Significant Size discussions

[jerstlouis]

The equation in CDB Volume 1 8.3.6 Organizing Models into Levels of Detail:

which is exactly the same as the one Ryan shared in https://github.com/opengeospatial/CDBV2-2023-Summer-Workshop/blob/main/VectorData.md#cdb-1x-significant-size

and corresponds to Table 3-1: CDB LOD vs. Model Resolution.

And this also corresponds to what I think it should all be if the Significant Size is what I call the smallestFeatureSizeInMeters, or explained more clearly:

details (that are smaller than the model as a whole) that should be visible before showing the next LOD, because they are larger than the model size criteria to include them at all at the next LOD

And this definition of the Significant Size also corresponds to how the Volume 6 OpenFlight starts defining it:

When assigning a Significant Size to a model LOD, the modeler needs to answer the following question: When I created a new model LOD, I did so to create additional detail in my model. What is the largest dimensional change in geometry for this new model LOD? In other words, what is the largest dimensional difference of a surface between this LOD and the next coarser LOD? In effect, the value of Significant Size corresponds to the “modeling difference” between the LOD and the next coarser LOD.

but I believe this is inconsistent to what it then formally states as the definition of the significant size:

Definition of Significant Size

The Significant Size is defined as the “size” of the model, expressed in meters. By extension, it applies equally well to a submodel represented by an Additive LOD. In the case of an Exchange LOD, the Significant Size is the difference between two representations of the model or submodel. Estimating the Size of the Model

Many models have shapes that resemble a cube (with roughly equal length, width, and height), and thus their significant size can be simply estimated by the length of the diagonal of their bounding box. As the shape of a model departs from that of a simple cube, either with respect to aspect ratio, or with respect to the amount of negative space within its bounding box, the model’s significant size should be decreased proportional to the amount of departure.

Defining the Significant Size as both the "size of the model" (that I will call ModelSize) and the "dimensional difference of a surface between this LOD and the next coarser LOD" (that I will call ModelResolution) is really inconsistent with a refinement / replacement approach of the model LOD. If using a refinement approach, a model has the same ModelSize throughout all of its LODs, whereas its ModelResolution improves. If a model is first shown at level A, with ModeSizeA and ModelResolutionA, then next Level B (A+1) will also include a version of that model, which presumably will have a ModelResolutionB roughly twice as fine (twice the amount of details, features half as big now distinguishable) compared to ModelResolutionA, but roughly the same ModelSizeB as ModelSizeA.

So it seems that this definition of the Significant Size is heavily biased towards and only (somewhat) consistent with an additive approach, where the size of the model being added is also the smallest new feature that was not included in the coarser model LOD. Unless all we have is boxes being added at every level, the "Size of the model" cannot be the same thing as the small details being added to the same model.

But basically I think that an improved definition that separately considers model size (what I also called the sizeOfTheLargestFeatureOfTheModelInMeters) and model resolution (what I also called the smallestFeatureSizeInMeters) would be mostly consistent with the CDB 1.x original intent (though not necessarily any or all of the tools / produced content -- something to verify, at least for our own tools and the San Diego CDB in this sprint), and consistent with the current values in Table 3-1, but we would be clarifying that the lower bound in the Significant Sizes in that table refers to the model resolution and NOT to the overall model size.

Also, with the modern 3D content production pipelines, everything is typically laser-scanned at the highest resolution, and then simplified to the lower LODs with mesh simplification algorithms (e.g., as done with EPIC's Nanite pipeline, and I think also in the One World Terrain pipeline). This contrasts with how Volume 6 currently starts to describe Significant Size:

When a finer model LOD is created, the modeler typically adds additional geometric detail, additional features (such as markings), or refines the shape of curved surfaces (such as engines, wheels), etc.

[ccbrianf]

Various places in the CDB wording may need enhancement for better clarity, but significant size has always referred to the largest/average/median size of the detail refinement. For the first appearance of that model, the model size approximately meets that definition. The concept may need more accurate description, but I don't think there is any need to account for two sizes in the application of it.

Significant size can't be determined without knowing the "size" of the amount of change. As such, in a simplification approach to LoD, it has to be a post pass determination (where the initial size is the approximate model size with modification like accounting for negative space, and the subsequent LoDs are the size of the differences, be they additive or refinement).