GLSL - Gaussian Blur Artifact Issue

var readInput = true;
function changeEventHandler(event){
readInput = true;
}

(function loadscene() {

var resize, gl, progDraw, progBlur, vp_size, blurFB;
var bufCube = {};
var bufQuad = {};
var shininess = 10.0;
var glow = 10.0;
var sigma = 0.8;
var radius = 1.0;

function render(delteMS){

    if ( readInput ) {
        //readInput = false;
        var sliderScale = 100;
        sigma     = document.getElementById( "sigma" ).value / sliderScale;
        radius    = document.getElementById( "radius" ).value / sliderScale;
    }

    Camera.create();
    Camera.vp = vp_size;
        
    gl.enable( gl.DEPTH_TEST );
    gl.clearColor( 0.0, 0.0, 0.0, 1.0 );
    gl.clear( gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT );

    // set up framebuffer
    gl.bindFramebuffer( gl.FRAMEBUFFER, blurFB[0] );
    gl.viewport( 0, 0, blurFB[0].width, blurFB[0].height );
    gl.clear( gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT );

    // set up draw shader
    ShaderProgram.Use( progDraw.prog );
    ShaderProgram.SetUniformM44( progDraw.prog, "u_projectionMat44", Camera.Perspective() );
    var viewMat = Camera.LookAt();
    //viewMat = RotateAxis( viewMat, CalcAng( delteMS, 13.0 ), 0 );
    //viewMat = RotateAxis( viewMat, CalcAng( delteMS, 17.0 ), 1 );
    ShaderProgram.SetUniformM44( progDraw.prog, "u_modelViewMat44", viewMat );
    ShaderProgram.SetUniformF1( progDraw.prog, "u_shininess", shininess );
    
    // draw scene
    VertexBuffer.Draw( bufCube );

    // set blur-X framebuffer and bind frambuffer texture
    gl.bindFramebuffer( gl.FRAMEBUFFER, blurFB[1] );
    gl.viewport( 0, 0, blurFB[1].width, blurFB[1].height );
    gl.clear( gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT );
    var texUnit = 1;
    gl.activeTexture( gl.TEXTURE0 + texUnit );
    gl.bindTexture( gl.TEXTURE_2D, blurFB[0].color0_texture );

    // set up blur-X shader
    ShaderProgram.Use( progBlur.prog );
    ShaderProgram.SetUniformI1( progBlur.prog, "u_texture", texUnit )
    ShaderProgram.SetUniformF2( progBlur.prog, "u_textureSize", vp_size );
    ShaderProgram.SetUniformF1( progBlur.prog, "u_sigma", sigma )
    ShaderProgram.SetUniformF1( progBlur.prog, "u_radius", radius )
    ShaderProgram.SetUniformF2( progBlur.prog, "u_dir", [1.0, 0.0] )

    // draw full screen space
    gl.enableVertexAttribArray( progBlur.inPos );
    gl.bindBuffer( gl.ARRAY_BUFFER, bufQuad.pos );
    gl.vertexAttribPointer( progBlur.inPos, 2, gl.FLOAT, false, 0, 0 ); 
    gl.bindBuffer( gl.ELEMENT_ARRAY_BUFFER, bufQuad.inx );
    gl.drawElements( gl.TRIANGLES, 6, gl.UNSIGNED_SHORT, 0 );
    gl.disableVertexAttribArray( progBlur.inPos );

    // reset framebuffer and bind frambuffer texture
    gl.bindFramebuffer( gl.FRAMEBUFFER, null );
    gl.viewport( 0, 0, vp_size[0], vp_size[1] );
    gl.clear( gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT );
    texUnit = 2;
    gl.activeTexture( gl.TEXTURE0 + texUnit );
    gl.bindTexture( gl.TEXTURE_2D, blurFB[1].color0_texture );

    // set up pst process shader
    ShaderProgram.SetUniformI1( progBlur.prog, "u_texture", texUnit )
    ShaderProgram.SetUniformF2( progBlur.prog, "u_dir", [0.0, 1.0] )

