Detect the edges in a photograph

Red, green and blue are first reduced to weighted luminance. Two 3×3 Sobel kernels measure change across x and y; their Euclidean magnitude is multiplied by strength and compared with the threshold. Inversion changes presentation only, turning bright-on-black into black-on-white.

Threshold

The minimum gradient magnitude allowed to remain.

Gradient strength

A multiplier on all measured edge magnitudes.

Black lines on white

An inversion after measurement, not a different detector.

Sobel locates contrast, not meaning. Noise, compression blocks, skin texture and patterned fabric can be stronger than the outline a person wanted. Blur the source beforehand when the job is structural edges; keep the unblurred source when fine texture is the subject.

The preview, draft and export all run the same kernel. Parameters measured in pixels are scaled with the source, so a chosen tile, band or radius does not become unexpectedly finer in the downloaded file.

What counts as an edge?

A rapid change in Rec. 709 luminance across neighbouring pixels. The Sobel pair measures horizontal and vertical change and combines their magnitudes.

Why does texture fill with lines?

Texture is made of local brightness changes too. The operator knows nothing about objects, so a leaf vein and a face outline are both gradients.

What does threshold remove?

Gradient magnitudes below the chosen value are written as zero before inversion. Raising it suppresses weak texture and also discards faint real boundaries.

• Blur stylesSoft, horizontal and vertical averaging with a stated radius.
• PosteriseContinuous tone reduced to measured channel levels, with optional pre-dither.
• HalftoneDots on a rotated screen, one plate or four, with the angles a press would use.
• The effects labAll six effects, chosen from your own photograph rather than from a list of names.

Rendered on your computer, not on a server.