Inspiring

Question

After changing only ppi setting, how to save without data change

Forum|Forum|8 years ago
September 10, 2017
4 replies
1675 views

There are a number of pages on the web that explain how to change the "resolution" in the metadata of an image without doing anything to the actual data. For example:

Quick Tip: How to Change the Pixels Per Inch of An Image in Photoshop — Medialoot

https://indesignsecrets.com/why-is-my-file-size-so-huge.php

Fine, but... none of them tell you how to save your file after you do that. When I try to save, the "JPEG Options" dialog pops up, and the quality setting does not have a "no change" choice or anything. No matter what quality number I choose, the projected filesize is never exactly what I started with, so I know it's resampling the image data. What am I missing?

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T

Test Screen Name

Legend

Thanks for the confirmation, it's an interesting theoretical problem (as well as a very real practical one!)

T

Test Screen Name

Legend

No, it's true. Ish. So long as you understand that every save of a JPEG causes damage. There is no possible change to the instructions to stop that.

Abambo

Community Expert

https://forums.adobe.com/people/Test+Screen+Name wrote
No, it's true. Ish. So long as you understand that every save of a JPEG causes damage. There is no possible change to the instructions to stop that.

It's a good assumption, but it's not 100% true.

There should be no degradation by opening and saving a JPEG file again with the same program and the same quality parameter. Only: if there is a small change in the image (or a programming error, who randomizes some parameters), the JPEG compression will again shuffle bytes some other way and compression errors (the difference between the original and the saved image) will accumulate!

To be on the safe side, best is not to save as JPEG until all manipulations have been done.

What is needed here, is the program to change the exif data. There exists a GUI for the exif tool. I never used the exif tool as anyhow, when I get JPEG files, it's rare that I will use them as recieved. And then I save the file as a TIFF.

ABAMBO | Hard- and Software Engineer | Photographer

T

Test Screen Name

Legend

I am not convinced this is true. "There should be no degradation by opening and saving a JPEG file again with the same program and the same quality parameter. " JPEG compression has multiple parts: colour conversion to YCbCr, downsampling of the Cb and Cr components, quantitisation and entropy encoding. The JPEG specification defines the maths, but not the implementation, especially accuracy and rounding rules. None of these transformations is lossless. I don't understand the entropy encoding.

So, while I can imagine that certain JPEG implementations will be stable if used multiple times (stable meaning that a given image, compressed and recompressed often enough will eventually converge on a particular set of colours that remain the same), other implementations may see oscillating colours, or slow progression.

Abambo

Community Expert

Changing only the ppi makes very little sense. But if you want doing this and you are on Windows, you may use this program: https://www.sno.phy.queensu.ca/~phil/exiftool/

You have to know, that the ppi value is a irrelevant information in a picture. The only use I have for it is having Indesign generating correctly sized preview files.

ABAMBO | Hard- and Software Engineer | Photographer

OsakaWebbieAuthor

Inspiring

That is exactly the application I need this for: placing the photos in InDesign and not ending up with a giant preview file (it's also nice to have the initial size of the photo be appropriate for the intended print resolution rather than starting out huge). I personally already use exiftool on Windows, but I'm advising a fellow member of a magazine design team, who uses a Mac and is more of an artist, not a geek - exiftool is cumbersome, and he already has Photoshop.

I was seeking clarification of what the two pages I linked seem to be saying. For example, the last sentence of my first link says:

You now have an image of the exact same quality and resolution, but the pixels per inch is now 300 instead of 72.

Sure, that's true, until you try to save it, and then you don't have the same quality. It seems that the instructions stop one step too soon.

Stephen Marsh

Community Expert

ExifTool works great on the Mac OS and using Apple Automator it is “easy” to hide the command line stuff:

Prepression: Automator – DIY ExifTool GUI Services

T

Test Screen Name

Legend

JPEG is an image editor first, not a metadata editor, so it is set up to expect you did stuff to the image, and resave it. You probably need a specialist metadata tool. This is important for JPEG especially as each time you save the data is damaged more.

T

Test Screen Name

Legend

And by the way, it's recompressing NOT resampling.

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