Unimportant question about importing images into InDesign

[Jason Burnett]

In my last book, I thought I would get ahead of the game by standardizing the size and resolution of all of the images I was importing. So rather than sizing to fit, they would come setup at the right size.

When I did this with .psd documents, they would import at their correct size and dpi. 

But when I decided to make a web-compatible version by converting all the images to .jpg at 150dpi, they imported into  InDesign at 73dpi. Even opening them in Photoshop shows them as 72dpi when Photoshop was the app I used to resize them. Is this expected behavior?  A bug in Photoshop, a bug in InDesign ?  I've worked around the issue, but wandered for future reference.

[leo.r]

well this info is incomplete unless you also specify the physical dimensions of your images as listed in both Photoshop and InDesign. regardless, their pixel dimensions should stay the same anyway.

but in short, yes: when you're dealing with jpgs you can run into all kind of discrepancies in the way InDesign and Photoshop (as well as other apps) report their resolution and physical size. once again, their pixel dimensions remain the same. the difference is in the way individual apps report the resolution and physical dimensions.

[Jason Burnett]

Thanks for the explanation.

It seems weird to me that in the Export As dialog, you are asked the physical dimensions and the ppi you wish to export as. Then it completely ignores what you put in those fields and just saves the actual pixels without telling you. I get why they did it, but they could at least have done the math and let you know tha that the new physical dimensions of your exported jpg were ____ x ____ and the resolution was 72ppi. 

I, apparently mistakenly, assumed that the physical dimensions and the preferred ppi would be saved in the jpg meta data (like they are when saving a copy as a jpg) so that it would be easier for other apps to rely on the physical dimensions. 

I was experimenting with using jpgs in InDesign to make it easier to Export to jpg for a web app that requested 150 ppi jpg documents. It thought maybe it would generate the images faster if the linked images were already 150 ppi and in jpg format. Then the web app I was playing with could create an interactive flip book from the resulting images. Needless to say, none of this went as expected since importing the .jpg versions of the content were twice the physical dimensions and half the resolution of their .psd counterparts.

I'm sure there's a reason why Adobe chose not to include the intended ppi and dimensions in the jpg meta data, but it still seems weird to me. Photoshop looks for that same data when importing a jpg, why not export it as well? Why would they make you "save a copy" to include the intended information? Why even ask the ppi and dimensions you wish to export if they aren't including that in the document somewhere? 

Thanlks for the information and explanation.

[Derek Cross]

The Effective PPI of an image you place in InDesign depends on the size (dimensions) you choose for it. The larger the dimensions the lower the PPI and visa versa.

[rob day]

You have to be careful with InDesign’s Pixel definition. There is a Pixel Ruler Unit that has nothing to do with image resolution—an InDesign page is a vector object and has no resolution. InDesign Pixels are a static output ruler unit that is equal to 1/72".

For example if I Place a 5"x5" 300ppi image at 100% and check its output dimensions with my Ruler Units set as Inches it measures 5"x5" as expected:

If I change the Ruler Units to Pixels the image and its parent frame will measure 360 x 360 px, not the image’s 1500 x 1500 pixel dimension.

If I select the image and Export it as a JPEG at 72ppi, its exported pixel dimensions will match the page item’s 360 x 360 px dimensions. If I export at 150ppi the pixel dimensions will be 750 x 750 px, and if I export at the listed Effective PPI resolution (300ppi in this case) the pixel dimensions will match the original 1500 x 1500 px.

So for any placed image, I could Export a JPEG image at half the original pixel dimensions by taking the Effective Resolution listed in Link Info, dividing it by 2, and using that number for the Export Resolution.

[Derek Cross]

The resolution of an image is irrelevant in images for the web.

Also note, the correct nomenclature for resolution is PPI (Pixels Per Inch) not DPI (Dots Per Inch) which is used for resolution on printers, such as laser printers.

How are you using the "web-compatible" version?

