How the pencil sketch effect works
This effect uses a classic technique: the image is converted to grayscale, a second inverted and blurred copy is layered on top using a "color dodge" blend, and the result mimics the look of graphite pencil shading on paper. Adjust the softness slider to control how fine or soft the sketch lines appear, then download the result. Everything runs locally in your browser using canvas filters.
Why this specific combination of filters produces a sketch look
The color-dodge blend mode brightens the base layer based on how light the blend layer is - combining a normal grayscale image with its own inverted, blurred version using this blend mode causes edges and fine detail to pop out as dark "pencil lines" against a lighter background, which is the same underlying principle many manual Photoshop pencil-sketch tutorials use, applied here automatically through canvas filters.
Why the softness slider changes how the sketch looks
The blur amount applied to the inverted layer controls how far the color-dodge effect spreads around each edge - a smaller blur produces tighter, finer sketch lines closer to the original edges, while a larger blur produces softer, broader shading that looks more like loose pencil strokes than precise linework.
Why image choice affects the result so much
This effect fundamentally works by exaggerating contrast and edges, so images that already have clear subject-background contrast (portraits against a plain background, objects with defined outlines) translate into recognizable sketches, while busy, low-contrast, or very dark photos tend to produce muddier, less legible results since there's less edge information for the effect to exaggerate.