国际象棋棋盘

Question

i was making a program to draw a chess board the code is

#include "glut.h"

void myDisplay(void);
void myInit(void);


int main(int argc, char ** argv)
{
    glutInit(&argc, argv);
    glutInitWindowSize(640, 480);
    glutInitWindowPosition(100, 150);
    glutCreateWindow("hello");
    glutDisplayFunc(myDisplay);
    myInit();
    glutMainLoop();

    return 0;
}

void myInit(void)
{
    glClearColor(1, 1, 1, 0);
    glColor3f(0, 0, 0);
    glPointSize(10);
    glutInitDisplayMode(GLUT_SINGLE | GL_RGB);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(640, 0, 480, 0);
}


void myDisplay(void)
{
    glClear(GL_COLOR_BUFFER_BIT);
    int counter = 0;
    for (GLint x = 0; x < 120; x += 15)
    {
        for (GLint y = 0; y < 120; y += 15)
        {
                counter++;
            if (counter & 1 ){ 
                glColor3f(.6, .6, .6);
            }
            else{
                glColor3f(.2, .2, .2);
            }
            glRecti(x, y, (x + 15), (y + 15));
        }//end of y     
    }//end of x 
    glFlush();
}

and the output was

now if i just add the equal (=) sign to the condition in my (loops) in the myDisplay function it works fine but it also add one more row and one more colu. so it's a new chess bored 9X9

note: i already debug the program and the funny thing it works fine on the debug mode since it did switch between the conditions ...

Answer 1

It "works" when you use <= of course, since then, you are drawing an odd number of rectangles per column. But if you draw the correct number of 8, you get an even number of invocations of your inner loop,hence counter will always be even when you start a new column.

That is due to the fact that counter & 1 is just not the right condition to make a checkerboard pattern. The simplest approach in your case would be to drop counter alltogether and just use something like ((x+y)/15) & 1 to check which type of field you are at.