Determine Image DPI

determine_dpi.sh

# Based on https://apple.stackexchange.com/a/389067/453491

# In order to calculate DPI, determine units and resolution first.
identify -verbose warc2corpus@2x_300dpi.png | grep -E -i "resolution|units"  
#  Resolution: 118.11x118.11
#  Units: PixelsPerCentimeter
#    png:pHYs: x_res=11811, y_res=11811, units=1

# 1 PixelsPerCentimeter corresponds to 2.54 DPI.
# Calculate DPI using the following command.
identify -format "%[fx:resolution.x*2.54] DPI" warc2corpus@2x_300dpi.png
# 299.999 DPI

# Caveats
#
# If the command below does not return units/resolution,
# this information is missing from the image. This is the case 
# for images exported from whimsical.com, for example.
identify -verbose warc2corpus@2x.png | grep -E -i "resolution|units"   
#  Units: Undefined

# ... in this case, open and save the image to a new file using, 
# for example, Krita or GIMP. Then, try again:
identify -verbose warc2corpus@2x_fix.png | grep -E -i "resolution|units"   
  Resolution: 28.34x28.34
  Units: PixelsPerCentimeter
    png:pHYs: x_res=2834, y_res=2834, units=1