how to determine image size png

Hi all, anyone can help me how to know the pixel size of one pixel??

my image is medtest.png

>> info = imfinfo('medtest.png')

info = 

  struct with fields:

                  Filename: 'C:\Users\Akmal\Documents\MATLAB\Add-Ons\Functions\Region Growing\medtest.png'
               FileModDate: '25-Dec-2020 23:21:43'
                  FileSize: 53631
                    Format: 'png'
             FormatVersion: []
                     Width: 468
                    Height: 400
                  BitDepth: 8
                 ColorType: 'grayscale'
           FormatSignature: [137 80 78 71 13 10 26 10]
                  Colormap: []
                 Histogram: []
             InterlaceType: 'none'
              Transparency: 'none'
    SimpleTransparencyData: []
           BackgroundColor: []
           RenderingIntent: []
            Chromaticities: [0.31269 0.32899 0.63999 0.33001 0.3 0.6 0.15 0.05999]
                     Gamma: 0.45454
               XResolution: 2835
               YResolution: 2835
            ResolutionUnit: 'meter'
                   XOffset: []
                   YOffset: []
                OffsetUnit: []
           SignificantBits: []
              ImageModTime: []
                     Title: []
                    Author: []
               Description: []
                 Copyright: []
              CreationTime: []
                  Software: []
                Disclaimer: []
                   Warning: []
                    Source: []
                   Comment: []
                 OtherText: []
                 
                 

Answer 1

[Walter Roberson]

IF you can trust the headers

               XResolution: 2835
               YResolution: 2835
            ResolutionUnit: 'meter'

then the resolution is 2835 pixels per meter, which happens to correspond to 72 pixels per inch.

However, when you see 72 pixels per inch or 100 pixels per inch, you should pretty much assume that nominal values have been written into the headers, that might have no relationship to reality.

There is probably nothing in those headers that you should trust other than the height and width.

You either need external information (e.g., knowledge that it was taken with a microscope with particular settings), or else you need to know the true size of some object in the field of view in order to calculate scales.