trying to figure out cinder screenToWorld

TrianglePaintApp.cpp

#include "cinder/app/AppNative.h"
#include "cinder/gl/gl.h"
#include "cinder/Camera.h"
#include "cinder/params/Params.h"

using namespace ci;
using namespace ci::app;
using namespace std;


#define SQRT_3_2 0.8660254037844386

class TrianglePaintApp : public AppNative {
  public:
	void setup();
	void resize();
	void mouseDown( MouseEvent event );
	void update();
	void draw();
	void drawGrid( float size, float unit = 50 );
	Vec2f convert( int r, int g, int b, float unit = 50 );
	Vec2f worldFromScreen( Vec2i xy );

	cinder::CameraPersp mCam;
	int mR = 1;
	int mG = 0;
	int mB = -1;

	Vec2f touched = Vec2f::zero();

	params::InterfaceGl	mParams;
};

void TrianglePaintApp::setup()
{
	mParams = params::InterfaceGl( "Parameters", Vec2i( 220, 170 ) );
	mParams.addParam( "R", &mR, "keyIncr=R keyDecr=r" );
	mParams.addParam( "G", &mG, "keyIncr=G keyDecr=g" );
	mParams.addParam( "B", &mB, "keyIncr=B keyDecr=b" );

	mParams.addParam( "X", &touched.x, true );
	mParams.addParam( "Y", &touched.y, true );
}

// Replicate most of the defaultResize so we get a reference to the camera
// for converting screen to world.
void TrianglePaintApp::resize()
{
	Vec2i size = getWindowSize();
	mCam = cinder::CameraPersp( size.x, size.y, 60.0f );

	mCam.lookAt( Vec3f( 0.0f, 0.0f, 400.0f ), Vec3f::zero(), Vec3f::yAxis() );
	gl::setMatrices( mCam );

	// Invert Y axis so increasing Y goes down.
	glScalef( 1.0f, -1.0f, 1.0f );
}

void TrianglePaintApp::mouseDown( MouseEvent event )
{
	touched = worldFromScreen(Vec2i(event.getX(), event.getY()));
	console() << touched << endl;
}

void TrianglePaintApp::update()
{
	mB = - (mR + mG);
}

void TrianglePaintApp::draw()
{
	int step = 50;

	gl::pushModelView();

	gl::clear( Color::white() );

	drawGrid( 500, step);

	// Origin
	gl::color( Colorf(1.0f, 0.0f, 0.0f) );
	gl::drawSolidCircle(Vec2f::zero(), 10);

	// Mover
	gl::color( Colorf(0.0f, 0.0f, 0.0f) );
	gl::drawSolidCircle(convert(mR, mG, mB, step), 10);


	gl::popModelView();

	mParams.draw();
}

void TrianglePaintApp::drawGrid( float size, float unit )
{
	gl::pushModelView();

	float step = unit * SQRT_3_2;

	gl::rotate(30);

	gl::color( ColorAf(0.7, 0, 0, 0.3f) );
	for ( float i = -size * step; i <= size * step; i += step ) {
		gl::drawLine( Vec2f(-size, i), Vec2f(size, i) );
	}
	gl::drawSolidTriangle(Vec2f(unit, 0), Vec2f(unit * 0.8, unit * -0.2), Vec2f(unit * 0.8, unit * 0.2));
	gl::rotate(120);

	gl::color( ColorAf(0, 0.7, 0, 0.3f) );
	for ( float i = -size * step; i <= size * step; i += step ) {
		gl::drawLine( Vec2f(-size, i), Vec2f(size, i) );
	}
	gl::drawSolidTriangle(Vec2f(unit, 0), Vec2f(unit * 0.8, unit * -0.2), Vec2f(unit * 0.8, unit * 0.2));
	gl::rotate(120);

	gl::color( ColorAf(0, 0, 0.7, 0.3f) );
	for ( float i = -size * step; i <= size * step; i += step ) {
		gl::drawLine( Vec2f(-size, i), Vec2f(size, i) );
	}
	gl::drawSolidTriangle(Vec2f(unit, 0), Vec2f(unit * 0.8, unit * -0.2), Vec2f(unit * 0.8, unit * 0.2));
	gl::rotate(120);

	gl::popModelView();
}

// RGB to XY
Vec2f TrianglePaintApp::convert( int r, int g, int b, float unit )
{
	// Should throw or something?
	if ( r + g + b != 0 ) return Vec2f::zero();

	// FLAT TOP
	// The slope of of the Green axis to X and inverted Y axis:
	// 120 degree is -1/2, sqrt(3)/2,
	// Vec2f vr = Vec2f(r, 0);
	// Vec2f vg = Vec2f(-0.5f * g, SQRT_3_2 * g);

	// POINTY TOP
	// Red is 330 degrees
	Vec2f vr = Vec2f(SQRT_3_2 * r, 0.5f * r);
	// Green is 210 degrees
	Vec2f vg = Vec2f(-SQRT_3_2 * g, 0.5f * g);
	// Blue is 90 degrees (up)
	//Vec2f vb = Vec2f(0, -b * 1.0f);

	return unit * (vr + vg);
}

Vec2f TrianglePaintApp::worldFromScreen( Vec2i pos )
{
	// generate a ray from the camera into our world
	float u = pos.x / (float) getWindowWidth();
	float v = pos.y / (float) getWindowHeight();
	// because OpenGL and Cinder use a coordinate system
	// where (0, 0) is in the LOWERleft corner, we have to flip the v-coordinate
	Ray ray = mCam.generateRay(u , 1.0f - v, mCam.getAspectRatio() );

	float result = 0.0f;
	Vec3f planePos = mCam.getCenterOfInterestPoint();
	Vec3f normal = mCam.getViewDirection();
	if ( ray.calcPlaneIntersection( planePos, normal, &result ) ) {
		return ray.calcPosition( result ).xy();
	}
	return Vec2f::zero();
}

CINDER_APP_NATIVE( TrianglePaintApp, RendererGl )