Separable box blur in GLSL

box.fp

#version 410

#ifdef GL_ES
    precision mediump float;
    #define LOWP lowp
#else
    #define LOWP
#endif

uniform sampler2D texture0;

#if __VERSION__ >= 140
	in vec2 _texcoords[7];
	out vec4 fragcolor;
#else
    varying vec2 _texcoords[7];
#endif

void main ()
{
    LOWP vec4 color = vec4(0.0);

    color += texture(texture0, _texcoords[0]) * 0.08;
    color += texture(texture0, _texcoords[1]) * 0.13;
    color += texture(texture0, _texcoords[2]) * 0.18;
    color += texture(texture0, _texcoords[3]) * 0.22;
    color += texture(texture0, _texcoords[4]) * 0.18;
    color += texture(texture0, _texcoords[5]) * 0.13;
    color += texture(texture0, _texcoords[6]) * 0.08;
    
    fragcolor = color;   
}

box.vp

#version 410

#ifdef GL_ES
    precision mediump float;
#endif

#if __VERSION__ >= 140
    in vec4 position;
    in vec2 texcoord0;

    out vec2 _texcoords[7];
#else
    attribute vec4 position;
    attribute vec2 texcoord0;
    
    varying vec2 _texcoords[7];
#endif

uniform float radius;
uniform vec2  texelsize; // vec2(1,0) for horizontal, vec2(0,1) for vertical

void main()
{
    gl_Position = position;
    
	vec2 offset = vec2(texelsize.x, texelsize.y) * radius;
    _texcoords[0] = texcoord0 - offset * 3.0;
    _texcoords[1] = texcoord0 - offset * 2.0;
    _texcoords[2] = texcoord0 - offset * 1.0;
    _texcoords[3] = texcoord0;
    _texcoords[4] = texcoord0 + offset * 1.0;
    _texcoords[5] = texcoord0 + offset * 2.0;
    _texcoords[6] = texcoord0 + offset * 3.0;
}