[whatwg] img loading events - can load & error fire for the same image?

Ian Hickson ian at hixie.ch
Wed Sep 11 17:24:24 PDT 2013

On Mon, 22 Jul 2013, Jake Archibald wrote:
> Take http://jsbin.com/ifihex/1/edit - the image on this page receives a 
> valid & full HTTP request, but the received data isn't valid image data 
> (it's html).
> Starting from 
> http://www.whatwg.org/specs/web-apps/current-work/multipage/embedded-content-1.html#update-the-image-data, 
> here's how I see the image loading according to the spec
> (everything seems fine until step 14)
> [...]
> * this is the point where I start to get lost, I'm not sure which
> else-like statements (as in "On the other hand" and "Otherwise") are
> linked to which if-like statements

Oh, yeah, this is very confusing. My bad.

I've tried to fix it. Let me know how it is now.

> >> "load" is fired once the width & height can be determined, which 
> >> suggests that an un-decodable image, but with intact headers (which 
> >> give the width & height) will fire "load" but not "error". I agree 
> >> with this, as it means the browser can defer decoding to render time, 
> >> but do we need a way in JS to confirm an image is decodable?
> >
> > What's the use case? (Surely the server should check this on the 
> > server.)
> The use-case would be image loading that happened without a server. Eg, 
> an in-browser Photoshop that operated without user interaction. User 
> selects file on their machine, it has correct headers so fires a load 
> event, but cannot be painted onto the canvas (or paints incorrectly) due 
> to corrupt data. The in-browser app is unable to detect the image is 
> corrupt without building a decoder/validator in JS.

Well... there is one way to tell right now. You can try to draw it to a 
<canvas> and see if anything got drawn by examining all the pixels; if no 
pixels got drown, then it has some data integrity issues.

But really if you're implementing a Photoshop equivalent, you really 
_should_ implement your own decoder, so you can do things like EXIF 
manipulation, handle other image formats, do "pngcrush"-like work, etc.

Ian Hickson               U+1047E                )\._.,--....,'``.    fL
http://ln.hixie.ch/       U+263A                /,   _.. \   _\  ;`._ ,.
Things that are impossible just take longer.   `._.-(,_..'--(,_..'`-.;.'

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