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Affinity Publisher: What is the right resolution for an image intended for the screen?


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12 minutes ago, Chills said:

Because it does spilling and grimmer chicks.

My comment was about a Preflight message that the DPI was too low for printing, when the document wasn't going to be printed.

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10 minutes ago, Chills said:

Back on topic: In reality, this is true:-

1 hour ago, Twolane said:

Use 72 or 96 for your on-screen images. Anything else is overkill for a screen, be it lapatop, desktop, or phone.

I tend to go for 96-150 depending on the image, but that is it.

Are you changing the DPI in Affinity documents and resampling to change the file’s dimensions?

For on-screen viewing of PNG and JPG files, the file dimensions (pixel resolution) matters, DPI does not.

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4 minutes ago, walt.farrell said:

My comment was about a Preflight message that the DPI was too low for printing, when the document wasn't going to be printed.

You only know that after you have looked at the pre-flight. My point was you should always look at the preflight.  It applies to any sort of output, not just output to paper.

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There's a lot of discussion, but none of it answers the question of why, with the same 1280 × 800 image in a 1920 × 1080 area, we get an indication of 45 dpi with a document resolution of 72 DPI, and 250 dpi with a document resolution of 400 DPI.
See the video and the Affinity Publisher source.

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35 minutes ago, Pyanepsion said:

There's a lot of discussion, but none of it answers the question of why, with the same 1280 × 800 image in a 1920 × 1080 area, we get an indication of 45 dpi with a document resolution of 72 DPI, and 250 dpi with a document resolution of 400 DPI.

A document's pixel resolution is not the same as its DPI. The latter is the number of 'dots' per inch it has at any particular print size, where 'dots' may mean different things depending on the type of printer technology used to make the print.

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13 minutes ago, Pyanepsion said:

There's a lot of discussion, but none of it answers the question of why, with the same 1280 × 800 (Edit by bruce 1200 x 800) image in a 1920 × 1080 area, we get an indication of 45 dpi with a document resolution of 72 DPI, and 250 dpi with a document resolution of 400 DPI.
See the video and the Affinity Publisher source.

Because of the difference between 1200 and 1920.

Here at 100% in a 1920 x 1080 space:

ScreenShot2024-06-24at12_23_32PM.png.365933b7b91ac482f816578b1c1b720b.png

Lots of empty space above and below and left and right at 100%.

=================================================

Here at 160% in a 1920 x 1080 space:

ScreenShot2024-06-24at12_24_11PM.png.3622e23278cd1e8ab9c5c2183bbc3977.png 

See how the picture fills the width and is taller than the 1080 height.

============================

If I change the DPI/PPI for the document the document's size will remain 1920 x 1080 pixels. And the image will remain a bit bigger, 160% bigger. The Image's Pixel dimensions will not change, it will be 1200 x 800 pixels.

1920/1200 = 1.6, or 160%

Changing the DPI/PPI of a document will not change the number of Pixels in a Document that is defined by Pixels as its Units of Measurement. This means the size of the document will be the same. You'll need actual Physical units of measurement.

1920 pixels wide at 72 DPI/PPI is 26.667 inches. 

1920 pixels wide at 400 DPI/PPI is 4.8 inches.

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47 minutes ago, Pyanepsion said:

but none of it answers the question of why, with the same 1280 × 800 image in a 1920 × 1080 area, we get an indication of 45 dpi with a document resolution of 72 DPI, and 250 dpi with a document resolution of 400 DP

The DPI meta data is basically meaningless in this context. It just reflects the proportion of physical pixel dimensions between the placed image (1200px wide) and the document width specified in pixels (1920px) = 0.625, so the placed ppi of the image (45dpi) just reflects the same 0.625 proportion of the document dpi (72). When you increase the document DPI, nothing in the document changes but the DPI meta data., so when you change it from 72 to 400, the placed ppi increases to 250 (which is 0.625 x 400). 

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1 hour ago, Pyanepsion said:

[…] why, with the same 1280 × 800 image in a 1920 × 1080 area, we get an indication of 45 dpi with a document resolution of 72 DPI, and 250 dpi with a document resolution of 400 DPI.

One could make this comparison:

An image is defined by a number of pixels (e.g. 1200 × 800 pixels) as a text is defined by a number of characters.  

  • This short text: "Cordialement" contains 12 characters
  • This long text: "Je vous prie d’agréer, Monsieur, l’expression de mes sentiments cordiaux" contains 72 characters

None of these are related to a physical size, only an amount of information.

If you want to establish the relationship with a physical size, you'll use the resolution, i.e. the number of information elements (be it pixels or characters) per physical unit (e.g. inches).
This means when you change the number of "pixels per inch (ppi)" or "dots per inch (dpi)" for an image, it's like if you say for a text: "A line of 10 inches contains 72 characters" or "A line of 10 inches contains 12 characters". 
Quantity of information (number of pixels or characters) remains the same but physical size changes OR a same physical size will contain different amount of information. 

