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texture coordinates texture coordinates texture coordinates texture coordinates te


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tes texture coordinates texture coordinates texture coordinates compilation texture c

introduction This page aims to explain how opengl can be used to place texture maps onto polygons.  Opengl uses the concept of 'texture coordinates' to achieve texture mapping.  These are stored per-vertex and are interpolated in areas where there are no vertices.  Below, we go into detail, firstly on texturing your objects in a modeling package and then calculating texture coordinates step by step.

This is only the simplest case.  We have one image per texture map and only do planar texture mapping.  For more elaborate and efficient texture mapping see [texture tools extortion]

tes texture coordinates texture coordinates texture coordinates compilation texture c

The image to the right shows 4 vertices, arranged in a plane.  Notice how zero-zero is in the bottom left corner with one-one at the top right.
The 'imaginary' vertex in the middle (the darker blue blob) would have texture coordinates of 0.5, 0.5.

Repeating textures have no upper bound, IE the top-right value could be anything above 1.0

Many 3D modeling packages can export UV texture coordinates along with the model.  (opengl calls these coordinates S and T).  We're going to generate our own.  Besides, Lightwave doesn't export UV coords anyway, so we're going to do it ourselves.

tes texture coordinates texture coordinates texture coordinates compilation texture c

example We'll start by thinking about how we use our modeler to texture the object.  In the example below we setup a model and texture in Lightwave.  Don't worry if you don't have this package - most 3d modelers work in a similar way.

  • This (rather nice) spaceship has been loaded into Lightwave's modeler.  Notice how I have stretched the window containing the top view bigger than all the rest.

  • Notice how the top of the ship is purple.  This is because all the polygons on the top belong to the same surface (more on this later).  Read a lightwave tutorial if you don't know how to do stuff like this.

  • We need to take a screen shot of this window.  Use the package you are most familiar with to do this (e.g. paint shop pro)

  • Use your paint package's tools to crop the image as close to the wireframe mesh as possible. Note that this is very important.

 

  • You'll hopefully end up with a picture of your object not unlike the one to the right.

 

  • Next, using your paint package draw a texture over the top of it.

 

  • Firstly notice how I stole this object and texture map from the cool game [independence war]

 

  • More importantly you should notice how the two images fit perfectly over each other.  I can't stress how important this is for the process to work properly. 

 

  • We assign this texture map to the purple surface in lightwave's scene editor.  Again, if you don't know how to do this there are some great LW tutorials out there.
tes texture coordinates texture coordinates texture coordinates compilation texture c

science bit  We've got our model all textured up - now we need to generate texture coordinates from the vertex data.  Planar coordinates are actually very easy to generate if you think about it.  Read on to see the way I do it...

As I mentioned above, non repeating texture coordinates start at 0.0 (in the bottom left hand corner) and finish at 1.0 (in the top right hand corner).

So each vertex belonging to 'top surface' needs a S and a T coordinate that map directly to the image.  For example:

A vertex somewhere on the left wing would have a S coord of about 0.15 and a T coord of about 0.20.  These numbers dictate whereabouts opengl looks for a texel in the image.  That is why its very important to get everything lined properly when you draw your texture.

We need to use some simple maths to figure out what the S and T value of each vertex in 'top surface' should be.

Our purple 'top surface' is being texture mapped in the Y plane.  IE we are looking down on the ship from above, along the Y axis.  To generate the coordinates we need look at the values of vertices in the X and Z planes.

In this example, consider the X plane - the vertices in this plane will form our S coords.

  • We go through all the vertices in 'top surface' and find the lowest X value and the highest X value.

  • Next we take the absolute range of the highest and lowest (note: multiplying a number by -1 changes its sign)

range = (lowest - highest) * -1

  • The next step is to find to lowest number's 'offset' from 0.  So if our lowest X coordinate was -1, its offset would be 1.
    If our lowest X coordinate was 12, its offset would be -12.

offset = 0 - lowest

  • Now for each X coord in 'top surface' we add its value to the offset, and divide by the range.

Lets have a proper example of the maths...

For this example of generating S coordinates (from X coords) we'll say that:

  lowest  X: -12.00
  highest X: 134.56

So first we find the range and the offset:

   range  = (lowest  -  highest) * -1
  146.56  = (-12.00  -  134.56)  * -1 

   offset = 0 - lowest
     12   = 0 -   -12

Now we go through all the vertices in the X plane.  As an example, for a vertex that has an X value of 87.45 we do this:

   87.45 + offset = absolute position
   87.45 +   12   = 99.45

99.45 / range   = S coord for vertex at X: 87.45
99.45 / 146.56  = S : 0.679

So there you have it!  You do this for all the vertices in the X and Z planes and store the results with their corresponding 'parent' XYZ coordinates.

tes texture coordinates texture coordinates texture coordinates compilation texture c

Website and content, Paul Groves. 
Destroyer model taken from [Independence War] Copyright (c) 1999, 2000 [Particle Systems] and Infogrames.  Used with permission.