drawing thick, textured lines in OpenGL
- by NateS
I need to draw thick textured line segments in OpenGL. Actually I need curves made out of short line segments. Here is what I have:
In the upper left is an example of two connected line segments. The second image shows once the lines are given width, they overlap. If I apply a texture that uses translucency, the overlap looks terrible. The third image shows that both lines are shortened by half the amount necessary to make the thick line corners just touch. This way I can fill the space between the lines with a triangle. On the right you can see this works well (ignore the horizontal line when the crappy texture repeats).
But it doesn't always work well. In the bottom left the curve is made of many short line segments. Note the incorrect texture application.
My program is written in Java, making use of the LWJGL OpenGL binding (and minor use of Slick, a 2D helper framework). I've made a zip file that contains an executable JAR so you can easily see the problem. It also has the Java code (there is only one source file) and an Eclipse project, so you can instantly run it through Eclipse and hack at it if you like. Here she is:
http://n4te.com/temp/lines.zip
To run, execute "java -jar lines.jar". You may need "-Djava.library.path=." before -jar if you are not on Windows. Press space to toggle texture/wireframe. The wireframe only shows the line segments, the triangle between them isn't drawn. I don't need to draw arbitrary lines, just bezier curves similar to what you see in the program. Sorry the code is a bit messy, once I have a solution I will refactor.
I have investigated using GLUtessellator. It greatly simplified construction of the line, but I found that applying the texture was perfect. It worked most of the time (top image below), but long vertical curves would have severe texture distortion (bottom image below):
This turned out to be much easier to code, but in the end worse than my approach.
I believe what I'm trying to do is called "line tessellation" or "stroke tessellation". I assume this has been solved already? Is there standard code I can leverage? Otherwise, how can I fix my code so that the texture does not freak out on short, vertical curves?
