Change ISO sets the iso-value of a voxel volume: the voxel value that its visible surface passes through. Use it to make the surface show the structure you need, for example bone instead of skin on a CT scan, before you measure the volume or turn it into a mesh with Voxels to Mesh.
Open the Tool
Select exactly one voxel volume and click CT › Change ISO.
- The tool is not available while Volume Rendering is on for the volume ("Volume rendering should be disabled"). Turn volume rendering off first.
- While Change ISO is open, Volume Rendering is not available.
Every change rebuilds the surface at once, and Undo returns the previous one.
Settings
| Control | What it does |
|---|---|
| Dual Marching Cubes | On (default): the surface is built with the Dual Marching Cubes algorithm. Off: with the standard Marching Cubes algorithm |
| Iso-value | The surface passes through the voxels with this value. Drag the field or type a number. The range is from the smallest to the largest value in the volume |
| Histogram | How many voxels have each value. The axis below shows the values; the scale on the right shows the share of voxels in percent, and a line marks the current iso-value. Hover a bar to see its value range and number of voxels. Click a bar to set the iso-value to the middle of that bar |
Choose a Good Iso-Value
- Look at the histogram. Materials of different density usually form separate peaks, and a value between two peaks separates them.
- Click a bar between the peak of the background and the peak of the material you need, then check the surface in the viewport.
- Fine-tune by dragging Iso-value: a higher value keeps only denser material, a lower value adds lighter material and noise.
- To read the real value at a point, close Change ISO and use Volume Rendering with a clip plane: hold the mouse over the section plane to see the voxel value there, and Ctrl+click (Cmd+click on macOS) on the plane to use that value as the iso-value. Voxels Line Graph shows how the values change along a line, so you can see where the edge of the material is.
In the example, a dental CT scan was cropped to the jaws with Set Active Voxel Box. A click on the histogram set the Iso-value to 936.262: the surface shows the jaw bone and the teeth (the picture above). A click on a bar further right set it to 1 744.027, the middle of that bar: the bone is gone and mainly the teeth remain, because they are denser.
Voxels to Mesh copies the surface you see, so set the iso-value first and then convert. See How to convert voxel to mesh. For an overview of the CT tab, see CT & Voxels Basics.