    // draw full screen space
    gl.enableVertexAttribArray( progBlur.inPos );
    gl.bindBuffer( gl.ARRAY_BUFFER, bufQuad.pos );
    gl.vertexAttribPointer( progBlur.inPos, 2, gl.FLOAT, false, 0, 0 ); 
    gl.bindBuffer( gl.ELEMENT_ARRAY_BUFFER, bufQuad.inx );
    gl.drawElements( gl.TRIANGLES, 6, gl.UNSIGNED_SHORT, 0 );
    gl.disableVertexAttribArray( progBlur.inPos );

    requestAnimationFrame(render);
}

function resize() {
    //vp_size = [gl.drawingBufferWidth, gl.drawingBufferHeight];
    vp_size = [window.innerWidth, window.innerHeight]
    //vp_size = [256, 256]
    canvas.width = vp_size[0];
    canvas.height = vp_size[1];

    var fbsize = Math.max(vp_size[0], vp_size[1]);
    fbsize = 1 << 31 - Math.clz32(fbsize); // nearest power of 2

    blurFB = [];
    for ( var i = 0; i < 2; ++ i ) {
        fb = gl.createFramebuffer();
        fb.width = fbsize;
        fb.height = fbsize;
        gl.bindFramebuffer( gl.FRAMEBUFFER, fb );
        fb.color0_texture = gl.createTexture();
        gl.bindTexture( gl.TEXTURE_2D, fb.color0_texture );
        gl.texParameteri( gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.NEAREST );
        gl.texParameteri( gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.NEAREST );
        gl.texImage2D( gl.TEXTURE_2D, 0, gl.RGBA, fb.width, fb.height, 0, gl.RGBA, gl.UNSIGNED_BYTE, null );
        fb.renderbuffer = gl.createRenderbuffer();
        gl.bindRenderbuffer( gl.RENDERBUFFER, fb.renderbuffer );
        gl.renderbufferStorage( gl.RENDERBUFFER, gl.DEPTH_COMPONENT16, fb.width, fb.height );
        gl.framebufferTexture2D( gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, gl.TEXTURE_2D, fb.color0_texture, 0 );
        gl.framebufferRenderbuffer( gl.FRAMEBUFFER, gl.DEPTH_ATTACHMENT, gl.RENDERBUFFER, fb.renderbuffer );
        gl.bindTexture( gl.TEXTURE_2D, null );
        gl.bindRenderbuffer( gl.RENDERBUFFER, null );
        gl.bindFramebuffer( gl.FRAMEBUFFER, null );
        blurFB.push( fb );
    }
}

function initScene() {

    canvas = document.getElementById( "canvas");
    gl = canvas.getContext( "experimental-webgl" );
    if ( !gl )
        return null;

    progDraw = {}
    progDraw.prog = ShaderProgram.Create( 
    [ { source : "draw-shader-vs", stage : gl.VERTEX_SHADER },
        { source : "draw-shader-fs", stage : gl.FRAGMENT_SHADER }
    ] );
    if ( !progDraw.prog )
        return null;
    progDraw.inPos = gl.getAttribLocation( progDraw.prog, "inPos" );
    progDraw.inNV  = gl.getAttribLocation( progDraw.prog, "inNV" );
    progDraw.inCol = gl.getAttribLocation( progDraw.prog, "inCol" );

    progBlur = {}
    progBlur.prog = ShaderProgram.Create( 
    [ { source : "post-shader-vs", stage : gl.VERTEX_SHADER },
        { source : "blur-shader-fs", stage : gl.FRAGMENT_SHADER }
    ] );
    progBlur.inPos = gl.getAttribLocation( progBlur.prog, "inPos" );
    if ( !progBlur.prog )
        return;    
    
