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I am trying to set a specific resolution (PPI) to a PNG file, but I am always getting N/A value

img_png

I if I convert it to JPG I lose the bit-depth that I want to preserve

img_jpg

Notes

Other PNG images without 1-bit of bit-depth shows the PPI well

Is there a way to set a PNG file to

  • 1-bit of color-depth
  • Set a specific PPI, 150 for instance

I have been trying to do it in Photoshop and ACDSee. Should I use another program?

1 Answer 1

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Apply in Photoshop Image > Image size > 150 PPI, NO resample. Then save as PNG. Be sure you have color mode = Bitmap.

Here's my example in Photoshop. I started a new image 300 x 300 pixels, color mode = bitmap with bit depth=1.

I decided with no reason except you have mentioned number 150 let it have 150PPI resolution i.e. the print size I selected was 2 in x 2 in. That selection is arbitary and doesn't affect at all the image content.

As well I could select say 600 PPI. In that case someone who sees that PPI=600 and the size is 300x300 pixels may think that I perhaps want to print it only as 0,5 inch wide and high and continues: Proves nothing, PPI can be changed if needed and that does not affect what the image contains.

Here the PPI is set to 150 in Photoshop:

enter image description here

FastStone image viewer (freeware) digs from the metadata resolution 150 PPI. It says 150 DPI instead of 150 PPI which can make some people angry, because DPI is also printing term and means there a different thing.

enter image description here

You see the button DPI. By clicking it I can change the number to 600:

enter image description here

Reopening the image in Photoshop and checking the image size gives now height and width = 0,5 inch:

enter image description here

The image has stayed all the time same: 300 x 300 px PNG, color bit depth =1.

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  • Thanks for answering. I tried what you say, but I still get N/A in the ppi. What is happening? I am seeing that information in the ACDSee viewer, should I use another viewer? Do you recommend any?
    – ChesuCR
    Mar 26, 2021 at 20:35
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    Ceate a new image which has color mode = bitmap, 1 bit depth, the same pixel dimensions and 150 PPI resolution. Paste your problematic image to that new image. Photoshop should be able to save as metadata the wanted print size . If you have for ex. 300x300 pixels dimensions, 150 DPI, the image print size should be 2 in x 2 in.
    – user82991
    Mar 26, 2021 at 20:47
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    @ChesuCR ( Continued): Hopefully you know that PPI is metadata, not a part of the image content. For bitmap images the only important size thing is the dimensions in pixels. Print size and PPI do not affect how much information the image contains, they are only nice to know. Nice to know that someone has written them, but I can change the print size as I like. You can see the print size and PPI in Photoshop Image > Image size dialog for the opened image file.
    – user82991
    Mar 26, 2021 at 21:03
  • Aah! I see. Then, it seems that Photoshop does not export the ppi metadata well in this case. And I don't find a way to update it manually. Do you know if there is a way?
    – ChesuCR
    Mar 27, 2021 at 0:14

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