Trace the outlines in a photo with Sobel, Prewitt, Roberts, Laplacian or Canny. Tune the threshold, flip to dark lines on white, and download.
Converts the picture to luminance, measures how sharply brightness changes at every pixel with the operator you choose, and keeps only the pixels whose change is stronger than the threshold. The result is a two-colour map: edge or not. Sobel and Prewitt look at a 3 × 3 neighbourhood in two directions, Roberts uses a tiny 2 × 2 diagonal pair, Laplacian measures curvature in all directions at once, and the Canny option here applies a clean binary cut to the Sobel gradient.
A 3000 × 2000 photo of a house front: Sobel at 50 percent gives clear white outlines of the windows, roofline and door on black, with the brick texture mostly dropped. Lowering the threshold to 20 brings the bricks back as a dense mesh; raising it to 75 leaves only the strongest outlines, which is what you want for a colouring page. Switching on Dark lines on white and downloading as PNG produces a printable line drawing of about 90 KB.
Use it to make line art and colouring pages, to check focus (a sharp photo produces crisp thin edges, a soft one produces thick faint ones), to prepare stencils, or as a step in tracing a logo. Run Grayscale Image or Remove Noise first on grainy phone photos so texture does not swamp the outlines. If you want the edges emphasised inside the original photo rather than extracted from it, Edge Enhancement does that.
Sobel is the default and the most forgiving. Prewitt is similar with a little more noise. Roberts gives the thinnest lines but reacts to grain. Laplacian finds fine detail in every direction and often doubles lines. Canny smooths the photo, thins every edge to one pixel and keeps only connected lines, the cleanest choice for tracing and colouring pages. Threshold is a percentage of the strongest edge in the image, so it adapts to each photo. Dark lines on white inverts the map for printing and sketch looks. Output defaults to PNG, which keeps the lines crisp; JPEG blurs them slightly.
Why is the output mostly black? The threshold is too high for a soft or low-contrast photo. Drag it down toward 20 percent; the value is relative to the strongest edge in this image, not an absolute.
Which operator gives the cleanest colouring page? Sobel at 60 to 75 percent with Dark lines on white. Run Remove Noise first if the photo is grainy.
Is the Canny option a full Canny detector? Yes. It smooths the photo first, thins every edge to one pixel (non-maximum suppression) and keeps weak edges only where they connect to strong ones (hysteresis). The threshold sets the strong-edge level.
Can I get coloured edges? Not directly; the map is black and white. Combine it with the original in Overlay Image using a blend mode, or use Edge Enhancement to strengthen edges inside the colour photo.
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