    // create cube
    var cubePos = [
    -1.0, -1.0,  1.0,  1.0, -1.0,  1.0,  1.0,  1.0,  1.0, -1.0,  1.0,  1.0,
    -1.0, -1.0, -1.0,  1.0, -1.0, -1.0,  1.0,  1.0, -1.0, -1.0,  1.0, -1.0 ];
    var cubeCol = [ 1.0, 0.0, 0.0, 1.0, 0.5, 0.0, 1.0, 0.0, 1.0, 1.0, 1.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 1.0 ];
    var cubeHlpInx = [ 0, 1, 2, 3, 1, 5, 6, 2, 5, 4, 7, 6, 4, 0, 3, 7, 3, 2, 6, 7, 1, 0, 4, 5 ];  
    var cubePosData = [];
    for ( var i = 0; i < cubeHlpInx.length; ++ i ) {
    cubePosData.push( cubePos[cubeHlpInx[i]*3], cubePos[cubeHlpInx[i]*3+1], cubePos[cubeHlpInx[i]*3+2] );
    }
    var cubeNVData = [];
    for ( var i1 = 0; i1 < cubeHlpInx.length; i1 += 4 ) {
    var nv = [0, 0, 0];
    for ( i2 = 0; i2 < 4; ++ i2 ) {
        var i = i1 + i2;
        nv[0] += cubePosData[i*3]; nv[1] += cubePosData[i*3+1]; nv[2] += cubePosData[i*3+2];
    }
    for ( i2 = 0; i2 < 4; ++ i2 )
    cubeNVData.push( nv[0], nv[1], nv[2] );
    }
    var cubeColData = [];
    for ( var is = 0; is < 6; ++ is ) {
    for ( var ip = 0; ip < 4; ++ ip ) {
        cubeColData.push( cubeCol[is*3], cubeCol[is*3+1], cubeCol[is*3+2] ); 
    }
    }
    var cubeInxData = [];
    for ( var i = 0; i < cubeHlpInx.length; i += 4 ) {
    cubeInxData.push( i, i+1, i+2, i, i+2, i+3 );   
    }
    bufCube = VertexBuffer.Create(
    [ { data : cubePosData, attrSize : 3, attrLoc : progDraw.inPos },
    { data : cubeNVData,  attrSize : 3, attrLoc : progDraw.inNV },
    { data : cubeColData, attrSize : 3, attrLoc : progDraw.inCol } ],
    cubeInxData );

    bufQuad.pos = gl.createBuffer();
    gl.bindBuffer( gl.ARRAY_BUFFER, bufQuad.pos );
    gl.bufferData( gl.ARRAY_BUFFER, new Float32Array( [ -1.0, -1.0, 1.0, -1.0, 1.0, 1.0, -1.0, 1.0 ] ), gl.STATIC_DRAW );
    bufQuad.inx = gl.createBuffer();
    gl.bindBuffer( gl.ELEMENT_ARRAY_BUFFER, bufQuad.inx );
    gl.bufferData( gl.ELEMENT_ARRAY_BUFFER, new Uint16Array( [ 0, 1, 2, 0, 2, 3 ] ), gl.STATIC_DRAW );  
    
    window.onresize = resize;
    resize();
    requestAnimationFrame(render);
}

function Fract( val ) { 
    return val - Math.trunc( val );
}
function CalcAng( deltaTime, intervall ) {
    return Fract( deltaTime / (1000*intervall) ) * 2.0 * Math.PI;
}
function CalcMove( deltaTime, intervall, range ) {
    var pos = self.Fract( deltaTime / (1000*intervall) ) * 2.0
    var pos = pos < 1.0 ? pos : (2.0-pos)
    return range[0] + (range[1] - range[0]) * pos;
}    
function EllipticalPosition( a, b, angRag ) {
    var a_b = a * a - b * b
    var ea = (a_b <= 0) ? 0 : Math.sqrt( a_b );
    var eb = (a_b >= 0) ? 0 : Math.sqrt( -a_b );
    return [ a * Math.sin( angRag ) - ea, b * Math.cos( angRag ) - eb, 0 ];
}

function IdentityMat44() {
    return [ 1, 0, 0, 0,    0, 1, 0, 0,    0, 0, 1, 0,    0, 0, 0, 1 ];
};

function RotateAxis(matA, angRad, axis) {
    var aMap = [ [1, 2], [2, 0], [0, 1] ];
    var a0 = aMap[axis][0], a1 = aMap[axis][1]; 
    var sinAng = Math.sin(angRad), cosAng = Math.cos(angRad);
    var matB = matA.slice(0);
    for ( var i = 0; i < 3; ++ i ) {
        matB[a0*4+i] = matA[a0*4+i] * cosAng + matA[a1*4+i] * sinAng;
        matB[a1*4+i] = matA[a0*4+i] * -sinAng + matA[a1*4+i] * cosAng;
    }
    return matB;
}

