Custom geometry SCNprogram

Hi

I am trying to mess around in SceneKit and teach myself it. Basically, I am creating a quad with 3 rectangular sides and 1 sloping slide. I want my texture on it to stretch and warp/deform across the surface. Reading some stuff online, it seems that I need to make an SCNProgram with custom vertex and fragment shaders to get the effect. But I cant seem to get the texture to spread across the surface. Need help please. (I am new to graphics programming hence trying to teach it to myself).

My swift code to create the geometry and texture it is as follows:

func geometryCreate() -> SCNNode {

        let verticesPosition = [
            SCNVector3Make(0.0, 0.0, 0.0),
            SCNVector3Make(5.0, 0.0, 0.0),
            SCNVector3Make(5.0, 5.0, 0.0),
            SCNVector3Make(0.0, 3.0, 0.0)
        ]
        let textureCord =    [CGPoint (x: 0.0,y: 0.0), CGPoint(x: 1.0,y: 0.0), CGPoint(x: 1.0,y: 1.0), CGPoint(x: 0.0,y: 1.0)]

        let indices: [CInt] = [
        0, 2, 3,
        0, 1, 2
        ]

        let vertexSource = SCNGeometrySource(vertices: verticesPosition, count: 4)
        let srcTex = SCNGeometrySource(textureCoordinates: textureCord, count: 4)
        let date = NSData(bytes: indices, length: sizeof(CInt) * indices.count)

        let scngeometry = SCNGeometryElement(data: date, primitiveType: SCNGeometryPrimitiveType.Triangles, primitiveCount: 2, bytesPerIndex: sizeof(CInt))

        let geometry = SCNGeometry(sources: [vertexSource,srcTex], elements: [scngeometry])
        let program = SCNProgram()

        if let filepath = NSBundle.mainBundle().pathForResource("vertexshadertry", ofType: "vert") {
            do {
                let contents = try NSString(contentsOfFile: filepath, encoding: NSUTF8StringEncoding) as String
                program.vertexShader = contents
            } catch {
                print("**** happened loading vertex shader")
            }
        }


        if let fragmentShaderPath = NSBundle.mainBundle().pathForResource("fragshadertry", ofType:"frag")
        {
            do {
            let fragmentShaderAsAString = try NSString(contentsOfFile: fragmentShaderPath, encoding: NSUTF8StringEncoding)
            program.fragmentShader = fragmentShaderAsAString as String
            } catch {
                print("**** happened loading frag shader")
            }
        }
        program.setSemantic(SCNGeometrySourceSemanticVertex, forSymbol: "position", options: nil)
        program.setSemantic(SCNGeometrySourceSemanticTexcoord, forSymbol: "textureCoordinate", options: nil)
        program.setSemantic(SCNModelViewProjectionTransform, forSymbol: "modelViewProjection", options: nil)
        do {
            let texture = try GLKTextureLoader.textureWithCGImage(UIImage(named: "stripes")!.CGImage!, options: nil)

            geometry.firstMaterial?.handleBindingOfSymbol("yourTexture", usingBlock: { (programId:UInt32, location:UInt32, node:SCNNode!, renderer:SCNRenderer!) -> Void in
                    glTexParameterf(GLenum(GL_TEXTURE_2D), GLenum(GL_TEXTURE_WRAP_S), Float(GL_CLAMP_TO_EDGE) )
                    glTexParameterf(GLenum(GL_TEXTURE_2D), GLenum(GL_TEXTURE_WRAP_T), Float(GL_CLAMP_TO_EDGE) )
                    glTexParameterf(GLenum(GL_TEXTURE_2D), GLenum(GL_TEXTURE_MAG_FILTER), Float(GL_LINEAR) )
                    glTexParameterf(GLenum(GL_TEXTURE_2D), GLenum(GL_TEXTURE_MIN_FILTER), Float(GL_LINEAR) )
                    glBindTexture(GLenum(GL_TEXTURE_2D), texture.name)
            })
        } catch {
            print("Texture not loaded")
        }

        geometry.firstMaterial?.program = program
        let scnnode = SCNNode(geometry: geometry)
        return scnnode

    }

My vertex shader is:

attribute vec4 position;
attribute vec2 textureCoordinate;
uniform mat4 modelViewProjection;
varying highp vec2 pos;
varying vec2 texCoord;
void main() {
    texCoord = vec2(textureCoordinate.s, 1.0 - textureCoordinate.t) ;
    gl_Position = modelViewProjection * position;
    pos = vec2(position.x, 1.0 - position.y);
}

My Fragment Shader is:

precision highp float;
uniform sampler2D yourTexture;
varying highp vec2 texCoord;
varying highp vec2 pos;
void main() {
    gl_FragColor = texture2D(yourTexture, vec2(pos.x, pos.y));
}

I just cant seem to get the texture at the bottom left spread out across the surface. Could you please help?

EDIT: Ok so I messed around a bit more. I can mathematically force the vertex and fragment shaders to do what I want. But the code is very inelegant and specific. I am very sure this is not how to tackle the issue. The modified vertex and fragment shader code is below with an output photo at the link if interested.

Vertex Shader

attribute vec4 position;
attribute vec2 textureCoordinate;
uniform mat4 modelViewProjection;
varying highp vec2 pos;
varying vec2 texCoord;
void main() {
    /
    texCoord = vec2(textureCoordinate.s, 1.0 - textureCoordinate.t) ;

    /
    gl_Position = modelViewProjection * position;
    pos = vec2(position.x, position.y);
}

Frag shader where 0.4 is the slope of the top of polygon:

precision highp float;
uniform sampler2D yourTexture;
varying highp vec2 texCoord;
varying highp vec2 pos;
void main() {
  
    gl_FragColor = texture2D(yourTexture, vec2(pos.x/5.0, 1.0 - pos.y/(3.0+0.4*pos.x)));
/
/
}

(Hmm for some reason I cant attach a picture here?? A picture of the output can be found on same question posted on http://stackoverflow.com/questions/34104369/custom-shader-scnprogram-ios9-scenekit)

Many thanks