Author Topic: jpeg size too large  (Read 3606 times)

Offline heavymeister

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jpeg size too large
« on: August 22, 2006, 03:35:48 PM »
Hi Kirk,

I am testing PM right now and what stuns me a little is the output size of jpegs either in the "email" or "Save as" functions. I usually send baby photos to my mother in law (who is not connected to broadband) as 800x600 jpegs with 72dpi. The file size of one of these photos processed in PM is around 400KB with medium quality setting.
When I use Nikon View or Graphic Converter with the same settings the file size is around 200 KB and looks better. (I kept the metadata in all cases).

As I would like to use only one tool and PM is quite handy in all other aspects, it would be nice if you could check on that file size.

Cheers,
HM

Offline dennis

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Re: jpeg size too large
« Reply #1 on: August 22, 2006, 04:47:43 PM »
The reason JPEG sizes are large is that Photo Mechanic isn't doing any chrominance subsampling.  We used to do this (like most JPEG writers do) but at some point we had made a change to not subsample the chroma and it never got put back in (I think this was when we adjusted the quality settings internally to make the file sizes more linear per the quality setting).  I will be adding chroma subsampling back in for quality 8 and lower, and in 4.5 we will add some kind of preference to control when chroma subsampling is done.

See the following link regarding JPEG if you want more info:

http://en.wikipedia.org/wiki/JPEG

Essentially, most JPEG compressors subsample the chroma by 1/2 since the human visual system is less sensitive to chrominance than luminance.  Since chroma is 2/3 of your image data, removing 1/2 of that reduces the file size by 1/3.  And if you don't subsample the chroma, there is higher frequency data to encode which would require more aggressive quantization to reach a given file size.  If you have an image with vertical red lines, for example, you will notice that Photo Mechanic (right now without chroma subsampling) is better able to hold the sharpness on the color edges).

Look for the fix in the upcoming 4.4.3.3 update.

Regards,

--dennis

Offline heavymeister

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Re: jpeg size too large
« Reply #2 on: August 23, 2006, 03:10:25 AM »
Would be nice to have an option checkbox where you could select chromasubsampling and another one for removal of all (or selected) metadata (like Grapic Converter has).

Offline dennis

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Re: jpeg size too large
« Reply #3 on: August 23, 2006, 02:12:48 PM »
Removing meta-data, yeah...  That has gotten some people into trouble before, like the published article of (I think we all agree) a low-life spammer who was accidentally outed by the online version of the story that included photos with IPTC info of the city he lived in.  Oops.  In this particular case I wasn't too upset, but in general this is not good if you believe that the press should be able to protect their sources.  We do offer the ability to not include Exif data for those who don't want anyone to know their camera (or that they shoot in P mode  ;)).  But this is the subject of another thread of course...  We will probably add the ability to strip IPTC/XMP in the future.  In Photoshop, for example, you have to go to some extra work to hide this (e.g. select all, copy, new document, paste).

But regarding chroma subsampling, we are discussing how to best offer this option.  We are thinking about a preference slider that says at what JPEG quality to start subsampling.

BTW - I was wrong about an earlier post.  I had thought that only one dimension was commonly subsampled (perhaps it was the D1X JPEGs I was looking at).  But typically if chroma subsampling is done, it is done in both the horizontal and vertical dimensions.  So in this case, chroma subsampling will remove 3/4 of 2/3 of the data, which is a savings of 1/2.  Photoshop CS2 starts chroma subsampling at quality 6 and lower.

--dennis