OpenFrameworks Pie Chart Example for Jon

generatePieMesh.h

#pragma once
#include "ofMain.h"

ofMesh generatePieMesh( float value, float radius = 10.f, float segments = 32 ) {
    
    // Create our mesh.
    ofMesh mesh;
    mesh.setMode( OF_PRIMITIVE_TRIANGLE_FAN );
    
    // Add the center point
    mesh.addVertex( ofVec3f::zero() );
    // Say it is the first point
    mesh.addIndex( 0 );
    
    // Add the outer points
    for ( float i = 0; i < segments; ++i ) {
        
        // Use our matrix to plot the points.
        // Because I can't be bothered doing the trig :)
        ofMatrix4x4 matrix;
        // This is done backwards
        // 1. Move to the outside
        matrix.translate( 0.0f, radius, 0.0f );
        // 2. Rotate around the circle
        matrix.rotate( 360 * (i / segments), 0.0, 0.0, 1.0 );
        // 3. Invert the scale so it works clockwise
        matrix.scale( 1.0, -1.0, 1.0 );
        
        // Create a point at zero and multiple it by our matrix
        ofVec3f p = ofVec3f::zero() * matrix;
        // Add the point
        mesh.addVertex( p );
        // Set the point
        mesh.addIndex( i + 1 );
    }
    
    return mesh;
}

ofApp.cpp

#include "ofApp.h"
#include "generatePieMesh.h"

//--------------------------------------------------------------
void ofApp::setup(){
    // For Example 2
    // Pre-generate a pi
    pie = generatePieMesh( 0.4, 100.0, 64 );
}

//--------------------------------------------------------------
void ofApp::draw(){
    
    // Example 1
    // =========
    
    ofPushMatrix();
    ofPushStyle();
    ofTranslate( 100.0f, 100.0f );
    ofSetColor( 255, 0, 0 );
    
    // Generate and draw the pie every frame (inefficent)
    // Generate returns a mesh which we call draw on
    generatePieMesh( 0.8, 100 ).draw();
    
    ofPopStyle();
    ofPopMatrix();
    
    
    // Example 2
    // =========
    
    ofPushMatrix();
    ofPushStyle();
    ofTranslate( 300.0f, 100.f );
    ofSetColor( 0, 255, 0 );
    
    // Draw a pre-generated pie
    pie.draw();
    
    ofPopStyle();
    ofPopMatrix();
    
    
}

ofApp.h

#pragma once

#include "ofMain.h"

class ofApp : public ofBaseApp{
  public:
    void setup();
    void draw();

    // For Example 2
    ofMesh pie;
};