Level 2

150 points

Exported JPG resizing artefacts when set to 300dpi but not 72dpi

When I create a JPG Export Preset that Sizes to Fit within for example 1200 x 1200 pixels, I have found that the resulting file has square artefacts if I set the preset to 300dpi rather than 72dpi.

I have 2 otherwise identical Export Presets (one 72 and other 300 dpi) and they give different results. At first I thought it was JPG artefacts, but the same thing happens with PNG exports if I change the dpi setting. I am starting with an original RAW file that is 6048 x 4032 pixels.

I checked the exported file in Safari, Preview and GraphicConverter and the same artefacts are there at 100% view.

I always thought that dpi was only a label and didn't affect the file, but clearly somehow it does.

Has anyone else noticed this odd effect?

24" iMac 3.06GHz 4GB RAM, Mac OS X (10.5.7)

Posted on May 19, 2009 11:31 AM

27 replies

Jun 28, 2009 10:55 AM in response to D Anderson

Ah... but you are "assuming" this anomaly will present itself in print files. I'm betting not. The "quadrille" overlay you are seeing is due to screen mode coming back down to 72ppi. You won't see a moire like this in the print output.

In fact, Aperture has shown to do very, very well in up-rez situations. But even then, I would just leave it at 72ppi.

JT

Jun 28, 2009 10:56 AM in response to D Anderson

I'm sure we all have a little "magic" in us. LOL

D Anderson Author

Level 2

150 points

Jun 28, 2009 11:01 AM in response to John Thawley

I'm not quite with you now, if the exported files are the same pixel dimensions in both cases, then they should look the same on the screen regardless of the dpi figure, which we agree is meaningless.

If the exported files are then printed at the same size after export then the same problem we see with the pattern will be apparent in print as well, will it not.

Print out the two files and see.

Jun 28, 2009 11:08 AM in response to D Anderson

Well, not really.

If all three us share a dollar, we'll all get 33 cents... right? What will happen to the other penny?

I'm not going to tell you I "know" how it happens. I can only tell you it is unequivocally the wrong way to do it. You can not ask the software to something wrong and expect it to come out right.

It is an unexpected change so the outcome (on screen) will be unpredictable.

JT

D Anderson Author

Level 2

150 points

Jun 28, 2009 11:19 AM in response to John Thawley

John - if the dpi figure is not used for screen display purposes, I cannot see how there can be a 'wrong' figure. It just shouldn't change anything. It is only a hint for a printing program to use to calculate what size to print the image at. There is nothing sacrosanct about 72dpi, most screens aren't 72 ppi nowadays anyway, not that it should make any difference at all.

The essential figures are the export pixel dimensions, which were the same, so the same scaling should be applied, but somehow a different scaling algorithm appears to be being used dependant upon the DPI figure.

I have not experimented to find if this issue affects particular values of image down or upsizing.

It only came to light by chance and was a source of some consternation here, when I couldn't get a clean export for web purposes until I changed to the 72DPI export. I'm wondering if the MobileMe problem is caused by something similar.

Jun 28, 2009 11:37 AM in response to D Anderson

I don't have the details of his MobilMe issue.

But... seriously, you can't tell the software to contradictory instructions and expect a "normal" outcome. Seriously... think about it... you're telling the application in one click... "please make this file bigger while sizing it smaller."

And, while I understand the potential is there for a user to do it, so is the potential for "learning" the process and/or even reading the Manual. 🙂 After all, it is packaged as a PRO application. LOL Right?

I'm pulling your leg a little bit... but I think you can see my point. I'm not sure we can expect software to protect user's from their own misunderstanding of a process. We have to accept our part of the process and learn the program.

Like I said, I can't speak to the MobileMe. My galleries export fine.

JT

D Anderson Author

Level 2

150 points

Jun 28, 2009 11:57 AM in response to John Thawley

That's the thing John, I didn't think I was asking it to resize by changing the DPI figure.

Are you saying that Aperture uses the camera's bogus 72 DPI figure that it has in the EXIF and resizes its output depending on that? Some cameras have 300DPI in their EXIF I believe, so would you have to change the export presets to 300DPI in those cases?

If it is the case that Aperture attempts to resize the file output pixel dimensions with both the DPI figure and the dimensions in pixels specified at the same time, then it is a bug, it should only allow the user to change one or the other.

I have even resorted to reading the manual again -

Setting Image Resolution When Exporting
You can set the image resolution you want for exported images in the Export Presets
dialog in several ways: by specifying a dpi setting, by specifying a percentage of the
original, or by specifying image dimensions in inches, centimeters, or pixels.

Surely specifying image dimensions in pixels should not be affected by the dpi setting? - so why have a user editable field for dpi when using that option?

Jun 28, 2009 12:24 PM in response to D Anderson

I don't have an answer for you. All I can tell you is I would only the size I am exporting based on the original file size.

JT

D Anderson Author

Level 2

150 points

Jun 28, 2009 12:29 PM in response to John Thawley

Thanks John, as I say, I know what not to do now!

Anatoli

Level 1

9 points

Jul 1, 2009 1:22 AM in response to D Anderson

I'm sure that it's not a scalling issue! If you zoom to 100%, the artefacts are still there, they are real image data. Also, from what you have shown with your images in the second post, this is also not related to jpeg.

If there is a workaround by choosing 72 dpi, than ok. It's not logical, but seems to be a feature of this software. But in the case of mobile me it's a bug, because there is any setting I could change!

Btw. sometimes I'm experiencIng the discussed structures directly in screen-view, inside Aperture. If I restart the program, they vanish.

Maybe it's some driver related issue? Nvidia?
Strange is, that if it would occur for every user, we should be more than 2, who are posting this...

D Anderson Author

Level 2

150 points

Jul 1, 2009 1:47 AM in response to Anatoli

Anatoli wrote:

Btw. sometimes I'm experiencIng the discussed structures directly in screen-view, inside Aperture. If I restart the program, they vanish.

Maybe it's some driver related issue? Nvidia?
Strange is, that if it would occur for every user, we should be more than 2, who are posting this...

NVIDIA GeForce 8800 GS is what I have here. I suppose given that the graphics card is used for so much in Aperture, perhaps it does have some effect in producing these artefacts in the files, I think only Apple can tell us that.

We have eliminated JPG as the cause, it does the same with other types of files. I have also viewed the files on other computers with different monitors and graphics cards and they display the same real artefacts.

Most important thing is to give feedback to the Aperture Team as I don't think they pay much attention to these discussions.

D Anderson Author

Level 2

150 points

Feb 23, 2010 7:43 AM in response to D Anderson

Fixed in Aperture 3

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Exported JPG resizing artefacts when set to 300dpi but not 72dpi