Image Difference Checker

Spotting what changed between two versions of an image by eye is slow and unreliable — a moved button, a recoloured icon, a retouched spot. This checker compares the images pixel by pixel with a perceptual colour metric, paints every changed pixel in a highlight colour over a faded copy of the original, and reports how much of the image changed and where. It ignores one-pixel shifts of anti-aliased edges, so screenshots of the same page do not light up everywhere. Both images stay on your device.

How to use this tool

1. Choose the original image and the changed image.
2. Keep “scale the second image” on if their sizes differ slightly (for example a re-export).
3. Set the threshold: 0.1 for normal use, 0 for exact pixel checks, higher to see only big changes.
4. Read the changed area, pixel count and bounding box.
5. Open the highlighted image, the difference image and the side-by-side view.

Formula

ΔE² = 0.5053·ΔY² + 0.299·ΔI² + 0.1957·ΔQ²; a pixel counts as changed when ΔE² > 35215 × threshold²

Y, I and Q are the brightness and two colour axes of the YIQ colour space, weighted the way the eye notices them (brightness most). 35215 is the largest possible value (black against white), so a threshold of 1 never reports a change and 0 reports any change at all. Transparent pixels are blended on white first. A changed pixel is treated as anti-aliasing when it lies between a darker and a brighter neighbour that are part of flat areas in both images.

Example

Two 1000 × 800 screenshots where a 40 × 20 px button changed colour give 800 changed pixels, which is 800 ÷ 800,000 = 0.1% of the image, with the bounding box around the button (for example x 120–159, y 300–319, 40 × 20 px). Text that shifted by one pixel would add only yellow anti-aliasing pixels, not red ones.