Does anyone know how PPI is calculated?

[ethank59616381]

Hi,

I'm hoping to make a script that takes an image say at 200DPI and converts it to 300DPI, while also keeping the PPI above 300.

Does anyone know A. how to do this, and B. how it's calculated? 

[Frans v.d. Geest · Accepted solution]

You can not just 'convert' an image to a higher ppi, you must upscale it in Photoshop with all the risk for quality! You can resize to height and widht to get a higher ppi, but the image proportions will then be smaller. That said, is your question more about  using a script and reading the actual ppi or the effective ppi? That is not clear in your question...

[ethank59616381]

If you see my response to Andy below, it's basically just a frustrating process of going fron Indesign to photoshop over and over that i'm attempting to remedy.

[Randy Hagan]

A couple of questions:

1) Do you need effective resolution to bump up from 200 ppi to 3000 ppi because you're enlarging the pixel image to 15 times it's original size? If so, I'd suggest that you give up even trying and find a higher-resolution original because you're built to lose big if you even try to do this.

2) DPI is a print-output measurement for an output device. How does your 200ppi original image print at its original size?

Please let us know the answers to these questions and we can try to help you from there.

Randy

Update: After this posted, your original resolution bump read from 200-300ppi. I could've sworn I saw 3000 in the question before I posted my response. Did you edit it?

[ethank59616381]

Nope, didn't edit it.

Basically, I need all images for print ready output at 300DPI - when I bring up the DPI, the PPI (naturally) goes down. I then am taking images into photoshop, upscaling them 120%, and that is what works best. Sometimes, I have to go to photoshop again and upscale it another 110% or something. It's frustrating and time consuming, so I'm trying to find a good work around

[rob day · Accepted solution]

It‘s a waste of time—upsampling simply adds pixels via an interpolation algorithm, it doesn’t add any detail quality, or information to the image.

300ppi is a rule-of-thumb for generally acceptable quality when the output is to a 150LPI halftone screen.  To get true 300ppi resolution it has to be captured by the camera or scanning input device.

InDesign’s scripting resolution properties are read only, so if you do think upsampling will have some beneficial affect, the script will have to upsample from Photoshop. Photoshop also uses ppi—there is no image dpi property in either app.

[Frans v.d. Geest]

You say:
>when I bring up the DPI, the PPI (naturally) goes down

To be honest, that makes no sense at all. When you output to 2400 Dpi with a line screen of 133/150 Lpi, you will need about  300 Ppi effective resolution. If you really Output at 300 Dpi you need far less effective Ppi... 

image resolution = image line screen x 2

When scaling:

image resolution = (image line screen x 2) x magnification

I feel you are confusing a few things...

[rob day]

I don’t think it’s possible to set a printer’s DPI via scripting

[Eugene Tyson]

That's if you're multiplying x2.

In truth the LPI is multiplied by 1.41

The pixels are rotated 45 degrees in the imagesetter and with them being square, it's the classic theorem of a squared + b squared - which is always 1.41

So you multiply LPI x 1.41 - and depending on print method

Newspapers for example typically have a lower ppi of 80-120

Magazines - 120-150

High end books - 150-175 or even 200.

Most commercial printers would be set to 150 lpi and that idea of being twice the LPI is an old hangup - as 2x150 = 300.

It's just not the case, it's 1.41x150 = 211.5 for DPI.

[Frans v.d. Geest]

Yes, factor 2 is for safety 😉 Just trying to tell that 300 ppi effective resolution is more than enough.

[rob day]

It's just not the case, it's 1.41x150 = 211.5 for DPI.

There is no magic number. The amount of resolution needed depends on the content of the image. If an image has fine details or high contrast there might be little or no halftone interference.

A photograph of fine dark hair on a white background might have edges that drop off to white, and would need more than 300ppi to be fully resolved. An extreme example would be lineart with color, i.e. an engraving with small text. If there’s no halftone on the edges 211ppi would not fully resolve the image text.

The opposite might be a photograph of clouds with little or no edge defintion. Images like that need less than 300ppi or even 200ppi to be fully resolved—an extreme example would be a gradient blend where there are no edges.

[Frans v.d. Geest]

Not to mention FM rasters 😉

[Eugene Tyson]

Of course indeed - a foggy scene wouldn't even require 220 ppi for a 150LPI output. 

There's always exceptions to the general rules. 

Of course, I was just pointing out the x2 rule is not entirely correct and it also depends on substrates etc. 

[rob day]

Here’s the API for placed images in InDesign:

https://www.indesignjs.de/extendscriptAPI/indesign-latest/#Image.html

And the resizeImage() method in the Photoshop API

https://theiviaxx.github.io/photoshop-docs/Photoshop/Document/resizeImage.html#document-resizeimage

[rob day]

With Adobe applications, and the scripting API, image resolution is always referred to as PPI, not DPI.

DPI is sometimes confused with PPI, but DPI is a measure of an output device’s resolution—a platemaker or imagesetter might have a resolution of 2400 DPI. The high imagesetter resloution is needed to draw the halftone (LPI) or stocastic screen, which the press or printer outputs. A 150 LPI haftone screen would need a print device with least 2400 DPI in order to draw the 150LPI halftone screen correctly.

As Frans suggests your script would need to open the image into Photoshop in order to increase its actual PPI resolution. LPI and DPI resolutions are controlled by the print driver and are not a property of a placed image or the document.

Images placed in InDesign have an .actualPpi and .effectivePpi JavaScript read only property—.actualPpi is the resolution of the image at 100%, and .effectivePpi is the scaled resoultion. If an image with an Actual Resolution of 300 PPI is scaled by 50%, its Effective Resolution would be 600 PPI—the pixels are 50% smaller, so the output resolution is twice as high.

In your first post’s screen capture the last column is showing Actual PPI and the middle column is showing the Effective or scaled PPI

[Test Screen Name]

Increasing ppi is rather like trying to turn chicken soup back into chicken. You can’t get there from here. Or rather you can, but your chicken won’t be very convincing. 

[Sheena Kaul]

Hi there, 

I hope your issue has been resolved. But if the issue still persists, please let us know so that we can assist you.

If any of the above-mentioned solutions have worked for you, kindly mark the answer correct. 

Regards,

Sheena