Processing a Photobatch: The Dense Cloud

The dense cloud is the step at which we conclude whether or not a model is “good” and whether or not work can continue below the current context. It is therefore vital that you 1) produce the dense cloud as efficiently as possible, and 2) check it thoroughly for any problems before giving the go-ahead to the excavators.

The dense cloud itself will also be used for other tasks in the future, most notably the construction of context volumes. This is important to remember, as any problems with the dense cloud at this stage will impact the volume production process and the quality of the volumes down the road.

Dense Cloud Workflow #

  1. Make sure that the model is georeferenced before proceeding with this step. If you georeference the model after the dense cloud has been produced, the program will automatically delete the dense cloud.
  2. Before building the dense cloud, you should clip the model to remove extraneous data. Do this by moving, resizing, and rotating the bounding box.
    1. You should generally clip the model outside the targets so that they will be visible in the eventual orthophotos; these can serve as a helpful reference for excavators when drawing Day Plans. That said, clipping the model so that the targets are not visible will not impact the accuracy of the model. 
    2. In rare cases where a context extends into the scarp, under a wall, or otherwise has significant overhang, it is beneficial to clip the model in such a way that the overhang is removed and the full context is made visible. You should do this in consultation with the excavators so that the model is clipped correctly and meets their needs. (Note: in cases where you have clipped to remove overhang, make sure you disable interpolation in future steps, including when building the mesh and DEM, to prevent the computer from algorithmically adding data to the clipped area.)

Note: An important part of the modeling process is being aware of where the context begins and ends (in other words, what parts of the model are more important than others). Usually, the area within the CTs is the context; therefore, any area outside of the CTs is less important, and may contain gaps/distortions without causing problems. However, if a context is against a scarp or a wall, sometimes the CTs will be far away from the actual context in order to surround it on all sides. If you model a context where one or more CTs appear to be far away from the others and there is a large gap, make sure to communicate with the excavators about whether or not the gap might represent a wall or a scarp that is not part of the context.

  1. To produce the dense cloud, go to Workflow> Build Dense Cloud and use the following settings:
Depth filteringAggressive
  1. Building the dense cloud can be one of the more time-consuming steps in the process of building a 3D model. If you are running a particularly large or particularly small model, you might change the settings to produce better results.
    1. For models that will likely take a long time to process, you might give an update to the excavators so that they can plan their time more effectively. 
  2. Once the dense cloud is built, save the file and thoroughly check the model again for any holes.

If there are significant holes in the model…

…you will need to ask the excavators for more photos. It is recommended that you send a screenshot of the model to the excavators with the location of holes indicated. Once the excavators send more photos, you will need to add them, and redo the Align Photos step, but you do not need to Georeference the model again (unless you think the additional photos will significantly impact the accuracy of the model).

Note: Make sure that you check the model thoroughly before you contact the excavators about taking more photos. It is frustrating for excavators to finish taking additional photos only to be told about another hole not seen at first.

If there are no significant holes in the model…

…you can tell the excavators that the model is “good” and that they can continue working in the area. Remember that once the excavators know the dense cloud is ok, they will pick up the targets and possibly begin excavating in the area in question. Therefore, you must make sure to check the model carefully before giving the excavator the ok.

Once you have decided that the model is good, you can move to the next step.

If the model is particularly large or small… #

Producing a dense cloud is all about balancing competing concerns, particularly between the time it takes to produce the model against the quality of the model. The goal is to only run the dense cloud once and only run it again if there are errors that cannot be fixed, so you should plan accordingly. 

  1. For especially large/complex models with many photos (e.g., the closing model of a whole area), it is recommended that you change the Quality to Low. You might also adjust the depth filtering, but the Quality is likely to make the largest difference in processing time. 
  2. For especially small/simple models with few photos (e.g., a top of an in situ vessel), you might want to change the Quality to High depending on the importance and complexity of the context you are working on. However, processing on Medium will almost certainly produce a model that is sufficient for our needs.

How do I know if the holes in a model are “significant”? #

To determine if a hole is too significant to proceed, consider both its size and its location. It is useful to keep in mind that closing holes in later steps (e.g., building the mesh or volume production) requires interpolation, in which the computer builds on existing data to create new data where none exists. Note that you will often see holes in the model that are outside the given context, such as when a target needs to be placed on a scarp or wall that is far above the level of the context. Check with the excavators by sending a screenshot to determine if a hole is outside the context. 

Keep an eye out for holes that obscure important information or relationships. Take, for example, a model of a pithos; a hole that obscures where the edge of the pithos intersects the surrounding soil or a wall will be a problem for reconstructing that relationship in later analysis. Also keep the purpose of the photobatch in mind when assessing holes; for example, you should be willing to spend more time refining a top model of an important feature or an in situ object than you would a general fill.  Consulting with the excavators will be valuable in determining whether a hole is in a problematic location or if the model needs to be of especially high quality.

Holes that are not in important locations might still be too large to proceed. A good way to get a feel for this is to play with the interpolation settings when building a mesh (but please do this when excavators are not waiting on a model!). If a hole is easily closed by the “Close Holes” tool, it is probably small enough that it will not cause subsequent problems. If you need to use “Extrapolated” interpolation to close holes, they are too large and more photos should be taken.

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