PNG bug

[ZZTop]

Ok, so lets say I have PNG file that is 17x17cm with resolution of 72 dpi, when I open it in Paint.net (latest version), it automatically changes its size so it becomes shrink say 9x9 cm (not exact value - just example) and resolution becomes 96 dpi. So for some reason Paint.net is doing an automatic size change of PNG file. Is this an "feature" or there is problem with program behavior?

[IRON67]

I've tried to reproduce that. But all works fine for me. So be sure, that you really save this image with this dimensions. Switch the measurement between pixel/cm and pixel/in and between cm and inch.

 

An image with 17 x 17 cm and 72 dpi means for me 1224 x 1224 px and 122,88 pixel/in.

 

Maybe you have only changed the print size and not the image size.

Edited October 7, 2016 by IRON67

[Ego Eram Reputo]

Your monitor is 96dpi. Paint.net is showing your image at that resolution. Remember, images don't have a resolution built in. They just have a size (height x width) in pixels. This is like a can of paint. A fixed quantity. If you spread the paint really thin you can cover more. Paint.net is showing your image at 96dpi. That's thicker paint than what you've been using, so the image is not spread out so far.

[ZZTop]

Greetings, thanks for replays - however it seems to me that we dont understand eachother so I will provide you with screenshots and file to test. So here it is:

 

On screenshot 1, you can see just opened image in paint.net (on this machine is old version, but its same with new one).

On second image you can see same image just opened in Adobe Photoshop CS6 - tested also on CC2015 at work and it is same.

 

I have also uploaded test PNG to zip file so you can test it yourself.

(edit I just re-uploaded images, so now units are same in both programs.)

 

 

Test picture.zip

Edited October 7, 2016 by ZZTop

[IRON67]

Yeah ... and in both screenshots you can see that the image has the exact same size of 605 x 120 pixels. The "Document Size" is an completely different thing and you have to choose the right conditions for your purpose. BTW is the dpi in the 1st screenshot 96 and in the 2nd 72 so it is no wonder that the dimensions are different.

Edited October 7, 2016 by IRON67

[Red ochre]

Welcome to the forum ZZTop - great user name!

I'm not sure I understand what you think is wrong? - all seems correct to me, I hope the calculations below help.
I also recommend this tutorial resolution tutorial

 

screenshot 1, paint.net
width 605 pixels/96 p.p.i = 6.302083333... inches = correct

screenshot 2,PSCS6.
 width 605 pixels/72 p.p.i = 8.4027777777777... inches = correct

(p.p.i = pixels per inch).

[ZZTop]

Wrong is that Paint.net changes image printing size - If I open image in program and use save as -> PNG to different location and than open that image in Photoshop, than that image would have 96 DPI in Photoshop as well.

 

I understand that there is no quality loss in what Paint.net is doing - it makes higher pixel density with image shrink. But point is that program is making an undesirable and uncontrolled change to file with no apparent reason.

[Red ochre]

I just saved two .png files with paint.net. One at 96 ppi and one at 72ppi. I then opened them in Gimp (as I do not have photoshop on this machine), it reports the resolutions as they were saved (correct to the nearest whole number).

? Perhaps it's a preference setting in Photoshop?... long-shot.

[ZZTop]

You dont understand core of the problem. (please do excuse me Im not trying to be rude, EN is not my native language - I dont know how to express that sentence without being rude, but be sure that I dont have that intention)

 

Problem is happening to files that ware created outside of Paint.net. In test image, I provided in zip file, in my first post - you can observe bug happening.

-First open native file in PS or Gimp and see resolution - 72

-Than open in Paint.net and check there - 96

-Than save image in Paint.net without resizing it or doing anything - just "save as" a backup png copy

-finally open copy in PS/Gimp and copy resolution would be 96

 

From observation above - you can see that PS is displaying true resolution of native file whichever it may be 72 or 96, but Paint.net is changing resolution of native file to 96 without any notice.

 

I dont know in which program that png file is created. It may be Paint.net issue only with that kind of PNG, but it is still an issue, since PS or other programs dont do that automatic change of native file.

 

I hope that issue is now more clear.

[Red ochre]

'101unmanned' from your zip, opened in Gimp - resolution == 72dpi.
Opened in paint.net - resolution = 96 dpi.?

 

Ah - yes I understand now!
It seems paint.net ignores the saved resolution of that file and opens at 96 dpi. Then when it is saved it saves at default 96 d.p.i although the original file was at 72 d.p.i.

 

That does seem very strange to me! ... hopefully Rick (the developer) can explain what's happening?
(btw an image saved at 72 dpi in paint.net re-opens at 72 dpi)

[ZZTop]

Yes when saved as 72 dpi in Paint.net than it reopens file normally as 72 dpi. I didnt want to speak about it before we understand the problem in order not to make bigger confusion. So this issue is only occurring with at least one type of png file, while other programs open that file normally.

[toe_head2001]

It's probably a limitation of the Windows Imaging Component; which paint.net uses for reading most filetypes.

 

Just an educated guess.

[ZZTop]

That could be it, when I open file in regular Windows paint and make "save as" backup copy it changes resolution to 96, but than again way that is not happening when you save file within Paint.net to resolution of 72? It has to be some incompatibility with that png type.

[Rick Brewster]

My guess is that the DPI metadata entry is missing from the file, causing each app to load it and then apply their own default DPI. Photoshop (maybe?) assumes that a missing DPI value should be defaulted to 72 DPI, whereas Paint.NET uses 96 DPI.

 

You should be able to tell this by right-clicking on the PNG file in Explorer, then go to Properties, then Details tab. Look for the rows, "Horizontal resolution" and "Vertical resolution." If they don't exist, then that supports this hypothesis.

 

Another corollary possibility is that Photoshop may omit this metadata when saving if it would be the default value of 72.

[Red ochre]

Hi Rick,
I cannot find any reference to resolution in the Properties/Details tab for any .png files?
...but by placing ZZtop's original test image file into a folder and then selecting to show the V & H resolution in the columns, it reports 96 d.p.i. for both.

 

Using Photoshop Elements 5, on my Vista machine, it reports the resolution as 72 d.p.i.? (Gimp 2.8 reports 72 x 72 ppi).

I'm confused!

[ZZTop]

I have looked into this and Rick is absolutely right. So here is metadata shown by TweakPNG (freeware PNG metadata viewer):

http://entropymine.com/jason/tweakpng/

 

 

And here is the metadata os same image when saved in Paint.net:

 

Now this file would display 96 DPI in whichever program it is opened with, but if we delete metadata values, than say Photoshop would display same file with resolution of 72.

[Red ochre]

Irfanview reports the resolution correctly!

I'm surprised that windows shows 96 dpi when it should just leave those details blank as they aren't there?

[ZZTop]

As this file is software screen, I think both entries 72 and 96 are wrong and correct size is displayed on device screen that is used to show this image - based on resolution of that screen. So in my opinion only correct thing for any image editor would be to follow source file metadata structure and not write anything new there related to image resolution.