Get height and width dimensions from a Base64-encoded PNG image

Question

I have this:

data:image/png;base64,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

I would like to get the height and width from this string using JavaScript. How would I do this? Is it even possible?

You can assume access to jQuery, window.btoa, and window.atob.

Specifically, I am looking for something that will parse and read the base64 string and do two things: (1) Verify that it is a PNG (Note: to me, it appears that all PNGs have the initial substring 'iVBORw0KGgoAAAANSUhEUgAAA'), & (2) Determine from the IHDR header the width, height (& it would be great to get the rest as well: bit depth, color type, compression mode, filter, and interlace). If possible, I'd like to avoid canvas or Image. — Travis Smith, Mar 11 '13 at 17:02
It is not very likely that you will be able to write a PNG parser in Javascript that is faster than the browser's native parser. — SLaks, Mar 11 '13 at 17:27
I don't believe it is possible to get this information with javascript/jQuery, since this information is simply handled by the browser and not exposed to javascript in any way I know of. It makes me wonder where the data uri comes from, and whether you can't get the information before it arrives in the webpage. — Sygmoral, Mar 11 '13 at 17:28
@Sygmoral: It is certainly possible to parse the PNG file format in pure JS. — SLaks, Mar 11 '13 at 17:31

Ry- · Accepted Answer · 2020-05-12T23:40:12.597

I’m sure that it’s possible to parse that out of the PNG somehow, but assuming data URI support (since we can assume atob), you can just create an image and wait for it to load (this works in any format):

var image = document.createElement('img');

image.addEventListener('load', function() {
    // image.width × image.height
});

image.src = 'data:image/png;base64,…';

Here's a demo.

Okay, apparently you’d like to extract this information manually. A PNG file starts with the bytes 89 50 4E 47 0D 01 1A 0A, followed by the IHDR chunk that contains the width and height and must be the first chunk. (Yay, easier!) A chunk has a 4-byte length, a 4-byte type, and then a length-byte content. IHDR’s content starts with a 4-byte width and a 4-byte height, so a PNG’s width and height are always bytes 16–24! This can all be checked if you like, but for a simple way that assumes the PNG is valid:

function toInt32(bytes) {
    return (bytes[0] << 24) | (bytes[1] << 16) | (bytes[2] << 8) | bytes[3];
}

function getDimensions(data) {
    return {
        width: toInt32(data.slice(16, 20)),
        height: toInt32(data.slice(20, 24))
    };
}

var base64Characters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/';

function base64Decode(data) {
    var result = [];
    var current = 0;

    for(var i = 0, c; c = data.charAt(i); i++) {
        if(c === '=') {
            if(i !== data.length - 1 && (i !== data.length - 2 || data.charAt(i + 1) !== '=')) {
                throw new SyntaxError('Unexpected padding character.');
            }

            break;
        }

        var index = base64Characters.indexOf(c);

        if(index === -1) {
            throw new SyntaxError('Invalid Base64 character.');
        }

        current = (current << 6) | index;

        if(i % 4 === 3) {
            result.push(current >> 16, (current & 0xff00) >> 8, current & 0xff);
            current = 0;
        }
    }

    if(i % 4 === 1) {
        throw new SyntaxError('Invalid length for a Base64 string.');
    }

    if(i % 4 === 2) {
        result.push(current >> 4);
    } else if(i % 4 === 3) {
        current <<= 6;
        result.push(current >> 16, (current & 0xff00) >> 8);
    }

    return result;
}

function getPngDimensions(dataUri) {
    if (dataUri.substring(0, 22) !== 'data:image/png;base64,') {
        throw new Error('Unsupported data URI format');
    }

    // 32 base64 characters encode the necessary 24 bytes
    return getDimensions(base64Decode(dataUri.substr(22, 32)));
}

var dimensions = getPngDimensions('data:image/png;base64,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');

console.log(dimensions.width + ' × ' + dimensions.height);

This works great! So I apologize for being unclear in my question. See new comment above. — Travis Smith, Mar 11 '13 at 17:02
@TravisSmith: Okay :D After much arduous toil, difficult labour and hard work, I read a paragraph of specification and made a thing for that. You might want to add some error-checking, though. — Ry-, Mar 11 '13 at 20:37
For those banging their heads on the shortcut via DOM, the IMGs created via `new Image()` or `document.createElement("IMG")` won't have dimensions until _after_ it loads. I figured BASE64 images would "load" instantly... **WRONG!** Adding the "load" event listener and _then_ acting on the height and width was the key. Shameful oversight on my part. :( — Eric L., Jun 24 '13 at 19:37
[As per the spec](https://developer.mozilla.org/en-US/docs/Web/API/HTMLImageElement), there are also the `naturalHeight` and `naturalWidth` properties, which may be more appropriate in cases where you actually display and style the . — Deiwin, Mar 24 '15 at 11:59
a bit of a waste to parse the hole base64 data when you could just read a pice of chunk of it — Endless, May 12 '20 at 20:15

score 21 · Answer 2 · edited Jan 30 '23 at 16:17

And here is a new updated ES6 version three years later with a tiny bit of help from typed arrays that is synchronous and where you don't have to load the hole image to memory to figure it out. So it’s faster :)

Also, it doesn't require any DOM, so it can work inside Workers.

function getPngDimensions(base64) {
  const header = atob(base64.slice(0, 50)).slice(16,24)
  const uint8 = Uint8Array.from(header, c => c.charCodeAt(0))
  const dataView = new DataView(uint8.buffer)

  return {
    width: dataView.getInt32(0),
    height: dataView.getInt32(4)
  }
}

// Just to get some Base64 PNG example
const canvas = document.createElement('canvas')
const base64 = canvas.toDataURL().split(',')[1]

const dimensions = getPngDimensions(base64)
console.log(dimensions)

My recommendation is also that you should try to use typed arrays instead of Base64 and blobs instead of typed arrays when possible. Base64 is the worse container and uses more memory.

