What data does PSE2 add when rotating JPG files?

DD
Posted By
Duke_Dobby
May 17, 2004
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214
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5
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Contrary to its competitors, when making a loss-less JPG rotation (e.g., 90 degrees left or right), PSE2 does INCREASE the size of the file… whereby all other programs that do have a similar function and that I am aware of do actually lose some data.

That increase can be 20% or even more — not a small thing! What kind of data and for what purpose/necessity does PSE2 add to the original file?

Hope someone here can enlighten us about this surprising "swelling"…

THANKS!

Duke
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Grant_Dixon
May 17, 2004
Duke

To do a loss-less jpg rotation what is done is a matrix transformation on the file. What I am saying is that to get a loss-less jpg rotation the file is not opened and then rotated and resaved as a jpg. It is rotated without opening.

In Elements when a file is opened it ceases to be a jpg and becomes a raster image. Rotation of this images is loss-less. If you resave it as a jpg there is a loss in data not because of the rotation but because of the jpg compression. The size of the final file will depend on what parameters you have the jpg save set to.

Grant
DD
Duke_Dobby
May 17, 2004
Grant,

Thanks for the quick answer.
Am still wondering though…

To do a loss-less jpg rotation what is done is a matrix transformation
on the file. What I am saying is that to get a loss-less jpg rotation the file is not opened… <<

Yes, since a rotation should simply be a realignment of the pixels along a different axis or direction, technically, there should not be any need to open/resave the file — just a transposition of the pixels or blocks of pixels.

In Elements when a file is opened it ceases to be a jpg and becomes
a raster image. Rotation of this images is loss-less. If you re-save it as a jpg there is a loss in data not because of the rotation but because of the jpg compression. <<

From what you say it would seem that PSE2 does NOT make a JPG loss-less rotation but, instead, DOES open the file, rotates it using its own loss-less format (thus no loss), then saves the rotated data into a JPG format, a step that, obviously, always DOES lose data. And yet, despite that loss, the resulting JPG file DOES end up BIGGER than the original one. So, am still puzzled here. But then there is the last part of your message…

The size of the final file will depend on what parameters you have the
jpg save set to. <<

Are there any kinds of "JPG saving parameters" that do actually "deliver" more pixels than the original file had prior to rotation? Which kind of data do they keep/protect that a normal JPG-save does not protect/save?

Another obvious question (assuming I understood the "PSE2 rotation" right) is this: Does the whole thing mean that, technically, a JPG file rotated in PSE2 maintains a higher quality than the same file rotated with the "JPG loss-less rotation" found in many non-Adobe products?

Thank you, Grant, clarifying that somewhat murky point. (Am sure am not the only one who has been wondering about that for a while)

Duke
JH
Jim_Hess
May 17, 2004
JPEG is a file format used for storing your image. Once you have loaded that image into Photoshop Elements, you are not editing a JPEG image, you are editing pixels. The reason you may see an increase in the size of your edited photograph could be a result of a difference in the amount of compression you are using when you re-save your image in Photoshop Elements.
GD
Grant_Dixon
May 17, 2004
Duke

You’re correct Elements doesn’t do loss-less jpg rotation. Although it does loss-less rotation, and it also can saves as jpg.

Jpg is a complex algorithm that groups pixels into 8×8 matrix and then divides the image into luminosity and two chromes. Then the chromes are compressed more than the luminosity. How much the compression takes place depends on what parameters you choose. At the end the process is reversed. No matter how much or little you choose to compress no pixels are lost or added to the final image, but the information in these pixels are altered. The amount of loss depends on the compression that is selected and the complexity of the image. At moderate compression levels (10-20%) on first compression I dare anyone to tell the difference by eye. The more you compress or the more times you compress you are just compound the problems.

So if you want to save data using Elements then import your jpg. Do what ever you want to in Elements. When it comes time to save your hard work save it as tif or psd and you will maintain all your information. The same is true with any other piece of software when it comes to jpg.

Grant
DD
Duke_Dobby
May 18, 2004
Grant,
Thanks again for the details. I now have a better grasp of what the actual differences between the two approaches (true JPG loss-less rotation vs. rotation of a JPG file with a non-JPG process/format [e.g. psd or tif]) are and why the size of the resulting files moves in opposite directions.

Jim,
Thanks for the input. Yes, we were aware of the impact of the compression factor but we never could figure out the "why". Now, thanks to Grant’s enlightening above explanations, I, my colleagues, and every curious Soul reading this thread will be less puzzled by the "file size mystery".

Now am off to the PS-Album forum to find out how that program deals with with the files when rotating pictures.
See ya all soon…

Duke

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