function Cross( a, b ) { return [ a[1] * b[2] - a[2] * b[1], a[2] * b[0] - a[0] * b[2], a[0] * b[1] - a[1] * b[0], 0.0 ]; }
function Dot( a, b ) { return a[0]*b[0] + a[1]*b[1] + a[2]*b[2]; }
function Normalize( v ) {
    var len = Math.sqrt( v[0] * v[0] + v[1] * v[1] + v[2] * v[2] );
    return [ v[0] / len, v[1] / len, v[2] / len ];
}

var Camera = {};
Camera.create = function() {
    this.pos    = [0, 3, 0.0];
    this.target = [0, 0, 0];
    this.up     = [0, 0, 1];
    this.fov_y  = 90;
    this.vp     = [800, 600];
    this.near   = 0.5;
    this.far    = 100.0;
}
Camera.Perspective = function() {
    var fn = this.far + this.near;
    var f_n = this.far - this.near;
    var r = this.vp[0] / this.vp[1];
    var t = 1 / Math.tan( Math.PI * this.fov_y / 360 );
    var m = IdentityMat44();
    m[0]  = t/r; m[1]  = 0; m[2]  =  0;                              m[3]  = 0;
    m[4]  = 0;   m[5]  = t; m[6]  =  0;                              m[7]  = 0;
    m[8]  = 0;   m[9]  = 0; m[10] = -fn / f_n;                       m[11] = -1;
    m[12] = 0;   m[13] = 0; m[14] = -2 * this.far * this.near / f_n; m[15] =  0;
    return m;
}
Camera.LookAt = function() {
    var mz = Normalize( [ this.pos[0]-this.target[0], this.pos[1]-this.target[1], this.pos[2]-this.target[2] ] );
    var mx = Normalize( Cross( this.up, mz ) );
    var my = Normalize( Cross( mz, mx ) );
    var tx = Dot( mx, this.pos );
    var ty = Dot( my, this.pos );
    var tz = Dot( [-mz[0], -mz[1], -mz[2]], this.pos ); 
    var m = IdentityMat44();
    m[0]  = mx[0]; m[1]  = my[0]; m[2]  = mz[0]; m[3]  = 0;
    m[4]  = mx[1]; m[5]  = my[1]; m[6]  = mz[1]; m[7]  = 0;
    m[8]  = mx[2]; m[9]  = my[2]; m[10] = mz[2]; m[11] = 0;
    m[12] = tx;    m[13] = ty;    m[14] = tz;    m[15] = 1; 
    return m;
} 

