Re: Compressive images test

Hi Frédéric,

If I'd try to "translate" what you said so it'd be better understood by a
wider audience, it would something like:
'JPEG images that can be perfectly divided into 8x8 pixel "boxes" will
compress better'.
This is great, and need to backed up by a larger scale research (which is
on my todo list :) ), but is also slightly tangent to the issue discussed
here.

Correct me if I'm wrong, but I think the point you tried to make is "The
examples we're using are the wrong ones".
If so, I'm not sure I agree. Since many of the images on the web don't
perfectly fit MCUs (The 8x8 pixel "boxes"), I think we should test whatever
we want to test on a large number of images, extracted from the Web. Some
will be a perfect fit, but some won't.

Let me know if I got it wrong,
Yoav





On Fri, Sep 13, 2013 at 8:34 PM, Frédéric Kayser <f.kayser@free.fr> wrote:

> 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<http://www.impulseadventure.com/photo/jpeg-snoop.html> can
> display those but sadly only for sequential JPEGs).
>
>
> Regards
> --
> 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
>
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>