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DOC: Document overlapping shapes with same density in forbild phantom #887

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6 changes: 6 additions & 0 deletions documentation/docs/Phantom.md
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Expand Up @@ -134,6 +134,12 @@ Clip planes can be applied to any volume to restrict their extents.

The function (attenuation, emission, etc. depending on the context) value `rho` is defined absolutely in the shape. RTK automatically deduces how much must be added to the current background based on the center (`x`,`y`,`z`) of the shape (e.g. 0.5 if one adds a shape with `rho=1.5` in a shape with `rho=1`). This is order dependent so the same phantom shapes defined in different orders might result in different results. The background of a shape must be defined first.

### Overlapping shapes with same density

When a new shape's center lies inside an existing shape with the same `rho`, the new shape's contribution is not added to the phantom. The code uses a center-based background-subtraction method which, when applied to multiple shapes with the same `rho`, prevents the last shape from increasing the phantom value within that shape's volume.

The practical solution is to set the `rho` in the overlapping region to the expected combined value so the overlap reflects the sum of contributions. The calculation outside the overlap remains correct because the background-subtraction logic is applied relative to each shape's center.

## Union

The `union` parameter allows taking the union of two shapes. The second shape takes a negative integer value (`union=-N`), representing an index offset between the current shape and the shape it is being united with.
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