Turn Photo Into Line Drawing
A photo becomes a line drawing when you keep the edges and throw away the color, the shading, and everything in between. Open the ColoringZap converter, drop the photo in, and move three sliders. Which way to move them depends on the photo — and that is what the measurements below are for.
The short answer: settings by photo type
These are the winning settings from a parameter sweep we ran on four kinds of image — 32 slider combinations each, 128 runs total. Percentages are "ink coverage": the share of the page that came out as black line. Under about 1.5% the page is nearly blank; over about 12% it turns into a grey blob that is no fun to color in.
| Your photo looks like | Smoothness | Edge Detail | Line Width | Measured ink coverage |
|---|---|---|---|---|
| Bold shapes, cartoon, logo, strong contrast | 0 | 20 | 2 | 2.52% — clean, printable |
| Grainy, or shot in low light | 1 | 80 (top of the slider) | 1 | 4.02% at threshold 90; expect slightly heavier |
| Busy fine texture — foliage, fur close-up, knit, gravel | 3 | 80 (top of the slider) | 1 | 23.55% at threshold 90 — dense, hard to color |
| Mostly smooth gradients — sky, soft portrait, blurred background | — | — | — | 0.00% at every one of the 32 settings. Use a different photo. |
One honest caveat: the sweep ran on synthetic test images, not on your photo. Treat these as starting points, then nudge one slider at a time.
What the three sliders actually do
- Smoothness (0–4) blurs the photo slightly before any edge is detected. Blur erases fine detail — which is exactly what you want when the fine detail is sensor noise, and exactly what you do not want when it is the subject's outline.
- Edge Detail (10–80) is the strength an edge has to reach before it gets drawn. A higher number means fewer lines, not more. The label reads like a "more detail" dial, but it works the other way round, and it is the slider people set backwards most often: if your page looks like scribble, raise it.
- Line Width (1–4) thickens the lines that survived. This one does not change how much is on the page, only how heavy it prints.
Underneath, the pipeline is grayscale → blur → edge detection → threshold → cleanup. It does not invent anything: every line in the result is an edge that was already in your photo. The complete pipeline is published as open source, and the measurement scripts are in the same repository, so every number on this page can be reproduced.
What the sweep found, run by run
| Image class | Blur | Threshold | Dilate | Ink coverage | Isolated specks |
|---|---|---|---|---|---|
| flat-vector (bold shapes, hard edges) | 0 | 20 | 1 | 2.52% | 0.00% |
| smooth-portrait (soft gradients) | 0 | 20 | 0 | 0.00% | 0.00% |
| detailed-texture (fine repeating detail) | 3 | 90 | 0 | 23.55% | 0.00% |
| noisy-lowlight (grainy) | 1 | 90 | 0 | 4.02% | 0.00% |
Two things stand out in the full 128 rows. First, isolated specks came out at 0.00% everywhere — the cleanup step that drops lone pixels does its job, so "dirty looking speckle" is not the failure mode here. Density is. Second, the winners are not close calls: the same photo class can swing from 2.52% to 98% coverage just by moving the threshold, which is why guessing slider positions one at a time takes so long.
Settings for each kind of photo
Bold shapes, cartoon, logo (easiest case)
Set Smoothness 0, Edge Detail 20, Line Width 2. Measured coverage 2.52%, right in the printable band. Leave Smoothness at 0: there is no noise to remove, and blurring a hard-edged image only softens the outlines you came for. Line Width 2 makes the outlines easier to color inside without filling in the small gaps.
Grainy or low-light photos
Set Smoothness 1, Edge Detail as high as it goes, Line Width 1. The sweep's winning value was a threshold of 90, which is above the slider's maximum of 80 — so push it to the top and expect something a little heavier than the 4.02% measured at 90.
Be careful not to overdo Smoothness on this kind of photo. In the sweep, blur radius 2 with threshold 60 dropped coverage to 0.25%, and blur radius 3 with threshold 40 dropped it to 0.08% — the grain was gone, and so was the subject. If your page comes out almost empty after raising Smoothness, you erased the photo along with the noise. Back Smoothness off to 1 and raise Edge Detail instead.
Busy, fine-textured photos
Set Smoothness 3, Edge Detail as high as it goes, Line Width 1. Even then, expect a dense page: at threshold 90 the measured coverage was 23.55%, more than double the "clean" ceiling, and since the slider stops at 80 you should expect heavier than that (at blur 3, threshold 60 measured 70.02%).
The useful move here is usually not a slider at all — it is a different crop. Fur, leaves, and knit fabric are made of thousands of real edges, and the tracer has no way to know which ones you care about. Photograph the same subject from further away, against a plain background, and the drawing gets dramatically cleaner at the same settings.
Smooth gradients (the case that does not work)
There is no setting for this one. The soft-gradient test image produced 0.00% coverage at every one of the 32 combinations — no blur, no threshold, no dilate found anything to draw. A gradient has no sharp step for an edge detector to catch, so a sunset sky, a smooth cheek, or an out-of-focus background gives you a blank page. That is the method working correctly on a photo that has no edges to find. Pick a photo with a hard boundary between subject and background instead.
Two failure modes, and which slider fixes each
- Page comes out nearly black. Too many edges survived. Raise Edge Detail first (it is a cut-off, so raising it removes lines), then raise Smoothness if the photo was grainy. In the sweep, the busy-texture image went from 98.14% coverage to 23.55% exactly this way.
- Page comes out nearly blank. Either the photo has no hard edges (see smooth gradients above), or Smoothness erased them. Drop Smoothness to 0–1 and lower Edge Detail toward 20. If it is still blank, the photo is the problem, not the settings.
What to do with the line drawing
Most people print it. Raise Line Width to 2 or 3 for crayons and thick markers, since thin outlines disappear under a fat tip. If you have a stack of photos, the Coloring Book Maker turns them into one PDF with one drawing per page.
If you would rather have numbered blocks than open outlines, run the same photo through Picture to Color by Number — it quantizes the photo into a small palette and numbers each region. For something that is not a photo at all, the Name Coloring Page Generator draws any name in big outlined letters, with an optional photo converted to outlines behind it. The general walkthrough for photo conversion is on Convert a Picture Into a Drawing.
Questions
Does my photo get uploaded? No. Everything runs in your own browser tab and the picture is never sent anywhere, so there is no server copy of it. That is also why there is no account and no queue — see the Privacy Policy.
Is a line drawing the same as a coloring page? Here, yes — the output is black outlines on white, which is what you color in. The difference is only in what you do next: print it as is, number it, or bind several into a book.
Why do my results differ from the numbers above? The sweep ran on synthetic test images, and your photo has its own contrast, resolution, and noise. The ranking holds — bold shapes convert best, gradients do not convert at all — the exact percentages will not.
Is there a limit? No daily limit and no watermark. Convert as many photos as you want.
Can I check the numbers myself? Yes. The pipeline and the sweep script are open source on GitHub (coloringzap-algorithms), and the full 128-row sweep is archived on Zenodo (doi.org/10.5281/zenodo.22812174), so anyone can rerun it and get the same table.