How can I add multiple vertex shaders to a ShaderMaterial?

Question

When creating a ShaderMaterial in THREEJS, you pass the fragment and vertex shaders:

new THREE.ShaderMaterial({
    uniforms: {
        time: { value: 1.0 },
        resolution: { value: new THREE.Vector2() }
    },
    attributes: {
        vertexOpacity: { value: [] }
    },
    vertexShader: document.getElementById('v-shader').textContent,
    fragmentShader: document.getElementById('f-shader').textContent
});

But I was following a tutorial that used a two-pass vertex shader:

//Horizontal
attribute vec4 a_position;
attribute vec2 a_texCoord;

varying vec2 v_texCoord;
varying vec2 v_blurTexCoords[14];

void main()
{
    gl_Position = a_position;
    v_texCoord = a_texCoord;
    v_blurTexCoords[ 0] = v_texCoord + vec2(-0.028, 0.0);
    v_blurTexCoords[ 1] = v_texCoord + vec2(-0.024, 0.0);
    v_blurTexCoords[ 2] = v_texCoord + vec2(-0.020, 0.0);
    v_blurTexCoords[ 3] = v_texCoord + vec2(-0.016, 0.0);
    v_blurTexCoords[ 4] = v_texCoord + vec2(-0.012, 0.0);
    v_blurTexCoords[ 5] = v_texCoord + vec2(-0.008, 0.0);
    v_blurTexCoords[ 6] = v_texCoord + vec2(-0.004, 0.0);
    v_blurTexCoords[ 7] = v_texCoord + vec2( 0.004, 0.0);
    v_blurTexCoords[ 8] = v_texCoord + vec2( 0.008, 0.0);
    v_blurTexCoords[ 9] = v_texCoord + vec2( 0.012, 0.0);
    v_blurTexCoords[10] = v_texCoord + vec2( 0.016, 0.0);
    v_blurTexCoords[11] = v_texCoord + vec2( 0.020, 0.0);
    v_blurTexCoords[12] = v_texCoord + vec2( 0.024, 0.0);
    v_blurTexCoords[13] = v_texCoord + vec2( 0.028, 0.0);
}

and

//Vertical
attribute vec4 a_position;
attribute vec2 a_texCoord;

varying vec2 v_texCoord;
varying vec2 v_blurTexCoords[14];

void main()
{
    gl_Position = a_position;
    v_texCoord = a_texCoord;
    v_blurTexCoords[ 0] = v_texCoord + vec2(0.0, -0.028);
    v_blurTexCoords[ 1] = v_texCoord + vec2(0.0, -0.024);
    v_blurTexCoords[ 2] = v_texCoord + vec2(0.0, -0.020);
    v_blurTexCoords[ 3] = v_texCoord + vec2(0.0, -0.016);
    v_blurTexCoords[ 4] = v_texCoord + vec2(0.0, -0.012);
    v_blurTexCoords[ 5] = v_texCoord + vec2(0.0, -0.008);
    v_blurTexCoords[ 6] = v_texCoord + vec2(0.0, -0.004);
    v_blurTexCoords[ 7] = v_texCoord + vec2(0.0,  0.004);
    v_blurTexCoords[ 8] = v_texCoord + vec2(0.0,  0.008);
    v_blurTexCoords[ 9] = v_texCoord + vec2(0.0,  0.012);
    v_blurTexCoords[10] = v_texCoord + vec2(0.0,  0.016);
    v_blurTexCoords[11] = v_texCoord + vec2(0.0,  0.020);
    v_blurTexCoords[12] = v_texCoord + vec2(0.0,  0.024);
    v_blurTexCoords[13] = v_texCoord + vec2(0.0,  0.028);
}

But how can I add a two-pass vertex shader to a Threejs material?

I'm still trying to learn how to work with shaders, so please tell me if I'm going in the wrong direction...

Answer 1

A shader program always has only 1 vertexshader and 1 fragmentshader in webgl. When 2 pass renderings are nessecary then there are 2 programs that both gets executed over the same dataset from the scene.

Most likely in your 2 pass rendering, you get 2 programs that render to 2 rendertargets which then get combined in a single frame. A example of that is http://threejs.org/examples/#webgl_rtt.