GLSL fragment shader gets vertex position

Question

<script id="shader-triangle-fs" type="x-shader/x-fragment">
  precision mediump float;

  uniform vec4 colorVec;
  uniform sampler2D uSampler;

  varying highp vec2 vTextureCoord;
  varying vec3 vVertexPosition;


  void main(void) {
    float dist = length(vVertexPosition.xy - gl_FragCoord.xy);
    float att = 100.0/dist;
    vec4 color = vec4(att,att,att, 1);
    vec4 texture = texture2D(uSampler, vec2(vTextureCoord.s, vTextureCoord.t));
    gl_FragColor = color;
  }
</script>

<script id="shader-triangle-vs" type="x-shader/x-vertex">
  attribute vec3 aVertexPosition;
  attribute vec2 aTextureCoord;

  uniform mat4 uMVMatrix;
  uniform mat4 uPMatrix;

  varying highp vec2 vTextureCoord;
  varying vec3 vVertexPosition;

  void main(void) {
    gl_Position = uPMatrix * uMVMatrix * vec4(aVertexPosition, 1.0);
    vTextureCoord = aTextureCoord;
    vVertexPosition = uPMatrix * uMVMatrix * vec4(aVertexPosition, 1.0);
  }
</script>

In float dist = length(vVertexPosition.xy - gl_FragCoord.xy);, what I want to get is the distance from the current vertex coordinate to the fragment coordinate.

But always I get it.

http://puu.sh/6rbva.jpg
http://puu.sh/6rbzM.jpg

(I used triangleStrip.)

These images show vVertexPosition.xy - gl_FragCoord.xy = 0so vVertexPosition.xy == gl_FragCoord.xy.

Why is this happening?
And how to get the distance from the current vertex position to the current FragCoord?

Answer 1

vVertexPosition varying, . , , , , , , .

, ? , " ", , "" - , , .

OGL/GLSL flat varying ( flat ), .