var ShaderProgram = {};
ShaderProgram.Create = function( shaderList ) {
    var shaderObjs = [];
    for ( var i_sh = 0; i_sh < shaderList.length; ++ i_sh ) {
        var shderObj = this.CompileShader( shaderList[i_sh].source, shaderList[i_sh].stage );
        if ( shderObj == 0 )
            return 0;
        shaderObjs.push( shderObj );
    }
    var progObj = this.LinkProgram( shaderObjs )
    if ( progObj != 0 ) {
        progObj.attribIndex = {};
        var noOfAttributes = gl.getProgramParameter( progObj, gl.ACTIVE_ATTRIBUTES );
        for ( var i_n = 0; i_n < noOfAttributes; ++ i_n ) {
            var name = gl.getActiveAttrib( progObj, i_n ).name;
            progObj.attribIndex[name] = gl.getAttribLocation( progObj, name );
        }
        progObj.unifomLocation = {};
        var noOfUniforms = gl.getProgramParameter( progObj, gl.ACTIVE_UNIFORMS );
        for ( var i_n = 0; i_n < noOfUniforms; ++ i_n ) {
            var name = gl.getActiveUniform( progObj, i_n ).name;
            progObj.unifomLocation[name] = gl.getUniformLocation( progObj, name );
        }
    }
    return progObj;
}
ShaderProgram.AttributeIndex = function( progObj, name ) { return progObj.attribIndex[name]; } 
ShaderProgram.UniformLocation = function( progObj, name ) { return progObj.unifomLocation[name]; } 
ShaderProgram.Use = function( progObj ) { gl.useProgram( progObj ); } 
ShaderProgram.SetUniformI1  = function( progObj, name, val ) { if(progObj.unifomLocation[name]) gl.uniform1i( progObj.unifomLocation[name], val ); }
ShaderProgram.SetUniformF1  = function( progObj, name, val ) { if(progObj.unifomLocation[name]) gl.uniform1f( progObj.unifomLocation[name], val ); }
ShaderProgram.SetUniformF2  = function( progObj, name, arr ) { if(progObj.unifomLocation[name]) gl.uniform2fv( progObj.unifomLocation[name], arr ); }
ShaderProgram.SetUniformF3  = function( progObj, name, arr ) { if(progObj.unifomLocation[name]) gl.uniform3fv( progObj.unifomLocation[name], arr ); }
ShaderProgram.SetUniformF4  = function( progObj, name, arr ) { if(progObj.unifomLocation[name]) gl.uniform4fv( progObj.unifomLocation[name], arr ); }
ShaderProgram.SetUniformM33 = function( progObj, name, mat ) { if(progObj.unifomLocation[name]) gl.uniformMatrix3fv( progObj.unifomLocation[name], false, mat ); }
ShaderProgram.SetUniformM44 = function( progObj, name, mat ) { if(progObj.unifomLocation[name]) gl.uniformMatrix4fv( progObj.unifomLocation[name], false, mat ); }
ShaderProgram.CompileShader = function( source, shaderStage ) {
    var shaderScript = document.getElementById(source);
    if (shaderScript)
    source = shaderScript.text;
    var shaderObj = gl.createShader( shaderStage );
    gl.shaderSource( shaderObj, source );
    gl.compileShader( shaderObj );
    var status = gl.getShaderParameter( shaderObj, gl.COMPILE_STATUS );
    if ( !status ) alert(gl.getShaderInfoLog(shaderObj));
    return status ? shaderObj : null;
} 
ShaderProgram.LinkProgram = function( shaderObjs ) {
    var prog = gl.createProgram();
    for ( var i_sh = 0; i_sh < shaderObjs.length; ++ i_sh )
        gl.attachShader( prog, shaderObjs[i_sh] );
    gl.linkProgram( prog );
    status = gl.getProgramParameter( prog, gl.LINK_STATUS );
    if ( !status ) alert("Could not initialise shaders");
    gl.useProgram( null );
    return status ? prog : null;
}

var VertexBuffer = {};
VertexBuffer.Create = function( attributes, indices ) {
    var buffer = {};
    buffer.buf = [];
    buffer.attr = []
    for ( var i = 0; i < attributes.length; ++ i ) {
        buffer.buf.push( gl.createBuffer() );
        buffer.attr.push( { size : attributes[i].attrSize, loc : attributes[i].attrLoc } );
        gl.bindBuffer( gl.ARRAY_BUFFER, buffer.buf[i] );
        gl.bufferData( gl.ARRAY_BUFFER, new Float32Array( attributes[i].data ), gl.STATIC_DRAW );
    }
    buffer.inx = gl.createBuffer();
    gl.bindBuffer( gl.ELEMENT_ARRAY_BUFFER, buffer.inx );
    gl.bufferData( gl.ELEMENT_ARRAY_BUFFER, new Uint16Array( indices ), gl.STATIC_DRAW );
    buffer.inxLen = indices.length;
    gl.bindBuffer( gl.ARRAY_BUFFER, null );
    gl.bindBuffer( gl.ELEMENT_ARRAY_BUFFER, null );
    return buffer;
}
VertexBuffer.Draw = function( bufObj ) {
for ( var i = 0; i < bufObj.buf.length; ++ i ) {
        gl.bindBuffer( gl.ARRAY_BUFFER, bufObj.buf[i] );
        gl.vertexAttribPointer( bufObj.attr[i].loc, bufObj.attr[i].size, gl.FLOAT, false, 0, 0 );
        gl.enableVertexAttribArray( bufObj.attr[i].loc );
    }
    gl.bindBuffer( gl.ELEMENT_ARRAY_BUFFER, bufObj.inx );
    gl.drawElements( gl.TRIANGLES, bufObj.inxLen, gl.UNSIGNED_SHORT, 0 );
    for ( var i = 0; i < bufObj.buf.length; ++ i )
        gl.disableVertexAttribArray( bufObj.attr[i].loc );
    gl.bindBuffer( gl.ARRAY_BUFFER, null );
    gl.bindBuffer( gl.ELEMENT_ARRAY_BUFFER, null );
}

initScene();