For example, if I enlarge the physical size of a 72 characters line by 160%, the same length (let's say 10 inches) will contain only 45 characters. 

image.png.ecc295c27eb18f713c0989bc94d1edcb.png

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1 hour ago, Pyanepsion said:

There's a lot of discussion, but none of it answers the question of why, with the same 1280 × 800 image in a 1920 × 1080 area, we get an indication of 45 dpi with a document resolution of 72 DPI, and 250 dpi with a document resolution of 400 DPI.
See the video and the Affinity Publisher source.

To add to the info others have provided, when the 72 DPI and 400 DPI documents are exported as JPG using identical export settings, the resulting JPG files will be the same file size and dimensions.

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Just an empirical observation, here: While the recommended standard resolution for an image to be seen onscreen is either 72 or 96 ppi (depending if coming from a Mac or PC tradition – but 96 seems to be now standardized for the web), higher resolutions are better.

All or most displays are now Retina/HiDPI. They work much better with twice the pixel density. Unless the original image is a low-resolution one, use the higher resolution for a better display.

Paolo

 

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What determines the actual pixel density of images viewed on a computer display is the number of pixels in the image, not its PPI/DPI. 

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1 hour ago, R C-R said:

What determines the actual pixel density of images viewed on a computer display is the number of pixels in the image, not its PPI/DPI. 

There is a minimum to consider. If you refer to 96ppi when drawing/exporting an image, you'll get the minimum resolution. With a higher resolution in the original image, you'll always get a higher density both at the intended maximum size (say, a typical web page), and when downsizing it. 

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12 hours ago, PaoloT said:

There is a minimum to consider. If you refer to 96ppi when drawing/exporting an image, you'll get the minimum resolution. With a higher resolution in the original image, you'll always get a higher density both at the intended maximum size (say, a typical web page), and when downsizing it. 

Pixel density & pixel resolution are two different things.

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38 minutes ago, R C-R said:

Pixel density & pixel resolution are two different things.

Some people are too dense to resolve that?  🙂
Sorry, could not resist.

There is a difference between PPI and DPI https://photographylife.com/dpi-vs-ppi

It is easy to get confused until it is stuck, correctly, in your head.

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9 minutes ago, Chills said:

There is a difference between PPI and DPI https://photographylife.com/dpi-vs-ppi

Yes, & from that reference there is this (emphasis added):

Quote

DPI stands for dots per inch and refers to the resolution of a printer. It describes the density of ink dots placed on a sheet of paper (or another photographic medium) by a printer to create a physical print. DPI has nothing to do with anything displayed digitally! 

Web pages are displayed digitally on screens. Thus, DPI is (as the spotlight article & many other sources!) mention, completely irrelevant if you are just viewing the web page.

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2 minutes ago, R C-R said:

Web pages are displayed digitally on screens. Thus, DPI is (as the spotlight article & many other sources!) mention, completely irrelevant if you are just viewing the web page.

Correct, but what if you expect that some users will be printing some of the images on the website? 

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2 minutes ago, carl123 said:

Correct, but what if you expect that some users will be printing some of the images on the website? 

Then you have no control over how large (the actual physical dimensions) they will print it, & thus the printed DPI. For example, on my Mac, I could choose to print this web page or any part of it scaled to 10%, 50% or whatever.

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1 hour ago, R C-R said:

Pixel density & pixel resolution are two different things

This is so obvious that i wouldn't waste time to discuss it. But if you have a large image at 96ppi, and then resize it to a smaller size, it is as obvious that you'll get an image at a higher resolution. And I maintain my point: in an age of high-resolution displays and efficient display renderers, there is no reason to use 96ppi as a target. A target higher resolution is better, and it is just a matter of common sense to avoid making an image too small or too big.

 

 

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1 minute ago, PaoloT said:

But if you have a large image at 96ppi, and then resize it to a smaller size, it is as obvious that you'll get an image at a higher resolution.

First, as the article you cited says, ppi (pixels per inch) applies to physical things like the resolution of computer screens, not images displayed on them, so here we are not talking about ppi, which will be different for different users & has nothing to do with the pixel resolution of the image.

That said, if you resize it, you will change its DPI but unless you resample it you will not change its pixel resolution.

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24 minutes ago, R C-R said:

First, as the article you cited says

I didn't cite any article. In any case, ok, let's end it here. I'm not ready for the usual endless looping discussion leading to nothing.

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4 hours ago, PaoloT said:

I didn't cite any article.

Sorry for that. I confused you with this one cited by @Chills. But it is worth a read, as is the Spotlight article I referenced in this earlier post.

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