How can I insert Monospace fonts into an image with opencv?

Question

Currently, I am able to insert some texts of HERSHEY font into images with openCV API (putText). But it seems openCV are not supporting any monospace font.

I was wondering how I can insert some Monospace or fixed-pitch texts into the image.

Disputatious

Answer 1

You could use PIL/Pillow for that aspect quite easily. OpenCV images are numpy arrays, so you can make a Pillow Image from an OpenCV image with:

PilImage = Image.fromarray(OpenCVimage)

Then you can draw with a mono spaced font using code in my answer here. You only need the 3 lines after the comment "Get a drawing context".

Then you can convert back to OpenCV image with:

OpenCVimage = np.array(PilImage)

Note that you are not limited to monospaced fonts, you can use any Truetype font you like.

That might look like this:

#!/usr/local/bin/python3

from PIL import Image, ImageFont, ImageDraw
import numpy as np
import cv2

# Open image with OpenCV
im_o = cv2.imread('start.png')

# Make into PIL Image
im_p = Image.fromarray(im_o)

# Get a drawing context
draw = ImageDraw.Draw(im_p)
monospace = ImageFont.truetype("/Library/Fonts/Andale Mono.ttf",32)
draw.text((40, 80),"Hopefully monospaced",(255,255,255),font=monospace)

# Convert back to OpenCV image and save
result_o = np.array(im_p)
cv2.imwrite('result.png', result_o)

---

Alternatively, you could have a function generate a lump of canvas itself, write your text on it, and then splice it into your OpenCV image wherever you want. Something along these lines - though I have no idea of what flexibility you would require so I have not parameterised everything:

#!/usr/local/bin/python3

from PIL import Image, ImageFont, ImageDraw, ImageColor
import numpy as np
import cv2


def GenerateText(size, fontsize, bg, fg, text):
   """Generate a piece of canvas and draw text on it"""
   canvas = Image.new('RGB', size, bg)

   # Get a drawing context
   draw = ImageDraw.Draw(canvas)
   monospace = ImageFont.truetype("/Library/Fonts/Andale Mono.ttf",fontsize)
   draw.text((10, 10), text, fg, font=monospace)

   # Change to BGR order for OpenCV's peculiarities
   return cv2.cvtColor(np.array(canvas), cv2.COLOR_RGB2BGR)


# Open image with OpenCV
im_o = cv2.imread('start.png')


# Try some tests
w,h = 350,50
a,b = 20, 80
text = GenerateText((w,h), 32, 'black', 'magenta', "Magenta on black")
im_o[a:a+h, b:b+w] = text


w,h = 200,40
a,b = 120, 280
text = GenerateText((w,h), 18, 'cyan', 'blue', "Blue on cyan")
im_o[a:a+h, b:b+w] = text

cv2.imwrite('result.png', im_o)

Keywords: OpenCV, Python, Numpy, PIL, Pillow, image, image processing, monospace, font, fonts, fixed, fixed width, courier, HERSHEY.

Fatally

Since the first example does not convert OpenCV's BGR to RGB, passing, e.g., 'blue' to draw.text() will draw red text. Also, it seems the font locations are quite OS-specific; On Ubuntu 16.04, /usr/share/fonts/truetype/freefont/FreeMono.ttf worked for me. – Bruxelles

@UlrichStern Yes, that's correct. I was showing a concept more than a completely tested, generalised, production-ready project. It often turns out that OPs don't give a full and complete specification of their needs and some clue as to how to get started is enough. – Fatally

@UlrichStern I guess you could pass the colour as a BGR triplet in a tuple and reverse it... – Fatally

I appreciated your answer and did upvote. Due to the OS-dependency of monospace, however, I ended up adding adding "column support" to my OvenCV-based putText() utility function via measuring text widths instead. – Bruxelles

The fact that I used a macOS font is not a "system dependency" - you can grab and use any TrueType font you like from anywhere. – Fatally