Surface Deviation: compare a scan with CAD in a deviation map

Surface Deviation colors one object by its distance to another object, so you see at once where a scan or a changed part differs from its reference, for example a scan of a printed part against the CAD model. The panel also gives the minimum, maximum and average deviation and can save them to a PDF report.

You find the tool in Inspect › Surface Deviation.

Before You Start

• Two objects. Select exactly two objects: two meshes, two point clouds, or a mesh and a point cloud.
• Roles. The Reference object is the baseline. The Inspected object gets the color map. The object that is higher in the Scene tree becomes the reference, unless only the other object has the Reference Model tag (the tag that marks the reference part for quality control): then that object is the reference. The panel shows the reference name in red and the inspected object in blue. Click Swap to exchange the roles.
• Alignment. Both objects must already sit in the same place. Align a scan to its reference first with Registration; if you skip this, the whole surface turns red and blue and the numbers mean nothing.

Compare Two Objects Step by Step

1. In the Scene tree, click the reference object, then Ctrl+click (Cmd+click on macOS) the object to inspect.
2. Go to Inspect › Surface Deviation. The inspected object is colored, a color scale with the share of the surface in each color band appears in the viewport, and the Measure Points window opens next to the panel.
3. Check the Reference object and Inspected object names and click Swap if they are the wrong way round.
4. Set the color range to your tolerance: clear Auto Update Min/Max and type the value in Min/Max, for example 0.1 for ±0.1 mm.
5. Read the Statistics section. Click Show next to Minimum distance or Maximum distance to point the camera at that spot.
6. To keep the result, click Save report to PDF, or build a complete report with the Quality Control Report.

With Signed Distance on, a point of the inspected object that lies inside the reference gets a negative distance (blue end of the scale), a point outside gets a positive distance (red end). Green means little or no deviation. In the example above the letters are cut into the part, so they lie inside the original and show in blue; the statistics give a Minimum distance of −3.552 mm (the depth of the letters) and a Maximum distance of 0 mm.

Panel Settings

Setting	What it does	Default
Swap	Exchanges the reference and inspected objects.	—
Take Selected	Takes the two objects now selected in the scene as the new reference and inspected objects. Active when you select a different pair.	—
Signed Distance	Gives points inside the reference object a negative sign. Available when the reference is a mesh or a point cloud with normals. When off, all distances are positive.	On
Distance to Point Plane	Shown only when the reference is a point cloud: measures to the plane through the nearest reference point and its normal. The colors may look unintuitive.	—
Set Zero to Green	Keeps zero deviation green and makes the range symmetric around zero.	On
Discrete Palette	Splits the scale into color bands; the number next to it sets how many (2–100). Clear it for a smooth gradient.	On, 7
Palette Type	Even Space: bands of equal width across the range. Central Zone: a green tolerance zone around zero, with separate limits for the green zone and for the red and blue ends.	Even Space
Min/Max	The range of the color scale. With Set Zero to Green off, you set Max (red) and Min (blue) separately.	From the data
Save Palette as / Reset Palette	Saves the palette settings as a preset / returns to the default palette.	—
Auto Update Min/Max	Fits the range to the data automatically whenever the distances change. Clear it to keep your own range.	On

The Advanced section adds:

• Gray Out of Range draws distances outside the range in gray instead of the end colors. It can also speed up signed distance computation.
• Gray Projections on Boundary (mesh reference) grays out points whose closest point lies on the boundary of the reference mesh, for example where an open scan ends.
• Limit from Standard Deviation sets automatic limits to the signed average ±3 standard deviations. It works only when Set Zero to Green is off.
• Compute Distances Time shows how long the last computation took, in ms. On computers with a CUDA-capable NVIDIA GPU, a CUDA option lets the GPU compute the distances.

Statistics

The Statistics section shows:

• Minimum distance and Maximum distance, each with a Show button that points the camera at that spot;
• Mean squared error;
• Avg. signed distance and Signed distance stdDev (only with Signed Distance on);
• Avg. absolute distance and Absolute distance stdDev.

Save report to PDF saves these values, the color scale and views of the model from six sides to a PDF file, and the same values to a CSV file with the same name next to it. This button is in the desktop app.

Check Single Points with Measure Points

The Measure Points window opens together with Surface Deviation. Use it to check the deviation at exact spots, for example at the points from your drawing.

1. Click a point on the inspected object. The window shows its coordinates and its deviation from the nominal position.
2. Choose how to measure the deviation: 3D (full distance) or only along X, Y or Z.
3. Open Nominal & Tolerance. MeshInspector suggests the closest point of the reference as the Nominal position; click Accept guess to use it or type your own coordinates. Enter the allowed Tolerance.
4. Click New Measurement to keep this point and pick the next one. Reset clears the current pick.

The points you keep stay in the scene. The Quality Control Report lists them with their tolerance and an OK/NOK status.

Surface Deviation colors a whole surface. To get one number for the gap or overlap between two parts, use Signed Distance.