Updated September 2026
The stencil tool has three sliders and an invert checkbox. Each one maps to a specific, named step in the underlying image-processing pipeline (see how it works for the full technical breakdown) — knowing what each one actually does makes it much faster to dial in a clean result instead of dragging sliders at random.
This is the sensitivity of the edge detector. Internally, every pixel gets a "how much of an edge is here" score, and this slider is the cutoff: any pixel scoring above the threshold becomes a black line, anything below becomes white background.
There's no universally "correct" value — it depends entirely on how much internal detail you want the stencil to carry versus how clean and simple the lines need to be for your transfer method.
This runs before the edge detector, not after — it averages each pixel with its immediate neighbors in a square window, sized by this value. The purpose is narrow but important: real photos have sensor noise (tiny random brightness flickers) that a raw edge detector reads as thousands of stray dots. Smoothing removes that noise so only real contrast edges register.
Raw edge-detection output is frequently just one pixel wide — accurate, but too thin to survive printing at typical resolution, let alone a physical transfer process afterward. This slider runs a dilation pass: any background pixel touching a line pixel becomes a line pixel too, repeated for the number of passes you set. Each pass fattens every line by roughly a pixel in every direction.
Flips the final output's polarity. Most printing and thermal-transfer workflows expect standard black lines on a white background (the default), but some carbon-transfer techniques and specific gel/paper combinations are built around a reversed image — check what your transfer method expects. See transferring a stencil onto skin for which methods typically call for it.
Because these settings interact — more smoothing means you may need to lower the line-detail threshold to recover detail the blur softened, and thicker lines make a busy result look even busier — the fastest path to a clean stencil is usually to change one slider at a time and watch the live preview update, rather than trying to predict the combined effect in advance. Every adjustment re-renders instantly; there's no "apply" step to wait on.
Is there a "best" setting for tattoos specifically? No single answer — it depends on the design's complexity and your transfer method. A simple, bold design for thermal transfer wants different settings than a highly detailed portrait meant for careful hand-tracing.
Why does my stencil look sparse even at low line detail? The source photo likely lacks real contrast in that area — see choosing a good source photo; no slider combination can invent an edge that isn't in the original brightness data.