An example of a "Fruit Ninja" style swipe in LibGDX.
The tutorial can be seen here:
https://github.com/mattdesl/lwjgl-basics/wiki/LibGDX-Finger-Swipe
Note that, since writing this, I have been developing a more efficient and more robust line rendering system. By using a shader, we only need one triangle strip per line. Alternatively, triangles can be used to batch multiple lines.
The shader also allows us to create the anti-aliasing and stroke based on the line width (so its completely scalable). We do this by giving one edge of vertices a texcoord of 1.0, and the other edge a texcoord of -1.0. Then, in the fragment shader, we use abs() to ensure that the gradient goes from 0.0 (outer edges) to 1.0 (center of line).
The smoothing would be calculated a little like this:
//the mirrored gradient in the range 0.0 to 1.0
float aa = 1.0 - abs(vTexCoord.y);
//the exact smooth amount
vec4 color = vColor; //vertex color
color.a *= smoothstep(0.0, smooth/thickness, aa);
gl_FragColor = color;The technique can also be used to draw general lines, such as a circular progress bar:
http://www.java-gaming.org/topics/circular-health-bar/29791/msg/274139/view.html#msg274139
I hope to continue working on my line rendering system, to provide features like end cap style, proper line joins, and so forth.
Lots of useful information on OpenGL line rendering here:
http://artgrammer.blogspot.ca/search/label/vase%20renderer