How does glReadPixels works?

Question

I have just started using OpenGL i was writing a program for boundary fill algorithm ,i used glClearColor to change the background colour to red but when i am reading a pixel color using glReadPixels it gives RGB value 0,0,0 instead of 1,0,0

#include<stdio.h>
#include<math.h>
#include<GL/glut.h>
#include<GL/freeglut.h>

int WIDTH=500,HEIGHT=500;

void boundaryfill(int x, int y)
{
    int pixel[4];
    glReadPixels(x,y,1,1, GL_RGB, GL_FLOAT, &pixel);
    printf("%f %f %f %f\n",pixel[0],pixel[1],pixel[2],pixel[4]);
    if(pixel[0]==(float)1 && pixel[1]==(float)0 && pixel[2]==(float)0)
    {
        printf("njerngwneroingoierngoineriongioerngiernogineiorngoiern");
    }
    else 
    {
        glBegin(GL_POINTS);
        glColor3f(0,1,0);
        glVertex2i(x,y);
        glEnd();
        glFlush();
        //boundaryfill(x,y+1);
        //boundaryfill(x+1,y);
        //boundaryfill(x,y-1);
        //boundaryfill(x-1,y);
    }
}

void display()
{
    glClearColor(1.0,0.0,0.0,1.0);
    glClear(GL_COLOR_BUFFER_BIT);
    glColor3f(1.0,0.0,0.0);
    glBegin(GL_LINE_LOOP);
    glVertex2i(250,250);
    glVertex2i(-250,250);
    glVertex2i(-250,-250);
    glVertex2i(250,-250);
    glEnd();
    boundaryfill(250,250);
    glFlush();
}

void myinit_func()
{
    glLoadIdentity();
    glMatrixMode(GL_PROJECTION);
    gluOrtho2D(-250,250,-250,250);  
}

int main(int argc, char **argv)
{
    glutInit(&argc,argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutInitWindowPosition(0,0);
    glutInitWindowSize(WIDTH,HEIGHT);
    glutCreateWindow("Boundary fill");
    myinit_func();
    glutDisplayFunc(display);

    glutMainLoop();
    return 0;
}

Answer 1

First of all you call glReadPixels() with GL_FLOAT however pixel is an int[]. Additionally you call it with GL_RGB and since you want the alpha you need to pass GL_RGBA.

float pixel[4];
glReadPixels(x, y, 1, 1, GL_RGBA, GL_FLOAT, &pixel);

Next in printf() you get the alpha like this pixel[4], I suspect this is a typo, as you need to do pixel[3]. You also use %d instead of %f.

printf("%f %f %f %f\n", pixel[0], pixel[1], pixel[2], pixel[3]);

Last but not least, by default GLUT doesn't setup an alpha buffer, which you can check this by doing:

int bits = 0;
glGetIntegerv(GL_ALPHA_BITS, &bits);

To which bits should remain 0. So instead you need to call glutInitDisplayMode like this:

glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB | GLUT_ALPHA);

Note that in glutInitDisplayMode you don't need to pass GLUT_RGBA as it's the same as GLUT_RGB. Which additionally is default, so it can be omitted.

Additionally I would discourage doing pixel[0] == (float)1 due to floating point precision. I would instead recommend using:

GLubyte pixel[4];
glReadPixels(x, y, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, &pixel);

printf("%u %u %u %u\n", pixel[0], pixel[1], pixel[2], pixel[3]);

Then doing pixel[0] == 255 instead of course.