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Stuart444

Resetting the dpi of export images

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Like the link says, some digital file formats record a DPI value, but some do not. And even when they do, it just indicates the intended size of the image. But as we all know, OS X & Windows both allow us to override that when printing by scaling the print to a different size, printing multiple copies to one page, & so on.

 

Regardless, the point I'm trying to make is that the dpi of a print is a physical measurement that doesn't change once the file is printed. The way we normally use ppi for screen images is as a virtual measurement that has no fixed relationship to the pixels of the screen or to the dpi of a printed page. For example, we can resize the image non-distructively by stretching or shrinking it, which does not change its pixel count, or destructively by resampling it, which does. The application software or the rendering library of the OS may display the bitmap of an image file differently at different on-screen sizes using interpolation, dithering, or anti-aliasing algorithms to alter the relationship between the bitmap & its on-screen pixel representation. And of course, files may also contain vector objects & text that have no fixed bitmap representation to begin with.

 

So basically, when we talk about "the pixels of an image," or its dpi or ppi measurements, we could be talking about any of several things, all related but not quite the same thing. It should not be surprising that this so often causes confusion, even for experienced professionals.


Affinity Photo 1.7.3, Affinity Designer 1.7.3, Affinity Publisher 1.7.3; macOS High Sierra 10.13.6 iMac (27-inch, Late 2012); 2.9GHz i5 CPU; NVIDIA GeForce GTX 660M; 8GB RAM
Affinity Photo 1.7.3.155 & Affinity Designer 1.7.3.1 for iPad; 6th Generation iPad 32 GB; Apple Pencil; iPadOS 13.1.2

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So to you there is a difference between a printed and thus fixed dpi and an interchangeable dpi as in a digital image.

Of course there is. One is an unchanging property of a physical object that is the same for everyone. The other is not.

 

I don't think it has been mentioned in this topic but the "Affinity Photo - Understanding DPI" video tutorial emphasizes that dpi is a physical property of prints which when changed (without resampling) has absolutely no effect on the appearance of a digital image, its file size, or its pixel resolution.

 

Regarding the physical pixels of a monitor, they are not the same thing as the dots of ink, toner, or dye on a piece of paper or other physical media. They have different properties because they are created by different physical processes that among other things do not use the same method to create perceptible color or intensity (one is transmissive & the other is reflective). Because they are physically different, no measurement based on one is directly equivalent to a measurement based on the other -- it will always be an "apples to oranges" comparison that can only be stretched so far & retain any useful meaning.


Affinity Photo 1.7.3, Affinity Designer 1.7.3, Affinity Publisher 1.7.3; macOS High Sierra 10.13.6 iMac (27-inch, Late 2012); 2.9GHz i5 CPU; NVIDIA GeForce GTX 660M; 8GB RAM
Affinity Photo 1.7.3.155 & Affinity Designer 1.7.3.1 for iPad; 6th Generation iPad 32 GB; Apple Pencil; iPadOS 13.1.2

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Well actually dpi and ppi is just the very same.

Just in case there is a flicker of life left in this dead horse ...  :lol:

 

The most concise, clearest explanation of why this isn't true I have found so far comes from the Cambridge in Colour "Basics Of Digital Camera Pixels" tutorial (http://www.cambridgeincolour.com/tutorials/digital-camera-pixel.htm). Reduced to the essentials, the distinction is that ppi characterizes image resolution independently of the device, while dpi does not.

 

Sometimes that distinction is not significant, but for anything involving more than one device (like a display screen & a printer) it is.


Affinity Photo 1.7.3, Affinity Designer 1.7.3, Affinity Publisher 1.7.3; macOS High Sierra 10.13.6 iMac (27-inch, Late 2012); 2.9GHz i5 CPU; NVIDIA GeForce GTX 660M; 8GB RAM
Affinity Photo 1.7.3.155 & Affinity Designer 1.7.3.1 for iPad; 6th Generation iPad 32 GB; Apple Pencil; iPadOS 13.1.2

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