Trying to replicate https://github.com/openframeworks/openFrameworks/issues/1533

minimal-main.cpp

// trying to replicate https://github.com/openframeworks/openFrameworks/issues/1533
// this test has ofFbo and ofPixels as members of testApp, and does everything in setup()

#include "ofMain.h"
#include "ofAppGlutWindow.h"

struct testApp : public ofBaseApp {
	void setup();
	ofFbo fbo;
	ofPixels_<unsigned char> pixels;
};

void testApp::setup(){
	std::cout << "start" << std::endl << "------" << std::endl;
	const unsigned int w = ofGetWidth();
	const unsigned int h = ofGetHeight();
	fbo.allocate(w, h, GL_RGB);
	fbo.readToPixels(pixels);
	
	for(int y = 0; y < h; y++) {
		for(int x = 0; x < w; x++) {
			ofColor someColor = pixels.getColor(x, y);
		}
	}
	
	pixels.clear();
	std::cout << "------" << std::endl << "end" << std::endl;
	ofExit();
}

int main(){
	ofAppGlutWindow window;
	ofSetupOpenGL(&window, 1024,768, OF_WINDOW);
	ofRunApp( new testApp());
}

output.txt

The following is the output for each of these on my system (OSX 10.8, Xcode 4.5), tested with Debug and Release.

GNU gdb 6.3.50-20050815 (Apple version gdb-1822) (Sun Aug  5 03:00:42 UTC 2012)
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "x86_64-apple-darwin".tty /dev/ttys001
[Switching to process 13475 thread 0x0]
start
------
------
end
Program ended with exit code: 0

pixel-stack-main.cpp

// trying to replicate https://github.com/openframeworks/openFrameworks/issues/1533
// This creates ofPixels on the stack, and has ofFbo as a member of testApp
// It does color reading / writing and pixels.clear() in update(), and draws the fbo in draw()

#include "ofMain.h"
#include "ofAppGlutWindow.h"

struct testApp : public ofBaseApp {
	void setup();
	void update();
	void draw();
	ofFbo fbo;
};

void testApp::setup(){
	std::cout << "start" << std::endl << "------" << std::endl;
	fbo.allocate(ofGetWidth(), ofGetHeight(), GL_RGB);
}

void testApp::update(){
	const unsigned int w = floorf(fbo.getWidth());
	const unsigned int h = floorf(fbo.getHeight());
	
	fbo.begin();
	ofSetColor(rand()%255, rand()%255, rand()%255);
	ofFill();
	ofRect(0,0,w,h);
	fbo.end();
	
	ofPixels_<unsigned char> pixels;
	fbo.readToPixels(pixels);
	
	for(int y = 0; y < h; y++) {
		for(int x = 0; x < w; x++) {
			ofColor someColor = pixels.getColor(x, y);
		}
	}
	pixels.clear();
	
	if(ofGetFrameNum() > 120) {
		std::cout << "------" << std::endl << "end" << std::endl;
		ofExit();
	}
}

void testApp::draw(){
	fbo.draw(20, 20, 640, 480);
}

int main(){
	ofAppGlutWindow window;
	ofSetupOpenGL(&window, 1024,768, OF_WINDOW);
	ofRunApp(new testApp());
}

stack-main.cpp

// trying to replicate https://github.com/openframeworks/openFrameworks/issues/1533
// This creates ofPixels and ofFbo on the stack
// It does color reading / writing and pixels.clear() in update()

#include "ofMain.h"
#include "ofAppGlutWindow.h"

struct testApp : public ofBaseApp {
	void setup();
	void update();
};

void testApp::setup(){
	std::cout << "start" << std::endl << "------" << std::endl;
}

void testApp::update(){
	ofFbo fbo;
	fbo.allocate(ofGetWidth(), ofGetHeight(), GL_RGB);
	
	const unsigned int w = floorf(fbo.getWidth());
	const unsigned int h = floorf(fbo.getHeight());
	
	fbo.begin();
	ofSetColor(rand()%255, rand()%255, rand()%255);
	ofFill();
	ofRect(0,0,w,h);
	fbo.end();
	
	ofPixels_<unsigned char> pixels;
	fbo.readToPixels(pixels);
	
	for(int y = 0; y < h; y++) {
		for(int x = 0; x < w; x++) {
			ofColor someColor = pixels.getColor(x, y);
		}
	}
	pixels.clear();
	
	if(ofGetFrameNum() > 120) {
		std::cout << "------" << std::endl << "end" << std::endl;
		ofExit();
	}
}

int main(){
	ofAppGlutWindow window;
	ofSetupOpenGL(&window, 1024,768, OF_WINDOW);
	ofRunApp(new testApp());
}

update-main.cpp

// trying to replicate https://github.com/openframeworks/openFrameworks/issues/1533
// this test has ofFbo and ofPixels as members of testApp, does color reading / writing
// and pixels.clear() in update(), and draws the fbo in draw()

#include "ofMain.h"
#include "ofAppGlutWindow.h"

struct testApp : public ofBaseApp {
	void setup();
 	void update();
 	void draw();
 	ofFbo fbo;
	ofPixels_<unsigned char> pixels;
};

void testApp::setup(){
	std::cout << "start" << std::endl << "------" << std::endl;
	fbo.allocate(ofGetWidth(), ofGetHeight(), GL_RGB);
}

void testApp::update(){
	const unsigned int w = floorf(fbo.getWidth());
	const unsigned int h = floorf(fbo.getHeight());
	
	fbo.begin();
	ofSetColor(rand()%255, rand()%255, rand()%255);
	ofFill();
	ofRect(0,0,w,h);
	fbo.end();
	
	fbo.readToPixels(pixels);
	
	for(int y = 0; y < h; y++) {
		for(int x = 0; x < w; x++) {
			ofColor someColor = pixels.getColor(x, y);
		}
	}
	
	if(ofGetFrameNum() > 120) {
		pixels.clear();
		std::cout << "------" << std::endl << "end" << std::endl;
		ofExit();
	}
}

void testApp::draw(){
	fbo.draw(20, 20, 640, 480);
}

int main(){
	ofAppGlutWindow window;
	ofSetupOpenGL(&window, 1024,768, OF_WINDOW);
	ofRunApp(new testApp());
}