AprilTag Scale Bar (Beta)

An AprilTag bar gives your model a scale in metres. An L bar can also level it and set its ground plane. The feature is available in Refine Pro from v1.4.1. Replica can find the tags automatically and use a bar without a JSON file.

Prepare the bar

Use a straight bar with two tag36h11 tags, or an L bar with three. Each tag needs a different ID. When available on the site, Replica's bar generator supplies print files and a JSON definition with dimensions. You can also use a compatible bar and enter its details manually.

Keep the bar rigid and still in at least three sharp images that have been aligned by reconstruction. The required tags must appear together, and the camera must move between shots: rotating it from one fixed position does not provide enough parallax. Give every other bar or marker in the scene unique IDs too.

To level with an L bar, place its printed face on the plane you want to use as the ground. If you move or remove the bar during a video, leave that section out of detection.

Find the tags

Add AprilTag Scale Bar (Beta) in Refine. Use Aligned image range to select images where the bar stayed still. Replica sorts them by filename; leave Last at 0 to include the remaining images.

There are two ways to proceed:

Command	How to use it
Find tags automatically	Search the aligned images for tags. Choose the bar type and enter a known centre-to-centre distance in millimetres.
Import JSON… (optional)	Load the generator's JSON: v1 for a straight bar, v2 for an L. Replica reads the IDs, roles and nominal dimensions. You can use those dimensions without entering them again.

You can also enter the IDs yourself and click Detect selected IDs. Either way, you need a known dimension: the tag pattern contains an ID, but does not say how large the tag is.

Choose the tags in the viewer

Replica shows detected centres in 3D, with ID labels and coloured arms. Click a marker or its ID button, then assign its role with Use as A/B for a straight bar or Use as Origin/X/Y for an L.

For an L, Replica identifies the right-angle corner as Origin and suggests the lower endpoint ID as X. Check the orientation and swap X and Y if needed: the IDs do not determine the axis directions. When several bars are present, choose the two or three tags belonging to the bar you want to use.

Enter the measurements

Measure from one tag centre to the other. The bar's outside length and a tag's black-square width are different measurements. If the centres are 100 mm apart, enter 100, even if the bar is longer.

Without JSON, enter at least one distance. For an L, start with Origin-X; you can add the other arm or diagonal measurement to check the fit.

After importing JSON, open Correct printed dimensions (optional) to adjust for the actual print. Your measurements take precedence over nominal dimensions. For an L, entering one actual measurement makes Replica use only the actual measurements you supply. A resized print is then not also compared against unchanged nominal distances.

Check the printed dimensions before using the JSON values, especially if you changed the scale in your slicer. Replica does not apply another scale adjustment from print-compensation metadata.

Apply the scale

Review the consistent-view count, reprojection error, parallax and calculated scale. Click Apply metric scale… for a straight bar or Apply metric scale + level… for an L. The confirmation dialog lists the IDs, dimensions and transform to apply.

A straight bar converts the reconstruction's scale to metres. The L also makes its printed face horizontal, points the X arm along +X and puts the Origin tag at world zero. This is why the bar must sit on the intended ground plane.

Replica replaces the current scale. It keeps the inputs of earlier Refine operations but marks their transforms unapplied so they do not accumulate. The bar definition and result are saved with the project; the preview markers stay out of exports.

After applying, check another measured distance on the subject. The diagnostics help assess detection, but the feature is still Beta and does not certify metric accuracy. Check units in the application where you open the export too: STL, for example, does not specify them.

Troubleshooting

Replica finds no tags or too few views

Check the tag family and make sure the tags are large enough in the images, sharp and free of glare or obstruction. You need at least three aligned views showing the required tags together, taken from different positions.

The scene has several bars or repeated IDs

Choose the bar's tags in the viewer. Two visible tags with the same ID make detection ambiguous; use unique IDs and capture the scene again.

The scale is wrong

Check centre distances and enter them in millimetres. Check whether you resized the print too. Replica warns when the measurement differs from the generated distance by more than 10%; measuring the bar's outside length is a common cause.

The L bar cannot be leveled

Check tag roles and whether the print is square. Replica rejects arms more than 5° from a right angle. Multiple measured distances must meet the fitting tolerance of 2% or 2 mm.

The previous result is no longer valid

Run detection again if you change the images, image range or calibration. While the panel stays open, you can change the selected tags and measurements using detections already held in memory.

Refine operations: GPS alignment and manual Measure & Scale.
Video import and camera tracking: capture a calibration section and check the camera path.
USD for VFX: export the scaled model with its cameras.