How To Save Dpi Info In Py-opencv?

import cv2 def clear(img): back = cv2.imread('back.png', cv2.IMREAD_GRAYSCALE) img = cv2.bitwise_xor(img, back) ret, img = cv2.threshold(img, 120, 255, cv2.THRESH_BINA

Solution 1:

AFAIK, OpenCV doesn't set the dpi of PNG files it writes, so you are looking at work-arounds. Here are some ideas...

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Method 1 - Use PIL/Pillow instead of OpenCV

PIL/Pillow can write dpi information into PNG files. So you would:

Step 1 - Convert your BGR OpenCV image into RGB to match PIL's channel ordering

from PIL importImageRGBimage= cv2.cvtColor(BGRimage, cv2.COLOR_BGR2RGB)

Step 2 - Convert OpenCV Numpy array onto PIL Image

PILimage = Image.fromarray(RGBimage)

Step 3 - Write with PIL

PILimage.save('result.png', dpi=(72,72))

As Fred mentions in the comments, you could equally use Python Wand in much the same way.

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Method 2 - Write with OpenCV but modify afterwards with some tool

You could use Python's subprocess module to shell out to, say, ImageMagick and set the dpi like this:

magick OpenCVImage.png -set units pixelspercentimeter -density 28.3 result.png

All you need to know is that PNG uses metric (dots per centimetre) rather than imperial (dots per inch) and there are 2.54cm in an inch, so 72 dpi becomes 28.3 dots per cm.

If your ImageMagick version is older than v7, replace magick with convert.

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Method 3 - Write with OpenCV and insert dpi yourself

You could write your file to memory using OpenCV's imencode(). Then search in the file for the IDAT (image data) chunk - which is the one containing the image pixels and insert a pHYs chunk before that which sets the density. Then write to disk.

It's not that hard actually - it's just 9 bytes, see here and also look at pngcheck output at end of answer.

This code is not production tested but seems to work pretty well for me:

#!/usr/bin/env python3import struct
import numpy as np
import cv2
import zlib

defwritePNGwithdpi(im, filename, dpi=(72,72)):
   """Save the image as PNG with embedded dpi"""# Encode as PNG into memory
   retval, buffer = cv2.imencode(".png", im)
   s = buffer.tostring()

   # Find start of IDAT chunk
   IDAToffset = s.find(b'IDAT') - 4# Create our lovely new pHYs chunk - https://www.w3.org/TR/2003/REC-PNG-20031110/#11pHYs
   pHYs = b'pHYs' + struct.pack('!IIc',int(dpi[0]/0.0254),int(dpi[1]/0.0254),b"\x01" ) 
   pHYs = struct.pack('!I',9) + pHYs + struct.pack('!I',zlib.crc32(pHYs))

   # Open output filename and write...# ... stuff preceding IDAT as created by OpenCV# ... new pHYs as created by us above# ... IDAT onwards as created by OpenCVwithopen(filename, "wb") as out:
      out.write(buffer[0:IDAToffset])
      out.write(pHYs)
      out.write(buffer[IDAToffset:])

################################################################################# main################################################################################# Load sample image
im = cv2.imread('lena.png')

# Save at specific dpi
writePNGwithdpi(im, "result.png", (32,300))

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Whichever method you use, you can use pngcheck --v image.png to check what you have done:

pngcheck -vv a.png

Sample Output

File: a.png (306 bytes)
  chunk IHDR atoffset0x0000c, length 13100 x 100 image, 1-bit palette, non-interlaced
  chunk gAMA atoffset0x00025, length 4: 0.45455
  chunk cHRM atoffset0x00035, length 32
    White x =0.3127 y =0.329,  Red x =0.64 y =0.33
    Green x =0.3 y =0.6,  Blue x =0.15 y =0.06
  chunk PLTE atoffset0x00061, length 6: 2 palette entries
  chunk bKGD atoffset0x00073, length 1
    index =1
  chunk pHYs atoffset0x00080, length 9: 255x255 pixels/unit (1:1). <-- THIS SETS THE DENSITY
  chunk tIMEatoffset0x00095, length 7: 19 Aug 201910:15:00 UTC
  chunk IDAT atoffset0x000a8, length 20
    zlib: deflated, 2K window, maximum compression
    row filters (0none, 1 sub, 2 up, 3 avg, 4 paeth):
      0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
      (100outof100)
  chunk tEXt atoffset0x000c8, length 37, keyword: date:create
  chunk tEXt atoffset0x000f9, length 37, keyword: date:modify
  chunk IEND atoffset0x0012a, length 0No errors detected in a.png (11 chunks, 76.5% compression).

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While I am editing PNG chunks, I also managed to set a tIME chunk and a tEXt chunk with the Author. They go like this:

# Create a newtIME chunk - https://www.w3.org/TR/2003/REC-PNG-20031110/#11tIMEyear, month, day, hour, min, sec =2020, 12, 25, 12, 0, 0    # Midday Christmas day2020tIME= b'tIME'+ struct.pack('!HBBBBB',year,month,day,hour,min,sec)
tIME= struct.pack('!I',7) +tIME+ struct.pack('!I',zlib.crc32(tIME))

# Create a new tEXt chunk - https://www.w3.org/TR/2003/REC-PNG-20031110/#11tEXt
Author = "Author\x00Sir Mark The Great"
tEXt = b'tEXt'+ bytes(Author.encode('ascii'))
tEXt = struct.pack('!I',len(Author)) + tEXt + struct.pack('!I',zlib.crc32(tEXt))

# Open output filename and write...
# ... stuff preceding IDAT as created by OpenCV
# ... new pHYs as created by us above
# ... newtIMEas created by us above
# ... new tEXt as created by us above 
# ... IDAT onwards as created by OpenCV
withopen(filename, "wb") asout:
   out.write(buffer[0:IDAToffset])
   out.write(pHYs)
   out.write(tIME)
   out.write(tEXt)
   out.write(buffer[IDAToffset:])

Keywords: OpenCV, PIL, Pillow, dpi, density, imwrite, PNG, chunks, pHYs chunk, Python, image, image-processing, tEXt chunk, tIME chunk, author, comment