Geometry Correction Strategies
The last several sections have shown you what geometry detection and correction tools are available through the Virtualwind SketchUp Plugin. Here are some suggestions on how to actually perform a geometry correction.
Start with Statistics: Before doing anything, use the Compute Watertightness Statistics Tool to get a sense of what you are dealing with – and make an informed decision as to whether you actually want to proceed with a geometry correction. As a general rule of thumb, an experienced correcter can probably expect to require two hours for every thousand invalid edges. Someone less familiar with correcting geometries should probably double this time.
Remove stray edges: Once you have decided to correct a model, use the Remove Zero-Faced Edges Tool. In some models, bunches of miniscule stray edges are left over as part of the original model-creation process. Sometimes, you can eliminate hundreds of non-watertight edges immediately. Additionally, use the Remove Zero-Faced Edges Tool often in the correction process, as the correction process itself often natually leaves leftover edges. Why delete them manually when you have a tool available to do it for you?
Unhide all: A lot of models have been saved with some geometry hidden. For example, nearly all SketchUp Warehouse models that originated from Google Earth have two terrains – one that is visible and one that's on a hidden Layer. Quickly scan for all such geometry, and decide if you want to delete it before you even start. (If it was hidden, it is perhaps not appropriate to export it.)
Identify Terrain: As mentioned, many models come with geometry that is clearly terrain. Group that and use the Make Terrain Tool right away to get it out of the way.
Delete what you do not need: Many models will have features that are cosmetically pretty, but are nearly impossible to correct. Fine-detail ornamentation on neo-Roman buildings are often troublesome examples. Do not be afraid to simply delete those elements. Keep in mind too that sometimes it is actually more appropriate to delete such details than to keep them. For example, chain-link fences are often modelled as a rectangles with transparent textures on them. No matter how you correct this, Virtualwind will see this essentially as a big, complex, wind-blocking structure – whereas its real-world equivalent would be mostly invisible to wind. It is actually more realistic to delete such features.
ReGroup (carefully): In many models, the original Grouping scheme does not match well with Virtualwind's "best-practice" paradigm. In some cases, a model is "underGrouped", meaning that there are only one or two Groups in the entire model. In other cases, a model is "overGrouped": every face has been turned into its own Group.
UnderGrouped models are hard to correct because the amount of geometry to tackle looks overwhelming. Such models should be subGrouped. The best way to start subGrouping is by separating each major structure (e.g. each building) into its own Group, then subGrouping as necessary. (Columns on classical-style buildings are typically particularly good elements to subGroup.)
In contrast, OverGrouped models are even harder to correct without regrouping, because a lot of geometry that should be valid (and even looks valid on first glance) is "artificially" invalid, because geometry does not connect (in the Virtualwind sense) across Group boundaries. OverGrouped models should be treated by locating the overGrouped components and exploding them, then reGrouping the components as necessary.
On the other hand, some models' Grouping structure are in fact very consistent with Virtualwind's expectations. In particular, some SketchUp modellers make very good use of the "Component" feature. (Components are virtually identical to Groups, except that they can be linked together.) Do not explode "Virtualwind-friendly Components" if you can avoid it – even though they may not be perfectly formed, you only have to correct one, and all of them are fixed.
Rely on the Check for Watertightness (Active) Tool: Although there are three flavours of Check for Watertightness Tools, the Check All and the Check Selected flavours are mostly special-situation tools. The Check Active Tool is far and away the most reliable and meaningful. (That's not to say that the other two are unreliable: they always work, but sometimes their results are difficult to interpret.) Unless you are doing something very specific, use the "Active Group" flavour of the Check for Watertightness tools.
Rely on SketchUp's Line tool: SketchUp automatically fills in as many faces as it can when you draw a line. Typically, you can close an entire floor, no matter how complicated its outline is, by drawing one single line. Because of this, the only geometry you should ever have to draw are lines. If you find yourself reaching for a drawing tool that isn't the Draw Line tool, you should highly suspect something unexpected is going wrong. Any drawing tool other than the Line tool will almost always make your model worse.
Rely on SketchUp's Auto-fill feature: Along with the Line tool, the Auto-fill feature will be your biggest friend. If you expect SketchUp to close a face when you draw a line and it does not, that is virtually always a sign of additional problem geometry. The more certain you are that the Auto-fill should close a face and it did not, the more you can bet that the problem will be a tricky one. The Auto-fill feature is virtually never wrong.
Always have expectations: Every time you draw a line or delete a face, form an expectation of what should happen. If the results don't match the expectation, be suspicious. Sometimes, it is a relatively benign mismatch. Often, it signifies something important. Be wary of writing off your expectations as "innocent mistakes". This is especially true with the Auto-fill feature.
Save often: It is very easy for something unexpected (and undesireable) to happen while you are correcting a model, but you only discover it much later. The most common example is that you think you are deleting a minor interior face, which you later discover is actually a major exterior wall. This happens far more often than you might think. Often, your only recourse for recovering from such a mistake is to simply reload from your last save point.
Check your progress often: Use the Compute Watertightness Statistics Tool and the Separate Solids and Canopies Tool often to see how you're doing. Do not be too literal when you interpret the "Non-Watertight Edges" data of the Statistic Tool though: it is natural for the non-watertight edges count to increase temporarily during the correction process. Overall though, it should trend downward.
Be willing to sacrifice visual accuracy: Sometimes, your only recourse will be to modify the geometry such that texture information gets modified. Often, the texture will only be modified a little bit; at other times, the effect can be quite dramatic. Occasionally, you will lose the texture altogether. If you are not willing to sacrifice visual accuracy, correcting geometry will be dramatically more difficult.
Be willing to sacrifice geometrical accuracy: In addition to visual accuracy, you will sometimes find it easy to correct geometry by making small geometrical adjustments. For example, a house may have eaves modelled as a curve, but it would be so much easier if it was angled. If that happens, be willing to make the modification. Remember, Virtualwind's accuracy is only as fine as the computational grid, so most small-scale adjustments will be literally undetectable by the solver.
Familiarize yourself with SketchUp: Although the Virtualwind Plugin provides you with a lot of tools to use, most of what you will do will depend on native SketchUp functionality. Certainly, you will need to know how to draw lines and rectangles. However, you will often have to rely on more advanced features: switching SketchUp's view modes; showing hidden geometry; performing surgical click-and-drag selections. Knowledge of SketchUp will be your biggest ally in correcting models.
In addition, SketchUp has a few features that are helpful when creating general-purpose models, but can sometimes have adverse effects when creating to create watertight geometry. The Auto-fill feature (see above) is a good example of this: when it is not being your best friend, it's your worst enemy, creating interior faces you never intended – or expected. Become familiar with SketchUp so that you can anticipate problems before they happen – or at least be less surprised when "unexpected" geometry does appear.
Have lots of patience: The first time you try to correct a five-thousand edge model, you'll find yourself wishing at some point that computers had never been invented. This is normal. In fact, it isnormal even if you are working on your tenth five-thousand edge model. A model with thousands of non-watertight edges is going to take a long time to correct, regardless of how many models you've done in the past. You will also routinely come across scenarios – no matter how many times you've corrected models – that will leave you stumped, and you will find yourself staring at a non-watertight edge wondering what is so wrong about it ("Why does the check tool say there are three connected faces? I only see two!"). It will be a much enjoyable process if you expect the unexpected.
The next section deals with common watertightness problems with pre-created models.