Get pixel color from an ofPixels object.

main.cpp

#include "ofMain.h"
#include "ofApp.h"

int main()
{
    ofSetupOpenGL(512, 512, OF_WINDOW);
    ofRunApp(new ofApp());
}

ofApp-getCoord.cpp

#include "ofApp.h"

void ofApp::setup()
{
    // Load the shapes image.
    shapesImg.load("shapes.png");

    // Set the window size to match the image.
    ofSetWindowShape(shapesImg.getWidth(), shapesImg.getHeight());
}

void ofApp::draw()
{
    // Draw the image as the background.
    ofSetColor(255);
    shapesImg.draw(0, 0);

    // Get a reference to the image pixels.
    ofPixels shapesPix = shapesImg.getPixels();
    // Get the color value under the mouse.
    ofColor color = shapesPix.getColor(mouseX, mouseY);

    // Draw a rectangle under the mouse using the pixel color.
    ofFill();
    ofSetColor(color);
    ofDrawRectangle(mouseX - 25, mouseY - 25, 50, 50);
    // Add an outline so we can see the rectangle better.
    ofNoFill();
    ofSetColor(0);
    ofDrawRectangle(mouseX - 25, mouseY - 25, 50, 50);
}

ofApp-getData.cpp

#include "ofApp.h"

void ofApp::setup()
{
    // Load the shapes image.
    shapesImg.load("shapes.png");

    // Set the window size to match the image.
    ofSetWindowShape(shapesImg.getWidth(), shapesImg.getHeight());
}

void ofApp::draw()
{
    // Draw the image as the background.
    ofSetColor(255);
    shapesImg.draw(0, 0);

    // Get a reference to the image pixel data.
    unsigned char* shapesData = shapesImg.getPixels().getData();
    // Get the color value under the mouse.
    int numChannels = shapesImg.getPixels().getNumChannels();
    int pixelIndex = mouseY * shapesImg.getWidth() + mouseX;
    ofColor color = ofColor(
        shapesData[pixelIndex * numChannels + 0], // R
        shapesData[pixelIndex * numChannels + 1], // G
        shapesData[pixelIndex * numChannels + 2]  // B
    );

    // Draw a rectangle under the mouse using the pixel color.
    ofFill();
    ofSetColor(color);
    ofDrawRectangle(mouseX - 25, mouseY - 25, 50, 50);
    // Add an outline so we can see the rectangle better.
    ofNoFill();
    ofSetColor(0);
    ofDrawRectangle(mouseX - 25, mouseY - 25, 50, 50);
}

ofApp-getFrame.cpp

#include "ofApp.h"

void ofApp::setup()
{
    // Load the shapes image.
    shapesImg.load("shapes.png");

    // Set the window size to match the image.
    ofSetWindowShape(shapesImg.getWidth(), shapesImg.getHeight());
}

void ofApp::draw()
{
    // Draw the image as the background.
    ofSetColor(255);
    shapesImg.draw(0, 0);

    int shapeWidth = shapesImg.getWidth();
    int shapeHeight = shapesImg.getHeight();

    int frameIndex = ofGetFrameNum() % (shapeWidth * shapeHeight);
    int x = frameIndex % shapeWidth;
    int y = frameIndex / shapeWidth;

    // Get a reference to the image pixels.
    ofPixels shapesPix = shapesImg.getPixels();
    // Get the color value for the frame.
    int pixelIndex = frameIndex * shapesPix.getNumChannels();
    ofColor color = shapesPix.getColor(pixelIndex);

    // Draw a rectangle at the sampled pixel position.
    ofFill();
    ofSetColor(color);
    ofDrawRectangle(x - 25, y - 25, 50, 50);
    // Add an outline so we can see the rectangle better.
    ofNoFill();
    ofSetColor(0);
    ofDrawRectangle(x - 25, y - 25, 50, 50);
}

ofApp-getIndex.cpp

#include "ofApp.h"

void ofApp::setup()
{
    // Load the shapes image.
    shapesImg.load("shapes.png");

    // Set the window size to match the image.
    ofSetWindowShape(shapesImg.getWidth(), shapesImg.getHeight());
}

void ofApp::draw()
{
    // Draw the image as the background.
    ofSetColor(255);
    shapesImg.draw(0, 0);

    // Get a reference to the image pixels.
    ofPixels shapesPix = shapesImg.getPixels();
    // Get the color value under the mouse.
    int index = (mouseY * shapesImg.getWidth() + mouseX) * shapesPix.getNumChannels();
    ofColor color = shapesPix.getColor(index);

    // Draw a rectangle under the mouse using the pixel color.
    ofFill();
    ofSetColor(color);
    ofDrawRectangle(mouseX - 25, mouseY - 25, 50, 50); 
    // Add an outline so we can see the rectangle better.
    ofNoFill();
    ofSetColor(0);
    ofDrawRectangle(mouseX - 25, mouseY - 25, 50, 50);
}

ofApp.h

#pragma once

#include "ofMain.h"

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

    ofImage shapesImg;
};