W3C home > Mailing lists > Public > public-respimg@w3.org > September 2013

Re: Compressive images test

From: Frédéric Kayser <f.kayser@free.fr>
Date: Fri, 13 Sep 2013 20:34:25 +0200
Cc: Jason Grigsby <jason@cloudfour.com>
Message-Id: <2852CF21-C85D-469B-96FF-7FDC59762A8D@free.fr>
To: public-respimg@w3.org
Pardon by French, but apparently these people don't even have a clue of how JPEG compression internally works: no word about chroma subsampling effects and use of a 300x200 pixels image sample that doesn't fit nicely into MCUs.
Here are a few explanations about it:
- the file size of about 20 images at different sizes ranging from 240x160 to 300x200 pixels (resized using ImageMagick convert -resize XxY\! -quality 76% -sampling-factor 2x2 -unsharp 1.5x1+0.7+0.02 —a rough equivalent to Photoshop Save For Web quality 50—), notice the inflections point around 240x160, 264x176 and 288x192, some images at 288x192 or 264x176 weight even less than their 255x190 and 261x174 counterpart!
- a sample 288x192 image with its DCT matrices count
- the same at 291x194 where new DCT matrices have been added into the JPEG file to hold the 3 extra columns and the 2 extra rows (now guess why the file size made such a jump between 288x192 and 291x194)
- finally all the pixels the file holds in the outer matrices even those out of the visible frame (JPEGsnoop can display those but sadly only for sequential JPEGs).

Frédéric Kayser

Le 13 sept. 2013 à 01:17, Jason Grigsby a écrit :

> Compressive images[1][2]. I love the idea and fear it at the same time.
> Love it because it would be oh so easy and awesome. Fear it because even if the file size is smaller, I worry that a memory constrained UA would have trouble decompressing images that contain four times the pixels.
> I do not know if this is a rational fear or if I am simply depriving myself of the chance at a long term retina image love.
> So here's the question: 
> If we wanted to construct a test that would definitively answer whether or not compressive images are safe to use or not, what would that test look like?
> -Jason
> [1] http://filamentgroup.com/lab/rwd_img_compression/
> [2] http://www.netvlies.nl/blog/design-interactie/retina-revolution
> -- 
> +1 (503) 290-1090 o | +1 (503) 502-7211 m | http://cloudfour.com
Received on Friday, 13 September 2013 20:40:32 UTC

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