Convert blurhash to a base64 DataURL string (no canvas or node-canvas)

blurhashDataURL.ts

import { decode } from "blurhash"

export function blurHashToDataURL(hash: string | undefined): string | undefined {
  if (!hash) return undefined

  const pixels = decode(hash, 32, 32)
  const dataURL = parsePixels(pixels, 32, 32)
  return dataURL
}

// thanks to https://github.com/wheany/js-png-encoder
function parsePixels(pixels: Uint8ClampedArray, width: number, height: number) {
  const pixelsString = [...pixels].map(byte => String.fromCharCode(byte)).join("")
  const pngString = generatePng(width, height, pixelsString)
  const dataURL = typeof Buffer !== "undefined"
    ? Buffer.from(getPngArray(pngString)).toString("base64")
    : btoa(pngString)
  return "data:image/png;base64," + dataURL
}

function getPngArray(pngString: string) {
  const pngArray = new Uint8Array(pngString.length)
  for (let i = 0; i < pngString.length; i++) {
    pngArray[i] = pngString.charCodeAt(i)
  }
  return pngArray
}

function generatePng(width: number, height: number, rgbaString: string) {
  const DEFLATE_METHOD = String.fromCharCode(0x78, 0x01)
  const CRC_TABLE: number[] = []
  const SIGNATURE = String.fromCharCode(137, 80, 78, 71, 13, 10, 26, 10)
  const NO_FILTER = String.fromCharCode(0)

  let n, c, k

  // make crc table
  for (n = 0; n < 256; n++) {
    c = n
    for (k = 0; k < 8; k++) {
      if (c & 1) {
        c = 0xedb88320 ^ (c >>> 1)
      } else {
        c = c >>> 1
      }
    }
    CRC_TABLE[n] = c
  }

  // Functions
  function inflateStore(data: string) {
    const MAX_STORE_LENGTH = 65535
    let storeBuffer = ""
    let remaining
    let blockType

    for (let i = 0; i < data.length; i += MAX_STORE_LENGTH) {
      remaining = data.length - i
      blockType = ""

      if (remaining <= MAX_STORE_LENGTH) {
        blockType = String.fromCharCode(0x01)
      } else {
        remaining = MAX_STORE_LENGTH
        blockType = String.fromCharCode(0x00)
      }
      // little-endian
      storeBuffer += blockType + String.fromCharCode((remaining & 0xFF), (remaining & 0xFF00) >>> 8)
      storeBuffer += String.fromCharCode(((~remaining) & 0xFF), ((~remaining) & 0xFF00) >>> 8)

      storeBuffer += data.substring(i, i + remaining)
    }

    return storeBuffer
  }

  function adler32(data: string) {
    let MOD_ADLER = 65521
    let a = 1
    let b = 0

    for (let i = 0; i < data.length; i++) {
      a = (a + data.charCodeAt(i)) % MOD_ADLER
      b = (b + a) % MOD_ADLER
    }

    return (b << 16) | a
  }

  function updateCrc(crc: number, buf: string) {
    let c = crc
    let b: number

    for (let n = 0; n < buf.length; n++) {
      b = buf.charCodeAt(n)
      c = CRC_TABLE[(c ^ b) & 0xff] ^ (c >>> 8)
    }
    return c
  }

  function crc(buf: string) {
    return updateCrc(0xffffffff, buf) ^ 0xffffffff
  }

  function dwordAsString(dword: number) {
    return String.fromCharCode(
      (dword & 0xFF000000) >>> 24, (dword & 0x00FF0000) >>> 16, (dword & 0x0000FF00) >>> 8, (dword & 0x000000FF)
    )
  }

  function createChunk(length: number, type: string, data: string) {
    const CRC = crc(type + data)

    return dwordAsString(length) +
      type +
      data +
      dwordAsString(CRC)
  }

  function createIHDR(width: number, height: number) {
    const IHDRdata =
      dwordAsString(width) +
      dwordAsString(height) +
      // bit depth
      String.fromCharCode(8) +
      // color type: 6=truecolor with alpha
      String.fromCharCode(6) +
      // compression method: 0=deflate, only allowed value
      String.fromCharCode(0) +
      // filtering: 0=adaptive, only allowed value
      String.fromCharCode(0) +
      // interlacing: 0=none
      String.fromCharCode(0)

    return createChunk(13, "IHDR", IHDRdata)
  }

  // PNG creations

  const IEND = createChunk(0, "IEND", "")
  const IHDR = createIHDR(width, height)

  let scanlines = ""
  let scanline

  for (let y = 0; y < rgbaString.length; y += width * 4) {
    scanline = NO_FILTER
    if (Array.isArray(rgbaString)) {
      for (let x = 0; x < width * 4; x++) {
        scanline += String.fromCharCode(rgbaString[y + x] & 0xff)
      }
    } else {
      scanline += rgbaString.substr(y, width * 4)
    }
    scanlines += scanline
  }

  const compressedScanlines = DEFLATE_METHOD + inflateStore(scanlines) + dwordAsString(adler32(scanlines))
  const IDAT = createChunk(compressedScanlines.length, "IDAT", compressedScanlines)

  const pngString = SIGNATURE + IHDR + IDAT + IEND
  return pngString
}

Nice! Thank you for sharing this <3 I wanted to optimize it a bit and added simple hashmap caching in my fork

Thank you ! awesome !

Thanks! Really helpful. I just removed some unnecessary things like transforming Uint8ClampedArray to string and simplify png generating to support only Uint8ClampedArray instead of both - array and string.

I think we should create a package for this blurhash to dataUrl util and maybe useBlurHashDataUrl hook in addition.

Thank you. I used the fork by @oleggrishechkin in a Nuxt.js app which now supports SSR and Blurhash thanks to this gist.

I tried thinking of a way to make this so that the base64 string that comes out if it would be webp (for even smaller delivery from SSR to client) but didn't have any luck and don't understand some parts of this gist enough 🙈, so I decided for now png is good enough. Even having SSR blurhash is incredible.

One thing I did change is that I am trying to use fast-blurhash for the decode. It's supposedly faster, even though I did not really benchmark that 🙈.

Thanks for the code! I have adapted and incorporated it into unlazy – a universal lazy loading library for placeholder images:

• 🎀 Native: Utilizes the loading="lazy" attribute
• 🎛️ Framework-agnostic: Works with any framework or no framework at all
• 🌊 BlurHash support: SSR & Client-Side BlurHash Decoding
• 🪄 Sizing: Automatically calculates the sizes attribute
• 🔍 SEO-friendly: Detects search engine bots and preloads all images
• 🎟 <picture>: Supports multiple image tags
• 🏎 Auto-initialize: Usable without a build step

Thanks a lot!