Unit System
How to retrieve and interpret the unit information of your CAD file
Getting Model File Unit
To retrieve the unit used for scaling a model file properly, use A3DAsmModelFileGetUnit().
This function returns the display unit used to scale the whole model file:
A3DDouble pdUnit = 0.;
A3DAsmModelFileGetUnit(pModelFile, &pdUnit);
The value stored in pdUnit represents the first valid unit found in pModelFile (A3DAsmModelFile*).
It is expressed relative to millimeters, as described in Interpreting Units.
In addition to retrieving direct scale information, you should also consider any scaling applied at the entity’s individual transformation level.
Interpreting Units
The length unit determines the scale at which a model file should be displayed.
Length units are expressed either using the A3DEUnits enumerator or as a double factor representing a scale relative to millimeters. The latter is used for storing units within a model file.
The following table provides a correspondence between common units and their values in millimeters.
For a complete list, refer to the A3DEUnits reference.
| Unit | A3DEUnits |
Value in millimeters (A3DDouble) |
|---|---|---|
| Millimeter (mm) | kA3DUnitMillimeter |
1.0 |
| Inch (in) | kA3DUnitUSSurveyInch |
100000./3937. |
| Meter (m) | kA3DUnitMeter |
1000.0 |
| Centimeter (cm) | kA3DUnitCentimeter |
10.0 |
| Foot (ft) | kA3DUnitFoot |
(12.0*25.4) |
| Micron (µm) | kA3DUnitMicron |
1.e-3 |
| Nanometer (nm) | kA3DUnitNanometer |
1.e-6 |
See full table... |
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Two utility functions provide you with converting in-between floating-point decimal representation and A3DEUnits:
Unit in PRC Tree
Unit length information can be found in multiple locations within a PRC tree, starting from the model file.
Both model files and product occurrences contain unit-related fields:
m_bUnitFromCADindicates whether the entity’s unit information comes from the original CAD file.- If
m_bUnitFromCADisA3D_TRUE,m_dUnitholds the unit scale (encoded as described in Interpreting Units).
When the unit is set at the model file level, it is referred to as the global unit scale. Retrieving this information can be complex, but A3DAsmModelFileGetUnit() simplifies the process:
The function first checks A3DAsmModelFileData::m_bUnitFromCAD.
If A3D_TRUE, it returns m_dUnit as the unit for the entire model.
If A3D_FALSE, it traverses the model file until it finds a product occurrence where m_bUnitFromCAD is A3D_TRUE.
It then returns the corresponding m_dUnit.
The traversal is pre-order and depth-first. Once all child product occurrences have been visited, prototypes and external data are traversed in that order.
For example, in the following scenario, A3DAsmModelFileGetUnit() would return the unit from Product Occurrence 3.
Format-Specific Behaviors
Not all CAD formats handle units the same way. A usual scenario is cases where an assembly is made of parts that do not encode the same unit.
Since HOOPS Exchange needs to handle an entire model file consistently, it determines a single unit for the whole file using A3DAsmModelFileGetUnit().
In this situation, the unit information attached to individual parts or sub-assemblies is purely informative and does not influence the actual geometry.
In most cases, this process is straightforward. Formats like SolidWorks always store geometry using a single and uniform unit. Thus, no special adjustments are needed.
Other formats, such as Inventor, handle things differently. Each part or sub-assembly can be stored in its own unit, meaning an assembly might contain a mix of millimeters, inches, and feet. To resolve this, a scale factor must be applied to ensure that all components align with the model’s primary unit, allowing everything to be displayed correctly. This operation is automatically performed by Exchange.
Like Inventor, JT allows different parts and sub-assemblies to have their own units. However, JT handles scaling internally by applying the necessary adjustments at the instance level. As a result, when reading JT files, HOOPS Exchange does not need to perform any additional unit conversion. The format itself takes care of it.
Unit with B-rep Data
In B-rep structures, unit information is stored in the A3DTopoContextData structure.
The m_dScale value defines the unit as a scale factor relative to millimeters and applies to all geometric elements under the same topological context.
For more details, see Unit Information.
Unit with PMI Data
Semantic PMI may store unit information only relevant to the involved PMI.
For more details, see Units in Markups
Unit with Feature Trees
In feature trees, units are defined using A3DFRMFeatureData entries of type kA3DFRMDoubleUnit.
These units describe the measurement context (e.g., diameter, angle) of sibling features and are stored in the associated A3DFRMDoubleData structure
For more details, see Units in Feature Trees.
