Resized images remain at original dpi

[Ken Cope]

I know that dpi doesn't really mean much when talking about pixel dimensions, but I wonder whether the resize document is working as expected.

 

1) Open a photo whose dpi is 150 dpi or greater

2) Resize Document to 72 dpi

3) Export resized document

 

Observe:

Export dialogue reports the document as 72 dpi

 

4) Locate the resized document and right click>Properties

 

Observe:

The Details show the document as the new pixel dimensions but the dpi shows the original size, not 72 dpi. Shouldn't this be reported as 72 dpi?

[Alfred]

Is this perhaps something to do with the DPI being stored in more than one place, Ken? If you change the DPI setting for a JPEG which has Exif data, I believe the info in the file header is changed but the Exif data is left alone.

[DarkClown]

Ken, this is maybe related to a common misunderstanding of the meaning of dpi (or ppi). Changing the dpi does not change the dimensions of the picture! It only tells the output media what size each pixel or point should be on this media. Example: You have a picture size 1200pxx800px (stating 300dpi) and you change the dpi to 72 the pixel still will be 1200x800px. The only difference will be if you print it it will 4 be times as large as the version with 300dpi.

With regards to AP: did you resize your document with the checkbox "recalculate" ticked on? In that case AP seems to preserve the "real" output size and reduced pixels accordingly. If you uncheck the "recalculation" box pixelsize remains the original.

[Alfred]

Ken, this is maybe related to a common misunderstanding of the meaning of dpi (or ppi). Changing the dpi does not change the dimensions of the picture! It only tells the output media what size each pixel or point should be on this media.

 

I think you may have skimmed over the first few words of Ken's post, Timo. :)

 

I know that dpi doesn't really mean much when talking about pixel dimensions, but....

[DarkClown]

I think you may have skimmed over the first few words of Ken's post, Timo. :)

Not really ... but it was a little bit vague :-) ... Maybe  I did not fully understand his question than ;-)

[Ken Cope]

Ken, this is maybe related to a common misunderstanding of the meaning of dpi (or ppi). Changing the dpi does not change the dimensions of the picture! It only tells the output media what size each pixel or point should be on this media. Example: You have a picture size 1200pxx800px (stating 300dpi) and you change the dpi to 72 the pixel still will be 1200x800px. The only difference will be if you print it it will 4 be times as large as the version with 300dpi.

 

With regards to AP: did you resize your document with the checkbox "recalculate" ticked on? In that case AP seems to preserve the "real" output size and reduced pixels accordingly. If you uncheck the "recalculation" box pixelsize remains the original.

Thanks, yes I do appreciate this has nothing to do with the dimensions of the picture. :)

 

Where is the checkbox 'recalculate'? Have I missed it? I just see the 'Resample' check box - see screenshot

 

[rah1861]

Concerning the "Recalculate" checkbox - perhaps this is a language issue. I know every time I try to talk to my friend in France about some menu item in software, we always get confused. For example, I ask him to open up Control Panel in Windows and he cannot find it (because he is using a French version of Windows).

 

As far as using dpi on a resize dialog, if you have resample checked, you are telling the software that you really want to resize the image - CHANGE the width and height in pixels. In this condition, dpi is being used as part of the calculation of the final pixel dimensions.

 

So, for example, if you have a 6000 x 4000 image directly out of your 24mp camera (set by the camera at 72 dpi), if you have resample checked and change the dpi to 144, it will double the pixel count (upsize it - "interpolation"). Many image editors allow you to specify a percent value (e.g. 200% to double the size), but Photo doesn't seem to.

 

BUT, if you have resample UNCHECKED, it only allows you to change the dpi, and will not affect the image pixel size, because you have not requested that the image really be resized in its true dimensions. I think this is very confusing for people.

 

I think this is done because when you say you want to "resize" an image, you MIGHT mean how large it prints, not how big the image actually is (in pixels). So just providing the ability to change the dpi (without affecting the true size of the image) makes this possible. But, as has been pointed out, dpi is essentially useless anyway, so what's the point of all this crap except to confuse everyone?!?

 

I mean, I don't think I've ever seen an image viewer/editor whose print dialog didn't provide the ability to specify the size the image will be printed at. You don't need the dpi and it is almost always ignored. IrfanView makes this pretty clear with a checkbox that says to use the dpi as an option.

 

There can be instances where you can use the dpi calculation to check if you have cropped an image too much to print at a certain size (for example, with injet printing, you probably don't want to print at less than say 150dpi). But generally it is just there to confuse.

[Chris_K]

Hi All

 

This is dues to the image using the DPI in the exif data instead of the one the user defined. It is a known issue. You are able to work around this by turning off embed metadata in the export options

 

Cheers

[Ken Cope]

Hi All

 

This is dues to the image using the DPI in the exif data instead of the one the user defined. It is a known issue. You are able to work around this by turning off embed metadata in the export options

 

Cheers

 

Thanks, Chris :)

[Alfred]

Is this perhaps something to do with the DPI being stored in more than one place, Ken? If you change the DPI setting for a JPEG which has Exif data, I believe the info in the file header is changed but the Exif data is left alone.

 

 

This is dues to the image using the DPI in the exif data instead of the one the user defined

 

Thanks for confirming my suspicions, Chris! :)

[DarkClown]

Where is the checkbox 'recalculate'? Have I missed it? I just see the 'Resample' check box - see screenshot

 

 

Upps, sorry, indeed a translation mistrake since I don't have an english AP (working with a german version) - So "Recalculate" was a close but wrong guess for the checkbox name.

[Ken Cope]

Upps, sorry, indeed a translation mistrake since I don't have an english AP (working with a german version) - So "Recalculate" was a close but wrong guess for the checkbox name.

 

I wondered if it may have been a translation issue, thanks for confirming. :)