三维建模之Making a Tree Symbol

Making a Tree Symbol

In this issue of the newsletter we will touch on some of the specific tasks and issues involved in creating a good 3D Symbol of a tree. There are a number of important considerations in this process, from the selection of a digital camera, to the construction of the actual symbol. We hope this will provide a good starting point for those thinking about entering the world of custom symbols.

Creating a tree symbol: The digital picture

A good picture of a tree is one that:

1. Is nicely illuminated and without shadows from adjacent trees or structures;
2. Does not have power/phone cables running across the tree canopy;
3. Has the proper perspective (e.g. the photographer is not looking up or down to image the tree);
4. Is silhouetted against a background that is distinctly different from the tree itself.

Taking the picture of a tree can be done with either a film camera or a digital camera. Since you will need the image in a digital format, using a digital camera will save a few steps and also allow immediate confirmation in the field that you have taken a good photograph. While any camera will do, using one with a good zoom lens will allow the photographer to step back from the tree to obtain a truer horizontal perspective. Our camera has an 8X optical zoom and has served us very well.

Using a zoom lens to capture a photograph from afar allows for a truer perspective.

If you are in the market for a digital camera, keep in mind that in 3D Symbol construction every effort is made to keep file sizes small. You will not need the highest pixel count camera to extract good textures for your 3D Symbols. It is more likely that whatever camera you buy, you will end up re-sampling your textures to make them smaller. Remember, your ultimate 3D scene will probably have hundreds of symbols and anything you can do to optimize the scene will improve performance. We suggest putting your money in the camera’s lens, not its megapixel number.

Creating a tree symbol: The texture

First of all, let's make sure that everyone understands that a texture is really just a digital image, usually of a specific object. In our discussion today, the texture that we are trying to create is one that only shows a tree, and nothing else. The image format of a texture can be of many different types. TIFF, JPEG, RGB and others will work, but for the purposes of creating a tree symbol only those formats that support an alpha channel (transparency) will produce the desired results. At this point, it seems as though RGB format images are the only ones that will reliably support alpha textures. We will explain the need for an alpha channel in the following text.

Your first step will be to open your digital image in a photo-editing program. Next, using the tools available to you, erase everything but the tree from the image. This includes gaps or holes in the tree canopy. Make sure that you have the editing settings set to create a sharp edge between the tree and the white background of the erased portion of the image. Many programs try to provide a transition softening between the erased area and the object of your interest. This can lead to a detracting halo effect around the tree symbol. Once you have erased the non-tree parts of the picture, crop the image to a bounding rectangle that only includes the extent of the tree. Remember, small file sizes are important for scene efficiency. Now save that cropped image to a new file name, including the alpha channel. We have found that some paint/photo-editing programs support alpha and some require the addition of a plug-in to add this capability. The image below shows a tree model using an alpha supported texture and one that does not have the alpha support.

A tree symbol using an alpha texture (left) and the same symbol built without an alpha texture (right).

Now that you have saved your first tree texture, it is time to step back and evaluate what you have done. Does the name of the texture tell you the species, season of the year or size? How big is the file size of the texture? If your texture is more than a couple hundred KB in size, consider re-sampling to a smaller size or a range of sizes. If you need a tree in the background, you can probably use a lower resolution texture. If the tree is to be in a very prominent location, a texture of higher resolution might be required. The point here is that saving your textures at varying resolutions is very quick and simple. The difficult task was extracting the tree from the background of the original photo. Expect to make more than one tree symbol from each texture.

Now you are finally ready to use a third-party software package to actually make the tree model. (Remember, these model-building programs come from the simulator and drawing communities where a graphic rendition of an object is called a model. We will convert these models to ESRI 3D Symbols when we import them into ArcGIS.) Hopefully, when you were in the field photographing the tree, you also recorded the height and canopy width of the tree. You will need this information to define the polygons on to which you will paste the tree texture. If you did not record these dimensions, estimate either the height or the width of the tree and use the ratio of the image dimensions to estimate the other dimension. Otherwise your texture might become distorted when pasted on the rectangular polygons.

A dimensionally correct tree symbol (left) and a distorted tree symbol using the same texture (right).

Creating a tree symbol: Additional considerations

Making the actual tree model is something that only takes a few minutes. You might consider creating several models at this time. Some would be just a billboard (single polygon) tree model and others might have two or three vertically intersecting polygons with textures pasted on all sides. Don’t forget to look at your multi-polygon model to make sure the tree trunks align correctly. Once you are satisfied with your model, save it, using a naming convention that you can use at a later time in ArcScene or ArcGlobe to select the proper 3D Symbol. You will need to save your model in an FLT, 3DS or VRML format to be able to import them into ArcScene and ArcGlobe. In a later issue we will cover the third-party model building programs that are available to you to make trees, buildings and other structures.

The library of 3D tree symbols that come with version 9 of 3D Analyst contains about 200 different trees. Most species are represented by a single example. Since ESRI must to meet the needs of customers’ world wide, the existing library is a very nice start. You will soon realize however, that you will want to augment the library with additional examples of the vegetation in your region. Not only will you want to vary the size of the trees, you may also wish to capture them in different seasons of the year. Just imagine how nice a scene of Washington D.C would look if the cherry trees were in blossom. Unfortunately, it is too late to photograph some of those trees this year, as the blossoms have already come and gone.

The Jefferson Memorial with cherry trees in full bloom.

What can you do to avoid waiting another 11 months to get the tree you need? As you will quickly discover, finding suitable trees to photograph is not an easy task. Extracting a good tree texture can be a slow process and creating more than two new tree models per day is difficult to do. The simulator development folks have also discovered these challenges and like all other adverse situations, someone saw it as an opportunity. In this case, a French company called Bionatics created a product called REALnat that is applicable to the GIS community. They “grow” tree models from species-specific mathematical seeds. In just a matter of minutes you can create a tree model, for a particular species, grown to a specific age and presented in a particular season. We found this company late last fall and have struck up a business relationship with them that allows us to sell their REALnat product to the GIS community. Next week we will cover this product in detail. In the interim, we would suggest that you point your browser to www.bionatics.com and look at their REALnat product.

We hope you find this information helpful and that you will feel free to contact us with questions, thoughts, comments and suggestions.