# Image Edge Detection

Find the edges in any photo or drawing with a Sobel or Laplacian operator. The image is converted to grayscale, the gradient strength of every pixel is computed and scaled by a strength setting, and the result is a grayscale edge map with bright lines on black or, inverted, dark lines on white. Useful for computer vision previews, tracing, masks and artistic effects.

## Frequently asked questions

### What is the difference between Sobel and Laplacian?

Sobel measures the brightness gradient in the horizontal and vertical directions and combines them, giving thick, smooth edges that respond well to real photos. Laplacian measures the second derivative with a four neighbor kernel and gives thinner, sharper lines that are more sensitive to noise. Sobel is the default.

### What does the strength setting do?

Strength multiplies the edge values before they are clipped to 255, from 0.5 to 10. The default of 2 makes typical photo edges clearly visible. Raise it for soft images and lower it when the map looks too busy.

### How bright is a full edge?

The Sobel response is normalized so that a hard step from black to white reaches 255 at strength 1. Softer transitions give lower values, which the strength setting can amplify. Flat areas are always 0.

### Can I get dark lines on a white background?

Yes, enable the dark edges on white option. The edge map is inverted so flat areas become white and edges become dark, which suits printing, tracing and coloring pages. For a cleaner line drawing with optional pure black lines, try the Image Outline tool.

### Is the output in color?

No, edges are detected on the luminance of the image, computed with the standard Rec. 709 weights for red, green and blue. The result is a grayscale picture saved as PNG. Transparent areas of the original keep their transparency.

### How are the image borders handled?

Pixels at the border reuse the nearest edge pixels as their missing neighbors. This prevents a false frame from appearing around the image. The output always has the same dimensions as the input.

### How can I reduce noise in the edge map?

Use Sobel rather than Laplacian, lower the strength, or blur the image slightly first with the Image Blur tool. JPEG artifacts and sensor noise create small speckles that disappear with a light blur. Higher resolution sources also give cleaner lines.

### Which formats can I upload?

Upload JPEG, PNG, GIF, WebP, TIFF or AVIF images up to 5 MB. Animated files use their first frame and images larger than 4000 pixels on a side are scaled down to fit. The edge map is saved as PNG.

### What can I use an edge map for?

Edge maps help with tracing artwork, creating masks and selections, checking focus and sharpness, preparing data for computer vision experiments and making graphic posters. Combined with a blend mode in an editor, they also add crisp line accents to photos.

### Is the result always the same for the same image?

Yes, the operators are fixed 3x3 kernels and the processing is deterministic. The same file and settings always produce an identical PNG.

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