How to improve image quality? [closed]

Question

I am making an OCR which reads ID cards. After getting region of interest by using YOLO, I am giving that cropped region to Tesseract to read it. As those cropped images are very small and blurry, Tesseract is unable to read them. When it can read them, it gives wrong predictions. I think that by improving image quality of cropped images, these problems can be solved.

One of the cropped images:

My question is, how would I go about improving such images?

Answer 1

The answer of @vasilisg. is a very nice sollution. One way to further improve upon this is to remove the remaining spots using a morphological opening operation. However, this will only work for spots smaller than the linethickness of the numbers in your image. Another option is to remove 'islands' of less than N pixels using the openCV connected components module. You could do this for example as following:

# External libraries used for
# Image IO
from PIL import Image

# Morphological filtering
from skimage.morphology import opening
from skimage.morphology import disk

# Data handling
import numpy as np

# Connected component filtering
import cv2

black = 0
white = 255
threshold = 160

# Open input image in grayscale mode and get its pixels.
img = Image.open("image.jpg").convert("LA")
pixels = np.array(img)[:,:,0]

# Remove pixels above threshold
pixels[pixels > threshold] = white
pixels[pixels < threshold] = black


# Morphological opening
blobSize = 1 # Select the maximum radius of the blobs you would like to remove
structureElement = disk(blobSize)  # you can define different shapes, here we take a disk shape
# We need to invert the image such that black is background and white foreground to perform the opening
pixels = np.invert(opening(np.invert(pixels), structureElement))


# Create and save new image.
newImg = Image.fromarray(pixels).convert('RGB')
newImg.save("newImage1.PNG")

# Find the connected components (black objects in your image)
# Because the function searches for white connected components on a black background, we need to invert the image
nb_components, output, stats, centroids = cv2.connectedComponentsWithStats(np.invert(pixels), connectivity=8)

# For every connected component in your image, you can obtain the number of pixels from the stats variable in the last
# column. We remove the first entry from sizes, because this is the entry of the background connected component
sizes = stats[1:,-1]
nb_components -= 1

# Define the minimum size (number of pixels) a component should consist of
minimum_size = 100

# Create a new image
newPixels = np.ones(pixels.shape)*255

# Iterate over all components in the image, only keep the components larger than minimum size
for i in range(1, nb_components):
    if sizes[i] > minimum_size:
        newPixels[output == i+1] = 0

# Create and save new image.
newImg = Image.fromarray(newPixels).convert('RGB')
newImg.save("newImage2.PNG")

In this example I have performed both the opening and connected component method, however if you use the connected component method you can usually omit the opening operation.

The results look like this:

After thresholding and opening:

After thresholding, opening and connected component filtering:

Answer 2

One way to do it, is to convert your image to grayscale and then use a threshold value to compare with every pixel in order to decide whether it should be black or white. Pillow is a library that you can use for this type of processing:

from PIL import Image

black = (0,0,0)
white = (255,255,255)
threshold = (160,160,160)

# Open input image in grayscale mode and get its pixels.
img = Image.open("image.jpg").convert("LA")
pixels = img.getdata()

newPixels = []

# Compare each pixel 
for pixel in pixels:
    if pixel < threshold:
        newPixels.append(black)
    else:
        newPixels.append(white)

# Create and save new image.
newImg = Image.new("RGB",img.size)
newImg.putdata(newPixels)
newImg.save("newImage.jpg")

Result image: