Can't get OpenGL code to work properly

Question

I'm very new to OpenGL, GLSL and WebGL. I'm trying to get this sample code to work in a tool like http://shdr.bkcore.com/ but I can't get it to work.

Vertex Shader:

void main()
{ 
    gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
    gl_TexCoord[0] = gl_MultiTexCoord0;
}

Fragment Shader:

precision highp float;
uniform float time;
uniform vec2 resolution;
varying vec3 fPosition;
varying vec3 fNormal;

uniform sampler2D tex0;

void main()
{
  float border = 0.01;
  float circle_radius = 0.5;
  vec4 circle_color = vec4(1.0, 1.0, 1.0, 1.0);
  vec2 circle_center = vec2(0.5, 0.5);

  vec2 uv = gl_TexCoord[0].xy;
  vec4 bkg_color = texture2D(tex0,uv * vec2(1.0, -1.0));

  // Offset uv with the center of the circle.
  uv -= circle_center;
  float dist = sqrt(dot(uv, uv));

  if ( (dist > (circle_radius+border)) || (dist < (circle_radius-border)) )
gl_FragColor = bkg_color;
else
gl_FragColor = circle_color;
}

I figured that this code must be from an outdated version of the language, so I changed the vertex shader to:

precision highp float;
attribute vec2 position;
attribute vec3 normal;

varying vec2 TextCoord;
attribute vec2 textCoord;

uniform mat3 normalMatrix;
uniform mat4 modelViewMatrix;
uniform mat4 projectionMatrix;
varying vec3 fNormal;
varying vec3 fPosition;

void main()
{

  gl_Position = vec4(position, 0.0, 1.0);
  TextCoord = vec2(textCoord);
}

That seemed to fix the error messages about undeclared identifiers and not being able to "convert from 'float' to highp 4-component something-or-other", but I have no idea if, functionally, this will do the same thing as the original intended.

Also, when I convert to this version of the Vertex Shader I have no idea what I'm supposed to do with this line in the Fragment Shader:

vec2 uv = gl_TexCoord[0].xy;

How do I convert this line to fit in with the converted vertex shader and how can I be sure that the vertex shader is even converted correctly?

Answer 1

gl_TexCoord is from desktop OpenGL, and not part of OpenGL ES. You'll need to create a new user-defined vec2 varying to hold the coordinate value.