A shader for implicit rendering of curves

Question

I tried to create a shader for implicit rendering of curves. Unfortunately, it malfunctions (problem most likely lies in the geometry shader), displaying merely a clear screen.

Apologies for possibly wrong terminology. Instead of aproximating a curve by a line strip, my program should proccess each individual pixel in a certain area and color it, accordingly to an implicit equation, defining the curve - hence, implicit rendering.

Vertex shader - takes a line strip as an input, two planar coordinates and the angle between the curve and x-axis at the said coordinates

#version 330 core

layout (location = 0) in vec2 x;
layout (location = 1) in float t;

out VS_OUT {
  float t;
} vs_out;

void main ()
{
  vs_out.t = t;
  gl_Position = vec4 (vec3 (x, 0), 1);
}

geometry shader - creates a triangle over a line, in which individual pixels are tested

#version 330 core

layout (lines) in;
layout (triangle_strip, max_vertices = 50) out;

in VS_OUT {
  float t;
} gs_in[];

out float k;

out vec2 R;

void main ()
{    
  vec2 A = vec2 (gl_in [0].gl_Position);
  vec2 B = vec2 (gl_in [1].gl_Position);

  float u = gs_in [0].t;
  float v = gs_in [1].t;

  float a = A.x - B.x;
  float b = A.y - B.y;
  float D = sin (u) * cos (v) - cos (u) * sin (v);

  vec2 V = A + vec2 (cos (u), sin (u)) * (- sin (v) * a - cos (v) * b) / D;

  float p = (cos (v) * a + sin (v) * b) / D;
  float q = (- cos (u) * a - sin (u) * b) / D;

  vec2 S = A + vec2 (sin (u), cos (u)) * p;

  gl_Position = vec4 (vec3 (V, 0), 1);
  R = V - S;
  EmitVertex ();

  gl_Position = gl_in [0].gl_Position;
  k = p;
  R = A - S;
  EmitVertex ();

  gl_Position = gl_in [1].gl_Position;
  k = q;
  R = B - S;
  EmitVertex ();

  EndPrimitive();
}

and fragment shader - colors pixel, that is located in a close proximity of the curve, white

#version 330 core

in float k;

in vec2 R;

out vec4 bv;

float lyn (const float x)
{
  if (abs (x) < 0.1)
    return 1;
  else
    return 0;
}

float kv (const vec2 x, const float r)
{
  return dot (x, x) - r * r;
}

void main ()
{
  bv = vec4 (lyn (kv (R, k)));
}

Can you define clearly what "implicit rendering of curves" is in the question? Otherwise it may be very hard for many readers to make sense of the question. — Chris Beck, Sep 15 '15 at 02:24
this might interest you [Draw Quadratic Curve on GPU](http://stackoverflow.com/a/31423105/2521214) — Spektre, Sep 15 '15 at 13:39

A shader for implicit rendering of curves

0 Answers0