skewcal — measure printer skew with a scanner or phone camera

Print a flat plate with a 9×9 grid of holes. Scan or photograph it, and skewcal finds every hole centre to a few micrometres. It then fits the grid to get:

• the XY angle: the true angle between your X and Y axes, where 90° is square
• the X / Y scale (how far apart the hole centres really are compared with how far apart they should be)
• a ready-to-paste PrusaSlicer post-processing line that corrects the skew

Beta. Skewcal has so far been validated on a single Prusa MINI+, against a flatbed scanner and calipers. Results on other printers, scanners and phones are not yet confirmed. Check any correction with a test print before relying on it, and please report problems or results in the issues or in the comments on Printables.

Hole centres stay put when holes print too small or too large. That makes the result insensitive to flow, elephant's foot and over-extrusion, which is the weak point of caliper-based tests.

Use it in your browser

https://alighomash.github.io/skewcal/ runs the same analysis with nothing to install. It runs on a phone too. Images are processed on your device and never uploaded. The page walks through printing the plate, scanning or photographing it, reading the results and correcting your G-code. On an iPad it also shows the tablet reference (section 3b). It opens on an example result from a Prusa MINI+.

The test plate, cube and reference sheets are also on Printables, where you can post your prints and results.

The rest of this README covers the command-line version.

1. Install (once)

cd "Skew Correction"
python3 -m venv .venv
.venv/bin/pip install -r requirements.txt
# optional, lets it read iPhone .HEIC photos directly:
.venv/bin/pip install pillow-heif

The G-code post-processor (skew_gcode.py) uses only the Python standard library and needs none of this.

2. Print the test plate

skewcal_plate.stl is included (114 × 114 × 1.6 mm, 4 mm holes on a 12.5 mm grid). Its holes are thin-rim holes: 4 mm only for the bottom 0.6 mm, then widening as a cone to 5.6 mm at the top. The visible hole edge is a thin first-layer rim, so lighting and viewing angle barely move it. In tests this halved the per-hole error in phone photos, and scans were no worse. To regenerate it or change its size:

.venv/bin/python skewcal.py plate -o skewcal_plate.stl

skewcal_plate_standard.stl (plate --standard) is the original plate with straight holes. To analyse it, pass --plate skewcal_plate_standard.json; otherwise the thin-rim plate is assumed. Plate .json files saved before thin-rim plates existed are still read as standard plates.

• Do not rotate or mirror it in the slicer. The small marker holes tell the software which way is X and which is Y on the plate. That only matches the printer's X and Y if the plate is printed exactly as exported. The origin hole is at the front left.
• Use your normal settings. Leave out a brim: it would make the edges hard to separate from the background.
• Dark filament is easiest because the holes show up bright. Light filament also works if there's something dark behind the holes.
• Let it cool fully on the sheet before you remove it. A warped plate gives poor results.

3. Choose how to capture the plate

Method	Angle	Scale	Needs
3a. Flatbed scanner	±0.006°	exact, no calibration	a scanner
3b. Phone + tablet	±0.003°	exact after a one-time 100 mm bar check	an 11-inch+ iPad, a phone with a 2× lens
3c. Phone + paper sheet	±0.01°	needs measured sheet spans	a printer for the sheet

The angle column is the combined result from two images with the plate turned 90°, measured on a Prusa MINI+. Take more images if you can: four are better than two.

3a. Measure with a flatbed scanner

1. Lay the plate face-down (first layer on the glass), roughly straight. Use dark filament with the lid closed, or light filament with black paper on top of it.
2. Scan at 600 dpi, grayscale, as PNG or TIFF. Turn off sharpening and descreening.
3. Turn the plate 90° and scan again. Doing it twice cancels the scanner's own squareness error (see "Why two images"). Optionally scan at 180° and 270° too; four scans average out more noise.
4. Run:

.venv/bin/python skewcal.py scan scan1.png scan2.png            # add --dpi 600 if the file lacks DPI info

3b. Measure with a phone and a tablet (recommended phone setup)

An iPad shows the reference markers instead of a printed sheet. A screen is perfectly flat and its pixels are evenly spaced. You don't need to print a sheet, and absolute scale needs at most a one-time calipers check of a bar shown on the screen. On a test plate it matched a flatbed scan of the same plate to 0.0015° in angle and 0.01% in scale (see Accuracy).

You need:

• the thin-rim plate (the default skewcal_plate.stl)
• an iPad with a screen at least 15 cm across the short side: 11-inch or larger (an iPad mini is too small)
• a phone with a 2× or 3× lens

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