Modern 3D PDF, documents based on the PRC (Product Representation Compact) standard include a range of possible geometric precision schemes. This guide offers help in selecting the most appropriate mode for your application.
PRC Encoding inside 3D PDF Files
First, the PRC 3D geometric encoding standard has two main categories of encoding, either pure or tessellated. In the “Pure” form, full fidelity geometric shapes are encoded preserving full numerical precision on vertex 3D location (X,Y,Z) coordinates. In addition, BREP (Boundary Representation) shapes are encoded including NURBS, B-Spline curves and other related curved surface types. The idea is that full fidelity re-use is possible, or precise measurement operations performed. Of course there is a cost – the file compression level used for Pure means that the 3D PDF document will be a little larger to accommodate the full precision coordinates.
Second, the PRC 3D geometric encoding can be in “Tessellation” mode, where the original surfaces are encoded into a triangle mesh. If the input model is also triangulated with some similar polygon structure, then this is a great mapping and preserves all facets. For curves and smooth BREP surfaces, a tessellated approximation is used.
As the most commonly used type of encoding by PDF3D users, by using a triangulated tessellation the compression can be much greater, in fact the PDF3D PRC “Highly Compressed Tessellation” or HCT mode yields the highest compression factors for 3D PDF files.
Geometric Precision Options
Using PRC-HCT tessellation, although each triangle is preserved in the final PDF, the actual X,Y,Z corner vertex 3D position is approximate to within a “Tolerance” factor. This means that the encoding process is rounding up or reducing the number of significant digits in the numerical coordinate values so the resulting position is nearby within the tolerance factor distance from the original position.
The options for specifying the “Tolerance” factor available to PDF3D users can either be “Absolute” or “Relative”. In Absolute mode, the actual 3D geometric tolerance distance can be specified to match up with document requirements, such as to make sure positions are within 1 mm, in case this is important for interpretation. In “Relative” mode, some percentage of the overall model bounding box extents is used, such as 0.00002 times the size of the model. This works fine for almost all cases, and can be adjusted lower if needed. Of course increasing tolerance can also further reduce the 3D PDF file size.
In a final twist, the PDF3D encoding will detect if entire triangles are smaller than the tolerance (“Micro-Facets”) and will use an edge-collapse method to removed them.
So PRC encoding offers a variety of 3D Geometric precision options, and gives users great flexibility over coordinate fidelity and final 3D PDF size.