glReadPixels Reads wrong color in opengl - c++

Question

There is a simple program which is actually from a bigger one,putting this aside,I tried to do a simple pixel read/write in Opengl.
The first section which involves reading a pixel color succeeds,But the second operation which involves writing a pixel and then reading it fails!.
Here is the program which demonstrates the problem :
#include <iostream>
#include <glut.h>
using namespace std;
void init(void)
{
glClearColor(1.0,0.0,0.0,0.0);
glMatrixMode(GL_PROJECTION);
gluOrtho2D(0.0,100.0,0.0,100.0);
}
void displayfnc()
{
glClear(GL_COLOR_BUFFER_BIT);
unsigned char current[3];
int r,g,b;
//x=0 to 6 is right
glBegin (GL_POINTS);
glColor3f (0.0,0.0,1.0);
glVertex2i (6,0);
glEnd();
//works well.
glReadPixels(6,0, 1, 1 , GL_RGB , GL_UNSIGNED_BYTE ,current);
r = current[0];
g = current[1];
b = current[2];
cout<<"read from x=6::::"<<"r:"<<r<<"g:"<<g<<"b:"<<b<<endl;
//x =7 and other isnt right
glBegin (GL_POINTS);
glColor3f (0.0,0.0,1.0);
glVertex2i (7,0);
glEnd();
//the problem is here in the reading.why this happen?
glReadPixels(7,0, 1, 1 , GL_RGB , GL_UNSIGNED_BYTE ,current);
r = current[0];
g = current[1];
b = current[2];
cout<<"Read from x=7::::"<<"r:"<<r<<"g:"<<g<<"b:"<<b<<endl;
glFlush();
}
int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize(100,100);
glutInitWindowPosition(0, 0);
glutCreateWindow("BoundaryFill");
init();
glutDisplayFunc(displayfnc);
glutMainLoop();
return 0;
}
Here is the output:
What is causing the second read procedure to fail?

Answer 1

At least on my system glutInitWindowSize(100,100) is just a suggestion. glutGet(GLUT_WINDOW_WIDTH) actually returns 104, not 100 as your gluOrtho2D() call is assuming.
Try using a dynamic projection instead:
#include <GL/glut.h>
#include <iostream>
using namespace std;
void displayfnc()
{
glClearColor(1.0,0.0,0.0,0.0);
glClear(GL_COLOR_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
double w = glutGet( GLUT_WINDOW_WIDTH );
double h = glutGet( GLUT_WINDOW_HEIGHT );
gluOrtho2D(0.0,w,0.0,h);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
unsigned char current[3];
int r,g,b;
//x=0 to 6 is right
glBegin (GL_POINTS);
glColor3f (0.0,0.0,1.0);
glVertex2i (6,0);
glEnd();
//works well.
glReadPixels(6,0, 1, 1 , GL_RGB , GL_UNSIGNED_BYTE ,current);
r = current[0];
g = current[1];
b = current[2];
cout<<"read from x=6::::"<<"r:"<<r<<"g:"<<g<<"b:"<<b<<endl;
//x =7 and other isnt right
glBegin (GL_POINTS);
glColor3f (0.0,0.0,1.0);
glVertex2i (7,0);
glEnd();
//the problem is here in the reading.why this happen?
glReadPixels(7,0, 1, 1 , GL_RGB , GL_UNSIGNED_BYTE ,current);
r = current[0];
g = current[1];
b = current[2];
cout<<"Read from x=7::::"<<"r:"<<r<<"g:"<<g<<"b:"<<b<<endl;
glFlush();
}
int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize(100,100);
glutInitWindowPosition(0, 0);
glutCreateWindow("BoundaryFill");
glutDisplayFunc(displayfnc);
glutMainLoop();
return 0;
}