PPI and resizing for printing Christmas gifts help, please

Hi, everyone. I posted here yesterday (the link to my original post is below) and have since taken the advice of the two people who replied - I contacted the company I'd ordered artwork from and asked them to redo it from scratch. I'd paid for 300 PPI when printed at 16"x20". The artwork they sent me is only 100 PPI when printed 16"x20". Now I don't trust them to really start from scratch. I'm afraid they're going to upsample it and that is not the quality I paid for. I'm also afraid I won't be able to tell just by looking at the new image that it's just been upsampled. So my question: Is there a objective way to tell if a new image they send has actually been redone from scratch at 300PPI (at 16"x20") or whether they are trying to pass off the original upsampled to 300 PPI? I do have that original image to compare the new one to. If there's a problem, I hope to have something less subjective to tell them other than "The new one looks bad."

Thanks in advance. Very happy to have found this community of experts willing to share their knowledge.

 https://community.adobe.com/t5/photoshop-ecosystem-discussions/ppi-and-resizing-for-printing-christmas-gifts-help-please/m-p/14269586#M768199

~Portraits #1 and #3 came up automatically to 300 PPI when I adjusted the image size to 16" x 20" with resampling unchecked. So I guess those are okay?

By @Robin with the crazy GSD

Yes, that was the right way to do that. They already had the correct number of pixels, they just needed to be distributed across the correct number of inches, and when that was done, it worked out to 300 ppi.

~Portrait #2 only came up to 100 PPI with that same method. So I guess I need to check the Resampling box. And then what?

By @Robin with the crazy GSD

I tried it out and I would recommend this:

1. With Resample off, set it to 16 x 20 inches, which like you said will change Resolution to 100 ppi.

2. Enable Resample.

3. Change Resolution to 300 ppi, and that’s it. Because you don’t want to change the 16 x 20 inches dimensions.

The real solution, of course, is to have the company go back and provide you with a file that really is 16 x 20 inches at 300 ppi without resampling. Because the problem with resampling up from 100 ppi to 300 ppi is that the software is “making up” details to reach 300% enlargement, and that won’t look quite as good as if the original had 300 ppi of real details.

Now, the reason for the odd sizes is probably the way they saved the files. That’s because the way a printed size and resolution is preserved is if the file preserves the PPI metadata. The problem is, some save/export commands in Photoshop include the PPI metadata, and some leave it out to reduce the file size. You may have to tell them this part…to preserve PPI metadata, choose File > Save a Copy, and then choose the format (PSD, TIFF, JPEG…).

If instead they use a command on the File > Export submenu, such as Export As or Save for Web (Legacy), those commands do not preserve ppi metadata and so when opened in another application, the image will probably come up showing some kind of default ppi value like 96 or 72, which screws up the assumed print dimensions. This is probably what you are seeing. So if they are doing the work in Photoshop and they swear that Image Size looks right on their side, encourage them to use File > Save a Copy to preserve the ppi metadata.

First of all, you must understand what pixels per inch, ppi, means. It's a lot simpler than people think: it means exactly what it says, no more, no less. Pixels-per-inch. Read that absolutely literally, word for word, those three words. How many pixels per inch of paper.

An image doesn't "have" a ppi number. The image is just pixels. The ppi number is assigned later, defining the pixel density on paper and thereby the print size.

You can use the ppi number as a guide to resampling. Then it's just simple math. Upsampling 100 ppi to 300 ppi means 3x the linear dimensions - or 9x the area; 9x as many pixels. Where do these pixels come from? That's right, nowhere. They have to be invented out of thin air.

You cannot just upsample a pixel image to 9x the area and expect it to look good. It will look awful. You need to get a better image!

Also see the last post in this thread from today (just so I don't have to type it again):

https://community.adobe.com/t5/photoshop-ecosystem-discussions/best-way-to-enlarge-the-image-without-creating-noise-or-cause-the-photo-to-pixelate/m-p/14268071#M768068