[Python] Converting Images to Grayscale with Pillow: Explanation of Color Spaces (Color Modes) and Conversion Methods

When converting color images to black and white (grayscale) or to formats suitable for printing, use the convert() method in Pillow.

I will explain the meaning of the “modes (strings)” specified during conversion and the code to actually convert images to grayscale.

Main Color Spaces (Color Modes) in Pillow

The following are the main mode identifiers used when loading images with Image.open() or converting them with convert().

Executable Sample Code

The following code is a script that loads a color image, converts it to grayscale (L mode), and saves it.

from PIL import Image, ImageDraw
import os

def convert_to_grayscale():
    input_filename = "vivid_landscape.jpg"
    output_filename = "monochrome_landscape.jpg"
    
    # Create a color image for operation verification (if file doesn't exist)
    if not os.path.exists(input_filename):
        create_test_color_image(input_filename)

    try:
        # 1. Load image
        with Image.open(input_filename) as img:
            print(f"Original Mode: {img.mode}")  # Usually 'RGB'

            # 2. Convert to Grayscale
            # Just calling convert("L") generates a natural black and white image based on luminance
            gray_img = img.convert("L")
            
            print(f"Converted Mode: {gray_img.mode}") # Confirm it changed to 'L'

            # 3. Save
            gray_img.save(output_filename)
            print(f"Saved: {output_filename}")

            # Reference: If you want to binarize (black or white only), specify "1"
            # binary_img = img.convert("1")
            # binary_img.save("binary_sample.jpg")

    except Exception as e:
        print(f"An error occurred: {e}")

def create_test_color_image(filename):
    """Create a colorful image for verification"""
    width, height = 400, 300
    img = Image.new("RGB", (width, height), "white")
    draw = ImageDraw.Draw(img)
    
    # Draw Red, Green, and Blue strips
    draw.rectangle((0, 0, width//3, height), fill="red")
    draw.rectangle((width//3, 0, width*2//3, height), fill="green")
    draw.rectangle((width*2//3, 0, width, height), fill="blue")
    
    img.save(filename)
    print(f">> Created test image: {filename}")

if __name__ == "__main__":
    convert_to_grayscale()

Explanation: Importance of the convert Method

image.convert(mode) does not just change the colors; it modifies the internal representation of the pixel data itself.

• File Size Reduction: Converting from RGB (24 bits per pixel) to L (8 bits per pixel) reduces the amount of information in the image, allowing for smaller file sizes when saving.
• Preprocessing for Image Processing: Before performing image analysis such as OCR (Optical Character Recognition) or edge detection, it is common to convert images to grayscale to reduce noise and computational load.