[whatwg] Image resize API proposal

Sterling Swigart sswigart at google.com
Tue May 11 11:58:41 PDT 2010

I'm working with David Levin, and based on the feedback received regarding
offscreen canvas, the proposal has been changed to address more specific
scenarios. The main use case was resizing images, so we are proposing an
asynchronous image resizing API. If you are curious about how we arrived at
our API below, take a look at the "appendix" to view the alternatives we

Let us know what you think. Thanks!

Use Cases:

Begin with a user giving a local image file to a webpage. Then:

1. In real-time chat, quickly give other users a thumbnail view of the image

2. Or, limit the size of an image file before uploading it to a web server.

Proposed Solution:

We propose adding image.getBlob. getBlob will be an instance function of the
javascript Image object which asynchronously gets a blob of the image,
resized to the given width and height, encoded into jpeg or png. The
function declaration will be:

getBlob(mimeType /* req */, width /* req */, height /* req */, successEvent
/* req */, errorEvent /* op */, qualityLevel /* op */, preserveAspectRatio
/* op */, rotateExif /* op */);

The blob will be passed as an argument to the success callback function, or
upon error, error data will be passed into the error callback function as an
argument. Quality level should be between 0.0 and 1.0, and any value outside
of that range will be reverted to the default, 0.85. If MIME type does not
equal "image/jpeg", then quality level is ignored. If null (or a negative
value) is passed in for the width or height, then the function will use the
source's measurement for that dimension. Default values for
preserveAspectRatio and rotateExif are true.

All EXIF metadata will be retained except for any saved thumbnails, and the
EXIF rotation property will be appropriately modified.


If the image source is of a different origin than the script context, then
getBlob raises a SECURITY_ERR exception.

Sample Code:

// url contains location of an image file

Image i = new Image();

i.src = url;

var successEvt = function (newBlob) { myDiv.innerHTML += "<img src='" +
newBlob.url + "' />"; };

var errEvt = function (err) { alert(err); };

i.getBlob("image/jpeg", 300, 350, successEvt, errEvt, .55);

// Image will retain aspect ratio and correct for EXIF rotation. If the
source image was 700x700,

// the blob will represent a new image that is 300x300.

That's all!Appendix: Alternatives considered

For reference, we've also included a list of other designs that we thought
of along with the reasons why they were dropped

Creating a new object for resizing

Summary of approach:


NamedConstructor=ImageResizer(blob, onsuccess),

NamedConstructor=ImageResizer(blob, onsuccess, onerror),

NamedConstructor=ImageResizer(blob, onsuccess, onerror, type),

NamedConstructor=ImageResizer(blob, onsuccess, onerror, type, width,

interface ImageResizer {

void start(); // starts resize operation

void abort(); // aborts operation

 attribute Blob blob;

attribute DOMString type; // default "image/png"

attribute unsigned long width;

attribute unsigned long height;

attribute float qualityLevel; // default 1.0, must be 0.0 to 1.0, else
reverts to default

 readonly attribute unsigned short started; // default 0

 attribute Function onsuccess;

attribute Function onerror;


Why it wasn't chosen:

Creating an entirely new object for this task made the task seem more
complicated and involved than necessary, and this problem could be solved
via modifications to the Image object.

Returning a SizelessBlob immediately from a method on image

Summary of approach:

var streamingBlob = image.toStreamingBlob(mimeType /* req */, width /* req
*/, height /* req */, qualityLevel /* op */, preserveAspectRatio /* op */,
rotateExif /* op */);

New Blob Interfaces:

interface SizelessBlob {

// moved from Blob

readonly attribute DOMString type;

readonly attribute DOMString url; // whatever name -- URL, urn, URN, etc.


interface StreamingBlob : SizelessBlob {

// at most one of the following functions will be called for a single

attribute Function onblobready;

attribute Function onerror;

readyonly attribute blob; // throws an exception if accessed before
onblobready is called.


interface Blob : SizelessBlob {

readonly attribute unsigned long long size;

Blob slice(in long long start,

in long long length); // raises DOMException


Why it wasn't chosen:

   - the disconnect of the error from the thing that caused it making
   failures hard to understand (e.g. An image load may fail but that may not be
   detected until the xhr is done using the resized image.)
   - the issues that result from passing a SizelessBlob, which has a
   reference to a loading image, to another document and closing the original
   document (thus killing the image loader)
   - introduction of multiple blobs which may be confusing to developers
   - the need to change all existing specs to use SizelessBlob instead of

Returning a Blob immediately from a method on image

Summary of approach:

var blob = image.toBlob(mimeType /* req */, width /* req */, height /* req
*/, qualityLevel /* op */, preserveAspectRatio /* op */, rotateExif /* op

blob.size and blob.slice throw until the blob is complete. There would need
to be a new event to say when the blob's size is ready.

Why it wasn't chosen:

It felt like it was changing how blobs worked to make the size throw. Also,
this has some of the same disadvantages of as the SizelessBlob approach.

Using CSS of Image to designate dimensions instead of putting it in the API

Summary of approach:

img.getBlob(mimeType /* req */, successEvent /* req */, errorEvent /* op */,
qualityLevel /* op */);

Why it wasn't chosen:

It would have been confusing to the user if getBlob took into account some
CSS attributes but not others, and using all CSS tags posed a lot of
unnecessary implementation complexity without any use cases.

Using the File object

Summary of approach:

FileReader readResized(). The result would be a data url with the resized

Why it wasn't chosen:

We also examined making this function an instance method of the FileReader
object, but this function did not fit well in the context of the File
object. FileReader's result is a string, posing a problem for large results
as well as an unnecessary conversion. One way another this problem is to use
a URN (like File.urn), it would have prevented us from posting the result to
a server as well as having the lifetime issues that File.urn does.

Having a very specialized method on FormData to do the resize and allow

Summary of approach:

FormData.appendResized(DOMString name, Blob value, mimeType /* req */, with
/* req */, height /* req */, qualityLevel /* op */, preserveAspectRatio /*
op */, rotateExif /* op */);

Why it wasn't chosen:

This approach was over specialized and didn't allow for using the result on
the web page at all which would be useful as a preview of exactly what is
being uploaded.

Using canvas in a worker

Summary of approach:

This is the OffscreenCanvas proposal which can be seen here:
ttp <http://www.mail-archive.com/whatwg@lists.whatwg.org/msg20297.html>:<http://www.mail-archive.com/whatwg@lists.whatwg.org/msg20297.html>
//www.mail-archive.com/whatwg at lists.whatwg.org/msg20297.html<http://www.mail-archive.com/whatwg@lists.whatwg.org/msg20297.html>

Why it wasn't chosen:

Due to making it rather complicated for the javascript user to accomplish
their goal and the code complexity and the speed trade-offs that may change
due to gpu acceleration, this idea didn't get traction and many folks
suggested something simpler for the intended use cases. A few more drawbacks
are that this doesn't obey exif automatically and it doesn't preserve the
exif metadata.
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