w * 4)
  }
  return out
}