[BobLevine]

What does "web-compatible version" mean? Version of WHAT?

The graphic? The document? Both?

[rob day]

A bug in Photoshop, a bug in InDesign ?

Hi @Jason Burnett , If you are using Photoshop’s Export or legacy Save for Web to save your JPEGs, the Export is to the image’s pixel dimensions with the Resolution set at 72ppi. So if your image is 1500x1500 pixels with an output dimension of 5" x 5" (300ppi), it will Export at the same 1500x1500px dimension, but with an output dimension of 20.83" x 20.83" (72ppi).

If you really want to Place JPEGs use File>Save As JPEG and the image output dimensions and resolution will not change. However, there’s no reason to change the format or resolution of the placed images because for a web destination you are going to have to Export your InDesign layout to either PDF, JPEG, or PNG and in all of those cases you choose an Export Resolution and images with a higher Effective Resolution will get sampled down on the Export.

[Jason Burnett]

I understand that it will be sampled down when I export to jpg for a web app that requests jpgs 150 ppi. I was thinking by sampling down the images prior to importing them, the resampling would be better (as Photoshop offers more resampling options) and the export would be faster and easier for InDesign. I was experimenting when I came across this previously unknown behavior., that's why I asked if it was the expected behavior or if not, was it a bug.
What I find ironic is that the Export to jpg option in InDesign allows you to set the target ppi (and provides the common standards for web images (300, 150, 96, 72) as presets. Then, when you open the exported jpgs created in InDesign, Photoshop lists the images as their intended ppi and dimensions not the 72ppi and resulting pixel dimensions. The behavior seems to be the opposite of how Photoshop exports jpgs. 

[rob day]

for a web app that requests jpgs 150 ppi

Also, HTML doesn’t recognize PPI as a property, all it considers is the image’s pixel dimension—its diplayed dimensions are dependent on the screen’s resolution. A 150ppi image could have any pixel dimension.

[Jason Burnett]

I get that. I'll complain to the service that requires the JPG files be submitted at 150ppi. LOL Thanks for your response.

[Robert at ID-Tasker]

You are confusing printing with web - for printing, 150ppi isn't wrong, but a bit too low.

Or maybe they were talking about LPI? That would make more sense...

[rob day]

You are confusing printing with web - for printing.

Hi Robert, I think @Jason Burnett ’s web developer asked for the 150ppi, which is an odd request for web work. I’m guessing they might be using 150ppi because general users might not know how to set a specific pixel dimension?

[rob day]

The behavior seems to be the opposite of how Photoshop exports jpgs.

In either case it doesn’t matter because both app’s exports assume a web destination where PPI is irrelavent—Photoshop’s Export replaced the legacy Save for Web.

I understand that it will be sampled down when I export to jpg for a web app that requests jpgs 150 ppi. I was thinking by sampling down the images prior to importing them, the resampling would be better (as Photoshop offers more resampling options) and the export would be faster and easier for InDesign.

It would certainly take more time and labor to sample down in Photoshop, save a copy, and replace; than letting it all happen automatically on the Export. You definately don’t want to place JPEG in ID for either print or web, because there will be a double compression on the export to JPEG, or an Export to PDF where the Compression format set to JPEG.

There is some question about InDesign’s general JPEG Export quality when there are linked images involved. If you are worried about quality, exporting to PDF and opening the PDF at the require res can be better.

For print applications I think you are wasting your time trying to keep the Effective Resolution (the output resolution) of all your links at 300ppi and giving up the flexibility to scale as needed. Keep in mind that print output is always going to be to some kind of screen—either halftone (LPI) or stochastic—which is going to interfere with the image pixels. Even if Photoshop does a better job at downsampling the interfering halftone screen isn’t going to be capbable of resolving those subtle differences. That’s why the PDF/X print presets default to Maximum Quailty JPEG—you just can’t see high quality compression artifacts in print.