Fixing image dpi without changing pixels

Help & Support
1

Howdy all,

I have a program that outputs tif files that are read by Photoshop as 72dpi instead of the correct 300dpi.
As a result, Acrobat refuses to OCR (it thinks the images are too large in inches).

Is there a way in Opus to display the dpi of an image? And to fix the dpi without touching the pixels (i.e., the pixel size remains the same, but the dpi data changes)?

Places I looked:

  1. Convert menu
  2. Metadata
  3. Picture Columns which have Res X, Res Y, Physical Height and Width (but could not find a way to edit)

It's easy enough to change the dpi for one image in Photoshop, but what I need is a batch fix.
Thanks in advance for any thoughts.

2

You can do that in Photoshop using "actions".

3

The Pictures > Resolution (X) and (Y) columns will show the DPI of TIFF images, or at least show "72 dpi" for the test image I just saved from Photoshop.

I don't think Opus has anything built-in for editing that field in TIFF images, but Photoshop has batch-processing, as Blueroly mentions.

4

Thanks guys!
For today I'll process in other software as suggested.

Would it make sense to make the resolution columns editable in the metadata section, or is that not how these files work?

5

The best tool for the job is probably Phil Harvey's ExifTool and a button like this:

exiftool.exe -Xresolution=300 -Yresolution=300 {allfilepath}
6

That would be a neat trick if you could pull it off.

What you need is an image editing program that is capable of changing the resolution of a picture. The average ink-jet printer has a dpi of 240, so as a general rule I change all my pictures to that resolution.

To understand how resolution works, consider any image that is 100 cms wide and 200 cms deep at 72 dpi.

If you were simply to convert it to 240 dpi and allow the image editing program to adjust the size automatically, you may well end up with a picture 45 cms wide by 65cms deep. (That's a guess)

The problem comes when you want to make that picture into a 240 dpi image that is 100 cms wide and 200 cms deep. Then the image management program has to interpolate the pixels and try to insert extra pixels to fill the vastly increased image area. Some make a better job than others, but none of them can really do it and hang on to the quality of the image, which is why you see an awful lot of "blown" images in the internet.

If you really want to do this sort of interpolation, you should look at a program that uses fractal technology to make a better job of the interpolation. Perfect Resize makes as good a job as most.

7

The aim is just to change the DPI number stored inside the TIFF headers, while keeping the pixel data unchanged.

(Resizing the pixel data is actually easier, but not what the OP wants to do in this case.)

8

I'm a big fan of IrfanView and have been using it for years. Its batch processing is pretty good.

Here are screenshots of the batch and advanced editing dialogs found in IrfanView. With all other values empty and/or unchecked, IrfanView will only modify the DPI as per the setting I outlined in red. With the first dialog, you can select many files (even with wildcard options) and have it output over the top of the same files or use a separate destination directory to make sure it didn't do bad stuff. If it's image related and I can't do it in DOpus, I can always get it done in IrfanView.

5e3378035d3d6afe267fdcd8ebd09eb8cb0ad9b2.png745×586 57.9 KB

7ffb1d91e1fee912e425c1748a6ddda80deeba19.png777×524 62.9 KB

9

@RicFischer Thank you for the terrific screenshots!
I had considered reinstalling IrfanView just for this purpose, and you've now sold me as the advanced settings are quite convincing.

@blueroly's Photoshop solution is also excellent, used it on the first batch of 600 files.

Cheers to you both!

10

Update: this solution by Leo is ideal.

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