pixelmatch-py

Python port of https://github.com/mapbox/pixelmatch.

A fast pixel-level image comparison library, originally created to compare screenshots in tests.

Features accurate anti-aliased pixels detection and perceptual color difference metrics.

from pixelmatch import pixelmatch

num_diff_pixels = pixelmatch(img1, img2, 800, 600, diff, {
  'threshold': 0.1
})

Implements ideas from the following papers:

• Measuring perceived color difference using YIQ NTSC transmission color space in mobile applications (2010, Yuriy Kotsarenko, Fernando Ramos)
• Anti-aliased pixel and intensity slope detector (2009, Vytautas Vyšniauskas)

Install

pip install pixelmatch

API

pixelmatch(img1, img2, width, height[output, options])

• img1, img2 — RGBA Image data of the images to compare. Note: image dimensions must be equal.

• width, height — Width and height of the images.

• output — Image data to write the diff to, or None if don't need a diff image. Note that all three images need to have the same dimensions. options is a dict with the following properties:

• threshold — Matching threshold, ranges from 0 to 1. Smaller values make the comparison more sensitive. 0.1 by default.

• includeAA — If true, disables detecting and ignoring anti-aliased pixels. false by default.

• alpha — Blending factor of unchanged pixels in the diff output. Ranges from 0 for pure white to 1 for original brightness. 0.1 by default.

• aa_color — The color of anti-aliased pixels in the diff output in [R, G, B] format. [255, 255, 0] by default.

• diff_color — The color of differing pixels in the diff output in [R, G, B] format. [255, 0, 0] by default.

• diff_mask — Draw the diff over a transparent background (a mask), rather than over the original image. Will not draw anti-aliased pixels (if detected).

Compares two images, writes the output diff and returns the number of mismatched pixels.

Example usage

PIL

from PIL import Image

from pixelmatch import pixelmatch


def pil_to_flatten_data(img):
    """
    Convert data from [(R1, G1, B1, A1), (R2, G2, B2, A2)] to [R1, G1, B1, A1, R2, G2, B2, A2]
    """
    return [x for p in img.convert("RGBA").getdata() for x in p]

img_a = Image.open("a.png")
img_b = Image.open("b.png")
width, height = img_a.size

data_a = pil_to_flatten_data(img_a)
data_b = pil_to_flatten_data(img_b)
data_diff = [0] * len(data_a)

mismatch = pixelmatch(data_a, data_b, width, height, data_diff, {
    "includeAA": True
})

img_diff = Image.new("RGBA", img_a.size)

def flatten_data_to_pil(data):
    return list(zip(data[::4], data[1::4], data[2::4], data[3::4]))

img_diff.putdata(flatten_data_to_pil(data_diff))
img_diff.save("diff.png")

Example output

expected	actual	diff

Changelog

v0.1.1

• fix: fix bug in fast path #18

v0.1.0

• Initial release