libgdx debugging shaders

Newbie & Debugging Questions
#1

I just stated working with shaders myself

I did a basic shader and then copied one from the internet to kinda play around with it and check it out.
But I just get nothing and I’m not sure how to debug shaders :smiley:

Vertex shader


void main()
{	
  gl_Position = ftransform();		
  gl_TexCoord[0] = gl_MultiTexCoord0;
}

Fragment shader

#version 120
uniform sampler2D u_texture; 

// Currently not used
uniform float time = 1.0; 

// Swirl effect parameters
uniform float radius = 80.0;
uniform float angle = 2.8;
uniform vec2 center = vec2(50.0, 50.0);

vec4 PostFX(sampler2D tex, vec2 uv, float time)
{
  vec2 texSize = vec2(512, 512);
  vec2 tc = uv * texSize;
  tc -= center;
  float dist = length(tc);
  if (dist < radius) 
  {
    float percent = (radius - dist) / radius;
    float theta = percent * percent * angle * 8.0;
    float s = sin(theta);
    float c = cos(theta);
    tc = vec2(dot(tc, vec2(c, -s)), dot(tc, vec2(s, c)));
  }
  tc += center;
  vec3 color = texture2D(tex, tc / texSize).rgb;
  return vec4(color, 1.0);
}

void main (void)
{
  vec2 uv = gl_TexCoord[0].st;
  gl_FragColor = PostFX(u_texture, uv, time);
}

its supposed to be like a swirl effect but I’m getting nothing.
It might also be that the vertex shader is wrong, since its very different from the default vertex shader