What is edge detection?

Edge detection finds the places in an image where brightness changes quickly, which are usually the outlines of objects. Computer vision uses it to find shapes. People use it to turn photos into line drawings and coloring pages, to get outlines for tracing, or to check what's in focus.

Tool description

Open an image and the tool shows its edges as light lines on black. Raise the strength to bring out fainter edges such as textures, lower it to keep only the main outlines. Turn on "Dark lines on white" for a look like a pencil sketch that also prints well. Download the result as PNG, JPG, or WebP at the original size.

Features

  • Sobel edge detection on the image's brightness
  • Light lines on black, or dark lines on white
  • A light blur first, so noise and JPG blocks don't show up as edges
  • Keeps transparency, and cutouts get an outline

How it works

The tool turns the image gray and blurs it a little. Then the Sobel operator compares each pixel's neighbors to the left and right, and above and below. That measures how fast brightness changes in each direction. The combined change is the edge strength: where brightness changes fast you get a bright line, where it's flat you get black. Edges in every direction count the same.

Tips

  • Sharp, well lit photos give clean lines. Blurry or noisy ones give soft, broken edges.
  • Use dark lines on white for coloring pages and tracing. It saves ink when you print.
  • To check focus, look for the area with the brightest, thinnest lines.