})();

html,body { margin: 0; overflow: hidden; }
#gui { position : absolute; top : 0; left : 0; }

<script id="draw-shader-vs" type="x-shader/x-vertex">
precision mediump float;

attribute vec3 inPos;
attribute vec3 inNV;
attribute vec3 inCol;

varying vec3 vertPos;
varying vec3 vertNV;
varying vec3 vertCol;

uniform mat4 u_projectionMat44;
uniform mat4 u_modelViewMat44;

void main()
{
    vertNV      = mat3( u_modelViewMat44 ) * normalize( inNV );
    vertCol     = inCol;
    vec4 pos    = u_modelViewMat44 * vec4( inPos, 1.0 );
    vertPos     = pos.xyz / pos.w;
    gl_Position = u_projectionMat44 * pos;
}
</script>

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

varying vec3 vertPos;
varying vec3 vertNV;
varying vec3 vertCol;

uniform float u_shininess;

void main()
{
    vec3 color = vertCol;
    vec3  normalV  = normalize( vertNV );
    vec3  eyeV     = normalize( -vertPos );
    vec3  halfV    = normalize( eyeV + normalV );
    float NdotH    = max( 0.0, dot( normalV, halfV ) );
    float shineFac = ( u_shininess + 2.0 ) * pow( NdotH, u_shininess ) / ( 2.0 * 3.14159265 );
    gl_FragColor   = vec4( color.rgb * (0.2 + NdotH), 1.0 );
} 
</script>

<script id="post-shader-vs" type="x-shader/x-vertex">
precision mediump float;

attribute vec2 inPos;

varying   vec2 pos;

void main()
{
    pos = inPos;
    gl_Position = vec4( inPos, 0.0, 1.0 );
}
</script>

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

varying vec2 pos;

uniform sampler2D u_texture;
uniform vec2      u_textureSize;
uniform float     u_sigma;
uniform float     u_radius;
uniform vec2      u_dir;

float CalcGauss( float x, float sigma )
{
    if ( sigma <= 0.0 )
        return 0.0;
  return exp( -(x*x) / (2.0 * sigma) ) / (2.0 * 3.14157 * sigma);
}

void main()
{
    vec2 texC     = pos.st * 0.5 + 0.5;
    vec4 texCol   = texture2D( u_texture, texC );
    vec4 gaussCol = vec4( texCol.rgb, 1.0 );
    vec2 step     = u_dir / u_textureSize;
    for ( int i = 1; i <= 32; ++ i )
    {
        float weight = CalcGauss( float(i) / 32.0, u_sigma * 0.5 );
        if ( weight < 1.0/255.0 )
            break;
        texCol    = texture2D( u_texture, texC + u_radius * step * float(i) );
        gaussCol += vec4( texCol.rgb * weight, weight );
        texCol    = texture2D( u_texture, texC - u_radius * step * float(i) );
        gaussCol += vec4( texCol.rgb * weight, weight );
    }
    gaussCol.rgb = clamp( gaussCol.rgb / gaussCol.w, 0.0, 1.0 );
    gl_FragColor = vec4( gaussCol.rgb, 1.0 );
}
</script>

<div>
<form id="gui" name="inputs">
    <table>
        <tr> <td> <font color= #CCF>radius</font> </td> 
                <td> <input type="range" id="radius" min="1" max="1000" value="1000" onchange="changeEventHandler(event);"/></td> </tr>
        <tr> <td> <font color= #CCF>blur</font> </td> 
                <td> <input type="range" id="sigma" min="1" max="100" value="5" onchange="changeEventHandler(event);"/></td> </tr>
    </table>
</form>
</div>

<canvas id="canvas" style="border: none;"></canvas>