ofxFX Addon Empty Vertex Shaders


I am trying to work out exactly how ofxFX works so I can recreate some of the ideas and use them in other projects.

I am finding it difficult to understand what this code in ofxFXObject.h does:

// Constructor Shader Source Strings: These strings can be specified
// by inheritors in their constructors, and will be loaded when
// appropriate. If any of the above strings are empty, the ones
// below are tried. If shaders are loaded from file, these are also
// ignored.

string          glESFragmentShader;
string          glESVertexShader;

string          gl2FragmentShader;
string          gl2VertexShader;

I can see that from this code in the selectShaderSource() function, that if we are not using the programmable renderer and a vertex shader is not specified then vertexShader is assigned the string gl2VertexShader:

if (ofIsGLProgrammableRenderer()){
        vertexShader = gl3VertexShader;
        // If the vertex shader for GL3 isn't specified, we fill
        // in a simple pass-through shader. This way, users can
        // give only fragment shaders, just like for GL2/ES. This
        // is necessary because having a vertex shader is mandatory
        // in GL3.
        if (vertexShader.empty()){
            vertexShader = "#version 150\n";
            vertexShader += STRINGIFY(
                    uniform mat4 modelViewProjectionMatrix;
                    in vec4 position;
                    void main(){
                        gl_Position = modelViewProjectionMatrix * position;
    } else {
        vertexShader = gl2VertexShader;

Then when compileCode() is called the vertex shader is loaded as follows:

shader.setupShaderFromSource(GL_VERTEX_SHADER, vertexShader);

As far as I can see gl2VertexShader is never assigned anything in ofxFXObject.cpp so this is the same as passing in an empty string.

However, if I pass in an empty string when attempting to recreate this in one of my own projects I get the following error:

ERROR: Compiled vertex shader was corrupt

I have tried to use a very simple vertex shader instead, i.e.

void main() {
    gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;

But then the output is not the same as what I am seeing with the ofxFX examples.