how to compress a base64 image to custom size

This was a fun challenge cuz it involved a binary search until it finds the right size. I’m not going to advice you to solve this with base64 instead of blob cuz you should really handle it as binary (blob) otherwise it takes up ~33% more data as base64

This code includes resizing that you can set a max width/hight and still be able to keep the aspect ratio and auto quality lookup until it finds the correct quality to match the MAX_SIZE

console.log('Downloading lorem ipsum image to simulate a file from user input')

fetch('https://picsum.photos/1920/1080/?random')
.then(res => res.blob())
.then(blob => {
  const img = new Image()
  img.src = URL.createObjectURL(blob)

  console.log(`Original image size (at 1920x1080) is: ${blob.size} bytes`)
  console.log('URL to original image:', img.src)
  
  img.onload = () => resize(img, 'jpeg').then(blob => {
    console.log('Final blob size', blob.size)
    console.log('Final blob url:', URL.createObjectURL(blob))

    console.log('\nNow with webp\n')

    resize(img, 'webp').then(blob => {
      console.log('Final blob size', blob.size)
      console.log('Final blob url:', URL.createObjectURL(blob))
    })
  })
}) 


const MAX_WIDTH = 1280
const MAX_HEIGHT = 720
const MAX_SIZE = 100000 // 100kb

async function resize(img, type="jpeg") {
  const canvas = document.createElement('canvas')
  const ctx = canvas.getContext('2d')
  
  ctx.drawImage(img, 0, 0)
  
  let width = img.width
  let height = img.height
  let start = 0
  let end = 1
  let last, accepted, blob
  
  // keep portration
  if (width > height) {
    if (width > MAX_WIDTH) {
      height *= MAX_WIDTH / width
      width = MAX_WIDTH
    }
  } else {
    if (height > MAX_HEIGHT) {
      width *= MAX_HEIGHT / height
      height = MAX_HEIGHT
    }
  }
  canvas.width = width
  canvas.height = height
  console.log('Scaling image down to max 1280x720 while keeping aspect ratio')
  ctx.drawImage(img, 0, 0, width, height)
  
  accepted = blob = await new Promise(rs => canvas.toBlob(rs, 'image/'+type, 1))
  
  if (blob.size < MAX_SIZE) {
    console.log('No quality change needed')
    return blob
  } else {
    console.log(`Image size after scaling ${blob.size} bytes`)
    console.log('Image sample after resizeing with losseless compression:', URL.createObjectURL(blob))
  }
  
  // Binary search for the right size
  while (true) {
    const mid = Math.round( ((start + end) / 2) * 100 ) / 100
    if (mid === last) break
    last = mid
    blob = await new Promise(rs => canvas.toBlob(rs, 'image/'+type, mid))
        console.log(`Quality set to ${mid} gave a Blob size of ${blob.size} bytes`)
    if (blob.size > MAX_SIZE) { end = mid }
    if (blob.size < MAX_SIZE) { start = mid; accepted = blob }
  }

  return accepted
}

PS/warning Canvas don’t do any good compression, if you paint a jpg picture on a canvas element and get the image back with no resizing, manipulation quality loss or changing the format toBlob('image/jpg', cb, 1) then you will most definitely get a larger file back since they probably already are well compressed and canvas dose none. I only change the quality & max width/height to reduce the size with the canvas api. You would need some compressor to reduce it even more without quality loss.

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