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Why is there a discrepancy of pixel and photo size when displayed on screen?

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I hope someone is kind enough to help me with my questions:

A) I am using a 825 pixel x 1125 pixel, 300 dpi photo (poker card size)

which shows in the 'resize window' a printed photo size of: 2.75 x 3.75 cm.

However when viewing the photo in painnet on the screen, the ruler on top,

shows something totally different.

A#1) Is it correct, that the ruler displayed, needs to be ignored, as it's not actual cm or inches?

A#2) Is it correct, that the size of the photo will be the one shown in the resize window only?

(Though I have experienced a discrepancy when printing, too).

B ) On the above mentioned 825 x 1125 photo I need to place another photo.

The size of that photo is in pixels: 213 x 312 pixel, 28 dpi.

Now, when I place this smaller photo onto the poker card size,

it's displayed, as if you place a stamp onto an A4 sheet of paper,

despite of the poker size actually only being 0.75 cm wider and the height being only 0.29 cm larger on each size.

B#1) Why am I experiencing such a discrepancy?

B#2) Is this normal, as the computer screen displays it differently, or am I doing something wrong?

My apologies for any confusion, it's difficult to describe, as I don't really understand what's going on with this,

and so might be describing it wrongly or from a wrong angle.

Who can help?

Thank you.

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Where are all the gurus today answering these kind of questions?

OK. Let's try. There are some threads in the forum about dpi, resolution, and printing.

A1) The ruler uses the unit you selected. Also you can see the used unit in the little square where vertical and horizontal ruler meet each other. But what you see in the ruler depends on the current zoom. And 100% just means that each pixel in the image matches one pixel on the display. So if you like to see the image 1 to 1 in cm you have to calculate the right zoom value depending on the dpi of the image and the dpi of the display.

A2) An image has no real world size. There are pixels and dpi values. The dpi describes how many pixels should be used for one inch in the real world. But this is just a hint for the visualization of the pixels on the display or the printer. The resize windows shows this size and the statusbar too.

B1) Because if you put one bitmap on the other the dpi value is not used. Just the pixels. You have to scale the pixels of one of the images.

B2) See above.

The biggest issue is that we still have to use pixel displays nowadays. It would be much better if we could switch to i.e. display postscript and use vector drawing only. You could avoid a lot of problems if you like to increase the dpi value of the display. That's the reason why a lot of applications have to be adapted now for Apples Retina display.

But on the other side there would be no reason to use Paint.NET any longer. We all would switch to Inkscape ;-)

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Just eliminate the confusion, you are mixing between DPI and DPCM "Dot Per Inch and Dot Per Centimetre" for printing.

In Paint.NET UI windows I can't see the term DPI, it is just Resolution, that covers both (dpi & dpcm)

So when you say 300 DPI then you are dealing with inches system and that equals 118.11 DPCM in metric system

I know if I spend all my life trying to explain this I will fail, I am not a good teacher, sorry.

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In answer to B:

B ) On the above mentioned 825 x 1125 photo I need to place another photo.

The size of that photo is in pixels: 213 x 312 pixel, 28 dpi.

You're mixing your information as to how they apply to the medium used.

Screen resolutions are generally quoted at 96DPI (Dots Per Inch). DPI is a piece of misinformation here because screens don't do dots, they do pixels. Better to refer to screens as having 96PPI (Pixels Per Inch). This is not adjustable*. The only resolution your screen can show is 96PPI*. Everything else you can fool in in to showing is just scaling*

* I'm not going to quibble - this is a gross generalization but probably quite valid for this discussion.

Printers do dots. It is correct to refer to a print resolution as 300DPI. Printers can cram many dots into an inch (600DPI), or just a few (50DPI).

Now that we have that clarified, lets look at your two images.

#1 825 pixels x 1125 pixels. If you were to view it at 1:1 on a 96PPI monitor, it would appear to be 8.59inches x 11.72inches (825/96=8.59 and 1125/96=11.718).

If you were to print this at 300 dpi it would be 2.75INCHES x 3.75INCHES (not cm as you stated).

#2 213 pixels x 312 pixels. If you were to view it at 1:1 on a 96PPI monitor, it would appear to be 2.21inches x 3.25inches (213/96=2.218 and 312/96=3.25).

If you were to print this at 300 dpi it would be 0.71inches x 1.04inches.

The fact that you quote these images as having a DPI is incorrect. They don't. They have a size in pixels. That is what Paint.NET shows, pixels, at 96PPI at 100% view size.

If you wish your screen size to match your print size then the cheap option is to adjust your printer to 96DPI. This will match your screen size of 96PPI and all will be well. Actually not, because things look awful when printed at this low resolution.

Better to allow for the printing resolution and make your image the print size (inches) x the resolution (300DPI) = pixels size.

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