Creating Linear Dimensions

This guide demonstrates how to create linear dimensions using openNURBS.


The annotation classes are different in openNURBS 6. A sample on how to create a linear dimension would be really helpful.


The annotation classes have been overhauled in Rhino 6. For an overview of what’s new and what’s changed, see the following:

Annotation Objects

Every annotation object needs a plane, dimension style, location, and text content. In addition, each specific type of annotation needs its specific information.


The following example code can be used to create linear dimensions using the openNURBS toolkit:

  Add an aligned linear dimension to the model. The dimension line is
  parallel to the segment connecting the dimension points.
  model - [in] 
    The model object.
  point0 - [in]
  point1 - [in]
    Locations of one of the points being dimensioned.
    The dimension line will be parallel to the segment
    connecting these points.
  line_point - [in]
    A point on the linear dimension line.
  plane_normal - [in]
    A vector perpendcular to the line between the extension points
    that defines the orientation of the dimension's plane.
  dim_style - [in]
    nullptr, model dimension style, or a valid override style.
    If invalid, then the current dimension style will be used.
  attributes - [in]
    nullptr or object attributes.
static bool AddDimLinear(
  ONX_Model& model,
  ON_3dPoint point0,
  ON_3dPoint point1,
  ON_3dPoint line_point,
  ON_3dVector plane_normal,
  const ON_DimStyle* dim_style,
  const ON_3dmObjectAttributes* attributes
  if (nullptr == dim_style)
    const ON_ModelComponentReference& dim_style_ref = model.CurrentDimensionStyle();
    dim_style = ON_DimStyle::Cast(dim_style_ref.ModelComponent());

  if (nullptr == dim_style)
    return false;

  const ON_UUID parent_id
    = dim_style->ParentIdIsNotNil()
    ? dim_style->ParentId()
    : dim_style->Id();

  const ON_ModelComponentReference& parent_dim_style_ref = model.DimensionStyleFromId(parent_id);
  const ON_DimStyle* parent_dim_style = ON_DimStyle::Cast(parent_dim_style_ref.ModelComponent());
  if (nullptr == parent_dim_style)
    return false;

  ON_DimLinear dim_linear;
  if (nullptr == ON_DimLinear::CreateAligned(point0, point1, line_point, plane_normal, parent_dim_style->Id(), &dim_linear))
    return false;

  if (dim_style->ContentHash() != parent_dim_style->ContentHash())
    ON_DimStyle* override_style = new ON_DimStyle(*dim_style);
    override_style->OverrideFieldsWithDifferentValues(*dim_style, *parent_dim_style);
    if (override_style->HasOverrides())
      delete override_style;

  if (nullptr == attributes)
    attributes = &ON_3dmObjectAttributes::DefaultAttributes;

  // 'model' will copy input and manage this memory
  const ON_ModelComponentReference& geometry_ref = model.AddModelGeometryComponent(&dim_linear, attributes);
  return !geometry_ref.IsEmpty();

You can test the above static functions by adding the following sample code to the Example_Write project included with the openNURBS toolkit:

ONX_Model model = ...;

model.AddDefaultLayer(L"Default", ON_Color::Black);
model.AddDefaultDimensionStyle(L"Default", ON::LengthUnitSystem::Millimeters, 0.001);

ON_3dPoint point0(0.0, 0.0, 0.0);
ON_3dPoint point1(10.0, 0.0, 0.0);
ON_3dPoint line_point(5.0, 5.0, 0.0);
ON_3dVector plane_normal = ON_3dVector::ZAxis;
bool rc = AddDimLinear(model, point0, point1, line_point, plane_normal, nullptr, nullptr);

model.Write(archive, 60);