Qimage ppi evaluation -- what's behind it?

jrsack

Newbie

Posts: 8

Qimage ppi evaluation -- what's behind it?

« on: October 15, 2024, 08:31:54 PM »

I've mentioned that recently I am printing contest-winning photographs (at 21x23") for excellent amateur photographers. I then mount the photographs in a public outdoor exhibit gallery. About 20-25 images/month. ( I print all the images at the same size, so that I can efficiently frame and hang them with reusable parts.)

I use Qimage exclusively for printing (which means I don't know squat about printing from Photoshop or Lightroom, etc.). I especially appreciate its assessment of what I'll call "real-pixel density" of the image file, relative to the size I am printing at. Image files I get will vary from 40ppi to 300ppi.

About 15-20% of the images are Poor or Fair quality according to Qimage. I have been going back to these photographers and asking them if perhaps they submitted a downsampled file for web use, etc. They will do their best to send me a new image file -- sometimes the files will go from 100K to 30megs! I guess they just ticked the box that said "superresolution" or such. Somehow, Qimage knows that those aren't real pixels. The ppi doesn't change.

Here are my questions:
1. Some images seem to work ok at "poor" ppi -- the nature of the image may not be something that demands precise detail. Are there some guidelines that might help me know when to work harder with a photographer to get a better image, and when to "let it go"?
2. Are there any tutorials on "preparing photoshop [or lightroom] image files for printing in Qimage" that I could offer the photographers? A good number of them are not technically sophisticated, and just click on buttons with promising-sounding names.
3. How does Qimage assess ppi when the files have been "up-rezed" in PS or LR? It seems to know that the pixels are made up! How does it do that? Just curious.
4. Any other suggestions for me in helping these photographers?

My next idea is to do a workshop environment where the photographers who get it right look over the shoulders of the photographers who get it wrong -- and adjust their practice. One workshop for LR users, one workshop for PS users, etc. I don't know this stuff well enough myself to help them.

Advice appreciated.


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admin

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Posts: 4229

Re: Qimage ppi evaluation -- what's behind it?

« Reply #1 on: October 15, 2024, 11:01:42 PM »

Qimage doesn't care about anything but pixels in the image and how large you print. If someone says they "enlarged" the image, they must have not added any pixels and maybe they just changed the embedded size because Qimage isn't trying to detect fake vs real pixels.

As to what you should tell people, that's super simple. Just tell them to do anything they want with respect to editing but DO NOT under any circumstances resize the image or add/remove pixels! Qimage will do a better job upsampling than Lightroom or Photoshop so just make sure the people submitting photos do not alter the pixel resolution of the images. Qimage will take care of that at print time and it is important that Qimage know the original size (without any resizing or resampling). If you add "fake" pixels with second rate interpolation like those in Lightroom or Photoshop, Qimage will assume those are real pixels and it defeats Qimage's interpolation algorithms.

Regards,
Mike


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jrsack

Newbie

Posts: 8

Re: Qimage ppi evaluation -- what's behind it?

« Reply #2 on: November 09, 2024, 02:58:01 PM »

Mike,

Do you know of any freeware tool (like a javascript tool or something) that one of the photographers I print for could run to get a PPI evaluation of his/her file before they send it to me? Something where they could input the dimensions I give them, and the tool responds with PPI.

I looked and couldn't find anything. The photographers use either Lightroom or Photoshop (mostly Lightroom), and I wonder if there's something in those tools. I'll ask my expert users about that. But wanted to check with you.


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admin

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Re: Qimage ppi evaluation -- what's behind it?

« Reply #3 on: November 09, 2024, 04:03:33 PM »

I don't use any Adobe products but I know Photoshop will show you the "size" of an image so LR probably will too. You can just set the size to, for example, 10 inches wide and it'll calculate the PPI for you. Just be careful not to resample: just change the size and not the pixels because you don't want PS or LR resampling the image.

It's a simple calculation so you can also just take the number of pixels and divide by the size you want. For example, if you have a 3600 x 2400 image and you want to print it at 10x8 inches, you'd have to crop the image a bit to make a 10x8 so 3600/10 is 360 PPI and 2400/8 is 300 PPI. Since it's cropped you take the lower of the two numbers and you know your 10x8 will be 300 PPI.

Regards,
Mike


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jrsack

Newbie

Posts: 8

Re: Qimage ppi evaluation -- what's behind it?

« Reply #4 on: November 09, 2024, 06:48:39 PM »

Thanks! I think this will get me started.

(I don't use any Adobe products anymore either except an old pre-subscription version of Acrobat 10 "Pro", and I only have the copy on my windows 7 OS left of that one! Smiley

I found this tool, which is fortunately independent of Adobe products:
https://fineartprinting.com.sg/image-analyser/

Seems to work ok for my purposes.


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