Photo Measuring Card: 240 mm ArUco Scale Reference for Phone Photos

What it is: a flat 240 mm C-shaped measuring card for phone photos. It has 7 ArUco markers (OpenCV DICT_4X4_50, ids 20-26) and mm rulers on thin 10 mm rails. Put the object inside the C, take a photo from any angle, and a script can undo the perspective and read real millimetres.

Key points

• Open C shape: the object sits inside, nothing to lift or line up; the open side lets long parts stick out.

• 7 markers: the script needs only 1 visible marker (2 or more recommended), so a hand or the object can cover some.

• Rulers you can read by eye: 1 mm zebra strip, 5 and 10 mm ticks, numbers every 50 mm.

• Two colours, flush: black is a 0.6 mm inlay in the top face, never raised, so the card lies flat and the edges stay sharp.

• Marker spec included: a JSON with every marker corner in card mm, for your own OpenCV code.

Tested

One real phone photo of the printed card (1512 x 2016 px, about 18 degrees off straight down, 34 cm away): all 7 markers found, fit error 1.3 px, clean top-down rectified view (the straightened photo in the gallery). On synthetic test photos the scale was within 0.04 mm over 100 mm.

Parts list

• 1 x printed card (white body + black inlay)

• A phone camera

• Python with OpenCV to run the free script (link below)

Use (about 1 minute)

1. Lay the card flat on a table.

2. Put the object inside the C.

3. Take the photo so that at least 2 markers are visible.

4. Run the script (link below): python photo_measure.py PHOTO.jpg --focal-auto. It writes a top-down view of the card plane at 10 px per mm and prints the scale and camera tilt.

Print settings

• PLA, 0.2 mm layers, 0.4 mm nozzle, 2 walls, 15 % infill

• Two colours: white body, black inlay (top 3 layers). Printed on a multi-tool printer; a single-nozzle printer with a filament changer or a colour swap at 2.4 mm also works

• Flat on the bed, no supports, outer brim recommended (a 240 mm flat part)

• About 29 g PLA (26 g white, 3 g black), about 1 h 10 min

• Needs a bed of at least 240 x 240 mm

Files

• photo_measure_card_pm2_x1.3mf: one object, two parts (white = extruder 1, black = extruder 2)

• photo_measure_card_pm2_white_x1.stl and photo_measure_card_pm2_black_inlay_x1.stl: the two parts, aligned

• photo_measure_card_pm2_x1.step: editable solid

• photo_measure_card_pm2_marker_spec.json: marker ids and corner positions in mm

Note: sizes are exact on the card plane only. The top of a tall object is closer to the camera and looks bigger; measure at the base, or take the photo straight down.

Script (Python, OpenCV) and marker spec: https://github.com/BorjaLL/cc2-toolkit/tree/main/photo-measure

AI disclosure

Designed by me with AI-assisted CAD scripts (Python generators written with an AI coding assistant) and tested on my own printer and phone. The photos are real photos of the print; the top-down view is one of them processed by the script; the last image is a CAD render, labelled as a render. No AI image or mesh generator was used.