// Wireframe shader based on:
// J. A. Bærentzen, S. Munk-Lund, M. Gjøl, and B. D. Larsen,
// “Two methods for antialiased wireframe drawing with hidden
// line removal,” in Proceedings of the Spring Conference in
// Computer Graphics, 2008.
//
// We are not using geometry shaders, though, as they are not
// available in GLES 2.0.

attribute vec3 position;
// Coordinates of the triangle vertices this vertex belongs to
attribute vec3 tvertex0;
attribute vec3 tvertex1;
attribute vec3 tvertex2;

uniform vec2 Viewport;
uniform mat4 ModelViewProjectionMatrix;

varying vec4 dist;

void main(void)
{
    // Get the clip coordinates of all vertices
    vec4 pos  = ModelViewProjectionMatrix * vec4(position, 1.0);
    vec4 pos0 = ModelViewProjectionMatrix * vec4(tvertex0, 1.0);
    vec4 pos1 = ModelViewProjectionMatrix * vec4(tvertex1, 1.0);
    vec4 pos2 = ModelViewProjectionMatrix * vec4(tvertex2, 1.0);

    // Get the screen coordinates of all vertices
    vec3 p  = vec3(0.5 * Viewport * (pos.xy / pos.w), 0.0);
    vec3 p0 = vec3(0.5 * Viewport * (pos0.xy / pos0.w), 0.0);
    vec3 p1 = vec3(0.5 * Viewport * (pos1.xy / pos1.w), 0.0);
    vec3 p2 = vec3(0.5 * Viewport * (pos2.xy / pos2.w), 0.0);

    // Get the vectors representing the edges of the current
    // triangle primitive. 'vN' is the edge opposite vertex N.
    vec3 v0 = p2 - p1;
    vec3 v1 = p2 - p0;
    vec3 v2 = p1 - p0;

    // Calculate the distance of the current vertex from all
    // the triangle edges. The distance of point p from line
    // v is length(cross(p - p1, v)) / length(v), where
    // p1 is any of the two edge points of v.
    float d0 = length(cross(p - p1, v0)) / length(v0);
    float d1 = length(cross(p - p2, v1)) / length(v1);
    float d2 = length(cross(p - p0, v2)) / length(v2);

    // OpenGL(ES) performs perspective-correct interpolation
    // (it divides by .w) but we want linear interpolation. To
    // work around this, we premultiply by pos.w here and then
    // multiple with the inverse (stored in dist.w) in the fragment
    // shader to undo this operation.
    dist = vec4(pos.w * d0, pos.w * d1, pos.w * d2, 1.0 / pos.w);

    gl_Position = pos;
}