So here is a solution for you who already have a blob:

document.createElement('canvas').toBlob(async blob => {
  // blob.arrayBuffer() is the new way to read stuff
  // It may not work in all browsers
  let dv = new DataView(await blob.slice(16, 24).arrayBuffer())

  console.log({
    width: dv.getInt32(0),
    height: dv.getInt32(4)
  })
})

// You could also try out the new experimental createImageBitmap.
// Don't use image or canvas, but this also works in web workers.
// And it also works for more than just PNG images.
// We could expect this is more heavier/slower than just reading bytes 16-24.
document.createElement('canvas').toBlob(async blob => {
  const bitmap = await createImageBitmap(blob)
  const { width, height } = bitmap
  bitmap.close() // GC
  console.log({ width, height })
})

This is great! Any chance you could help with a JPEG version of this function? The Specifications are over my head! — case2000, Jan 17 '17 at 15:39
I get the height in negative **{width: 4718592 height: -1962920}**. Resolution of the image if 2400x2400 — Musab Akram, Dec 25 '19 at 06:12
hmm. just tried changing the canvas to 2400x2400, works just fine. did you try it on anything other than png maybe? (this just works for png). — Endless, Dec 25 '19 at 16:40
Not working geting this errors: "Uncaught DOMException: Failed to execute 'atob' on 'Window': The string to be decoded is not correctly encoded" — Siraj Ali, Nov 01 '22 at 13:37
@SirajAli Are you working with a PNG? if atob don't work, then try any of the other methods i provided. try to get a arrayBuffer and read Int32 at pos 16 & 24 — Endless, Nov 02 '22 at 11:38

Roko C. Buljan · Answer 3 · 2014-09-01T12:17:53.463

7

LIVE DEMO

var imgData = 'data:image/png;base64,iVBORw0.........';    
var img = new Image();       
img.onload = function(){
  alert(img.width +' '+ img.height );
};
img.src = imgData;

edited Sep 01 '14 at 12:17

answered Mar 10 '13 at 21:48

Roko C. Buljan

196,159
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I apologize for being unclear in my question. See new comment above. – Travis Smith Mar 11 '13 at 17:03

Sygmoral · Answer 4 · 2014-09-11T14:11:31.763

5

With jQuery:

var width, height;
$('<img/>').load(function(){
    width = $(this).width();
    height = $(this).height();
    // Call whatever function here that requires the width/height
}).attr('src', datauri)

edited Sep 11 '14 at 14:11

answered Mar 10 '13 at 21:38

Sygmoral

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2
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13

"You can assume access to `jQuery`, window.btoa, and window.atob." – Sygmoral Mar 10 '13 at 21:39
need image to load before you can get dimensions – charlietfl Mar 10 '13 at 21:44
Note: if it had been a normal URL (instead of a data uri), the image might be cached, which means the load event would never happen. So in that case add `.each(function(){if($(this).prop('complete'))$(this).trigger('load')})` to the end. – Sygmoral Mar 10 '13 at 21:53
I apologize for being unclear in my question. See new comment above. – Travis Smith Mar 11 '13 at 17:03
Where do you define `img`? – Sean Kendle Aug 05 '14 at 17:21
@SeanKendle: `$('')` itself creates an Image element work with (note the tags: it creates, it doesn't select). The image does not actually appear in the document. – Sygmoral Sep 10 '14 at 20:34
1

I know that much, I meant the variable `img`, is that somehow implied, like `$(this)`? – Sean Kendle Sep 11 '14 at 13:24
Oh my, you're right... I misunderstood. Indeed, rather than `img.width()` I should have written `$(this).width()`! – Sygmoral Sep 11 '14 at 14:10

score 3 · Answer 5 · edited Jan 30 '23 at 16:21

3

In Node.js:

function getPNGSize (buffer) {
  if (buffer.toString('ascii', 12, 16) === 'CgBI') {
    return {
      'width': buffer.readUInt32BE(32),
      'height': buffer.readUInt32BE(36)
    };
  }
  return {
    'width': buffer.readUInt32BE(16),
    'height': buffer.readUInt32BE(20)
  };
}
getPNGSize(buffer)

edited Jan 30 '23 at 16:21

Peter Mortensen

30,738
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answered Apr 03 '18 at 13:14

аlex

5,426
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care to explain what byte 12-16 means? – Endless May 16 '20 at 21:29
What are all those magic numbers and magic strings? – Peter Mortensen Jan 30 '23 at 16:22

score 0 · Answer 6 · answered Feb 08 '23 at 02:17

You could simply get away with using Image. This returns a tuple of [<width>, <height>].

async function getBase64Dimensions(src) {
  return new Promise((resolve, reject) => {
    const image = new Image();

    image.src = src;

    image.onload = () => resolve([image.width, image.height]);
    image.onerror = () => reject();
  });
}

Type annotations if you can't live without them like me:

async function getBase64Dimensions(src: string) {
  return new Promise<[number, number]>((resolve, reject) => {
    const image = new Image();

    image.src = src;

    image.onload = () => resolve([image.width, image.height]);
    image.onerror = () => reject();
  });
}

Get height and width dimensions from a Base64-encoded PNG image

6 Answers6

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