.eval()

The .eval() method in Pillow processes an image by applying a given function to each pixel. This is useful for pixel-level transformations like adjusting brightness, inverting colors, or applying custom filters.

Syntax

Image.eval(image, function)

• image: The input image object to process.
• function: A function/lambda that takes one integer argument (the pixel value) and returns a modified integer.

Example

The example below converts the input image into a new image, where each pixel is transformed by a function:

from PIL import Image

# Open an image
img = Image.open('media/pillow-original.jpeg').convert("RGB")  # Ensure RGB mode

# Define a function to invert pixel values
def invert_pixel(x):
  return 255 - x  # Works for grayscale and individual RGB channels

# Apply the function to each pixel
new_img = Image.eval(img, invert_pixel)

# Display the new image
new_img.show()

to clipboard
Copy to clipboard

In this example:

• x: Represents the pixel intensity value, which typically ranges from 0-255 (for 8-bit grayscale or RGB images).
• 255 - x: Subtracts the pixel value from 255, effectively inverting its brightness: 0 → 255 (pure black becomes pure white), 255 → 0 (pure white becomes pure black), 100 → 155 (mid-gray becomes a lighter gray).

Here is the original image:

Here is the updated image: