#1 - Do Not Use Textures That Repeat

Due to the use of a single texture for an entire object (again, regions of it are actually used), you may not use textures that repeat. This means for example that you can't use a brick texture made up of say 9 bricks and then have that texture repeated many times over a polygon to create a wall.

Instead, use a higher-resolution texture with many more bricks (enough to look good), then apply this texture to each face / surface that needs bricks.

This is due to the way that texture repetition is implemented in OpenGL. When using a single texture with multiple regions (subtextures) as we are doing, the "wrapping" mechanism that normally works for separate textures/files causes portions of the single texture outside the subtexture to be drawn, which, to put it simply, draws the wrong thing.

#2 - Limit your "master" texture to a maximum of 1024x1024 pixels

This is approximately 3 megabytes (1024x1024x3), which is a lot of texture memory, so whenever possible, try to use as little texture memory as you can get away with to balance quality with memory usage.

#3 - Think About Textures

Study the Texture Use Limitations section from the Lexington Example Tutorial to see some examples of automatic and manual (via HTML image maps) merging.

The latest Liberty master texture, part of it shown below, uses the entire texture as the island, with the subtextures placed around it for a very efficient use of texture memory:

#4 - Keep Your Polygon Counts Low(ish)!

Although our largest XGL-based (non-textured) test object has over 30,000 polygons, Liberty7 (with the island) has only 3,500.

Anthony's current Brooklyn Bridge has about 4,000, primarily because he is using 3-sided polygons for the cables of the bridge for the most realism (lines or points are not currently supported in XGLTool and lines do not really render well in X-Plane especially as your distance from the object changes).

#5 - Use only Triangles, Quads, and Polygons in your models

These are the basics of 3D modeling which all map to the powerful IndexedFaceSet VRML entity, and allow the most realism. There are some conventions / requirements we have established for the X-Plane Special Types as mentioned above.

#6 - Use lowercase filenames and extensions

Do not put <space>, "/", or other "funky" characters in your model or texturenames. For example, for liberty4 ... the model name is liberty4.lwo, the vrml file is liberty4.wrl, and the texture names are of the form body-front.bmp, fort-roof.bmp, etc. The day master texture is liberty4.bmp and the night master texture is liberty4LIT.bmp. You may use underscore (_) which is my personal preference, so body-front.bmp would become body_front.bmp.

#7 - Put your model and all textures or other assets into a single directory with the same name as your model

Most important is that your VRML file (mymodel.wrl), optional HTML image map (mymodel.html), master textures (mymodel.bmp, mymodelLIT.bmp), and individual textures (tex1.bmp, tex2.bmp, etc.) all be in the same directory. XGL files (.xgl or .xpxgl) will be created in the same directory and be named mymodel.xgl or mymodel.xpxgl.

#8 - Your master texture must be named [model].bmp

Whether you use XGLTool to auto-merge it or you create it manually and use an HTML image map, your master (day) texture must be of the form [mymodel].bmp

For the night texture it is of the form [mymodel]LIT.bmp and must be the same size and dimensions as your day texture.

X-Plane merges these textures together between dusk and dawn.

#9 - Use 24-bit Windows Bitmap files (.bmp) for your individual textures

This is not an absolute requirement, but since XGLTool and X-Plane only fully support 24-bit .bmp files, you may want to do this.

You *can* optionally use 24-bit PNG (.png) for your individual textures so that you can view your VRML (.wrl files) in a VRML viewer such as Cortona, but since XGLTool can display both VRML and XGL files with an easier to use (IMHO) keyboard navigation interface, you might as well use 24-bit .bmp files.

Also, automatic texture merging only works with 24-bit .bmp files

#10 - Automatic Texture Merging Considerations

If you are going to use automatic texture merging, and you won't really know how well it will work until you try it, you need to understand some things about how it works.

First, it loads all of your individual textures to determine their dimensions. It keeps track of the tallest texture, and calculates the combined width of all of your textures.

Since X-Plane requires a power of 2 bitmap, it then creates a single bitmap that is a power of 2 in each dimension (usually much wider than it is tall).

It then copies each individual texture into this master bitmap, left to right based on the order of the texture file in the model (.wrl file). It then adjusts the individual texture parameters to refer to these subtextures within the merged texture.

This can do a great job, or be very wasteful, depending on your individual texture dimensions.

If you wish to use this mechanism, here are some suggestions to get a fairly good result:

1. Decide on a maximum height for your textures that is a power of 2 ... this could be as small as 32, or as large as 1024, but keep in mind that whatever height you choose, any textures that are less than this height will have wasted space above them... One version of Liberty that worked well had all textures 128 pixels tall, with varying widths to fit nicely into a 1024x128 bitmap
   
   
2. Tho not a requirement, it's probably a good idea to make your individual textures a power of 2 width (2, 4, 8, 16, 32, 64, 128, 256, etc.) - I would also suggest you don't use anything less than 8 or 16 for the width.
   
   
3. For height, you can use anything less than or equal to the "maximum height" texture of the group, but keep in mind anything less than that height will be wasted texture memory. Again, I would suggest the height also be a power of 2.

There will be cases where automatic merging won't work. The Lexington Example is one case ... the hull needed to be wide and not tall to apply correctly. The flightdeck needed to be tall and narrow. Using automatic merging for these results in a lot of wasted space, which is why the manual method (with an HTML image map) was chosen.

#11 - Use a single model file per object

This means do not use separate parts/objects (in lightwave separate .lwo files) and try to merge them into a single scene or object. This does not work correctly at this time and is not supported nor recommended.

#12 - Dimensional Considerations

Here are some important things to keep in mind as you build your model(s):

• If you are building objects meant to be on the ground, always build up from Y=0
  
  
• Place the Y=0 center of your object at X=0, Z=0
  
  
• Think in meters (X-Plane's basic dimensional unit), and draw your objects in meters (1 unit in your modeling app = 1 meter in X-Plane). To convert feet to meters, multiply by 0.3048. To convert meters to feet, divide by 0.3048.

You may find it very useful at the early stages of your model development to use XGLTool and XGLTester to test your dimensions and be sure your object "fits well" inside X-Plane.

Although most 3D modeling packages do allow you to resize things as you work, we have found that doing this after applying textures has problems (at least in LightWave), so we would recommend using XGLTester/XGLTool to get your dimensions right before you get too far along.

#13 - Using Materials

Most 3D modeling packages allow you to define things like color, luminosity, diffusion, and emission (or more). Exactly how these map to VRML / OpenGL materials is dependent on the VRML exporter.

VRML, which is our "intermediate" format, supports: