Photo to Coloring Page Converter

A colouring page needs clear, closed outlines and nothing else. This converter finds the outlines in a photo or drawing and turns them into black lines on white paper, ready to print. It smooths the picture first so skin, grass and sky do not turn into scribbles, traces the strongest edges, removes stray specks and thickens the lines so crayons stay inside them. You get a PNG and a PDF sized for A4 or US Letter, made entirely in your browser.

How to use this tool

1. Choose one or more photos; a clear subject with a plain background works best.
2. Pick the detail level: low for young children, high for older colourers.
3. Adjust line sensitivity, thickness and speck removal if needed.
4. Download the PNG, or the PDF with one picture per page, and print it.

Formula

Gaussian blur → Sobel gradient → non-maximum suppression → hysteresis (high = max(8, P98 × (0.5 − 0.4 × sensitivity)), low = 0.4 × high)

This is the Canny edge method. P98 is the 98th percentile of the thinned edge strengths, so the thresholds adapt to each photo. The blur radius is 3.2, 2.2 or 1.4 px per 1200 px of page for low, medium and high detail. Line pieces with fewer pixels than the speck limit are removed, then each line is widened to the chosen thickness. The PDF fits the picture inside the margins, turning the page to landscape for wide pictures.

Example

A 4000 × 3000 photo at the 1800 px setting becomes a 1800 × 1350 page. With medium detail the blur is 2.2 × 1800/1200 = 3.3 px, and with 3 px lines most photos end up with roughly 2–20% of the page in black (busier pictures more). On landscape A4 with 12 mm margins the 1350 px height sets the scale, so the picture is printed about 24.8 cm wide.