Question

I’m writing a MS paint-like application (I’m not allowed to use opengl primitives). So far it only draws lines and points. I don’t understand why when rendering a single point on click this code prints it where it should, based on the transformation on the renderPoint function:

void renderPoint(double x, double y)
    {
        W = glutGet(GLUT_WINDOW_WIDTH);
        H = glutGet(GLUT_WINDOW_HEIGHT);

        float X;
        float Y;
        glBegin (GL_POINTS);
            X = (2*x/W) - 1;
            Y = (-2*y/H) + 1;
            glVertex2f (X, Y);
        glEnd ();
        glFlush ();

    }

It doesn’t plot properly when called inside the bresenham function (which I believe it’s correct since I implemented it line by line from the article in wikipedia). Am I doing something wrong in the transformations?

#include "GL/glut.h"

#include <stdio.h>
#include <math.h>
int i;

int mainWindow, subWindow;
int X1, Y1, X2, Y2;
int W = 320, H = 320;
void menuApp (int value)
{
    if (value == 1) printf("Linea\n");
    if (value == 2) printf("Circulo\n");
    if (value == 3) printf("Elipsis\n");
    if (value == 4) exit(0);

}

void crearMenu()
{
    //inicio Creando el menu
    int submenu;
    submenu = glutCreateMenu(menuApp);
    glutAddMenuEntry("Linea", 1);

    glutAddMenuEntry("Elipse",3);
    glutAddMenuEntry("Salir",4);
    glutCreateMenu(menuApp);
    glutAddSubMenu("SubMenu", submenu);
    glutAttachMenu(GLUT_RIGHT_BUTTON);

    //fin Creando el menu
}

void renderPoint(void) /*REVISAR ESTO*/
{
    glClear (GL_COLOR_BUFFER_BIT);
    glBegin (GL_POINTS);
        glVertex2f  (-0.98, 0.98);
    glEnd ();
    glFlush ();
}





void renderPoint(double x, double y)
{
    W = glutGet(GLUT_WINDOW_WIDTH);
    H = glutGet(GLUT_WINDOW_HEIGHT);

    float X;
    float Y;
    glBegin (GL_POINTS);
        X = (2*x/W) - 1;
        Y = (-2*y/H) + 1;
        glVertex2f (X, Y);
    glEnd ();
    glFlush ();

}

/*wiki pseudo:

function line(x0, x1, y0, y1) //x1
     boolean steep := abs(y1 - y0) > abs(x1 - x0)//x2
     if steep then//x3
         swap(x0, y0) //x4
         swap(x1, y1) //x5 
     if x0 > x1 then //x6
         swap(x0, x1) //x7
         swap(y0, y1) //x8
     int deltax := x1 - x0 //x9
     int deltay := abs(y1 - y0) //x10
     int error := deltax / 2 //x11
     int ystep //x12
     int y := y0 //x13
     if y0 < y1 then ystep := 1 else ystep := -1 //x14
     for x from x0 to x1 //x15
         if steep then plot(y,x) else plot(x,y) //x16
         error := error - deltay //x17
         if error < 0 then //x18
             y := y + ystep //x19
             error := error + deltax //x20
*/


void bresenham1(GLint x0, GLint y0, GLint x1, GLint y1) //function line(x0, x1, y0, y1)
{


    //double result1 = fabs((double)y1 - y0); //abs(y1 - y0)
    //double result2 = fabs((double)x1 - x0); //abs(x1 - x0)


    int result1 = abs(y1-y0);
    int result2 = abs(x1-x0);

bool steep = (result1 > result2); //boolean steep := abs(y1 - y0) > abs(x1 - x0)

if (steep){ //if steep then

        GLint aux1 = x0; //swap(x0, y0)
        x0=y0;
        y0 = aux1;

        GLint aux2 = x1; // swap (x1,y1)
        x1=y1;
        y1=aux2;

}

if(x0>x1){ // if (x0>x1)
        GLint aux3=x0; //swap(x0,x1)
        x0=x1;
        x1=aux3;

        GLint aux4=y0;//swap(y0,y1)
        y0=y1;
        y1=aux4;

}

int deltax = x1-x0; // deltax = x1-x0
int deltay = abs(y1-y0); //  int deltay := abs(y1 - y0) - revisar 
int error = (deltax / 2); //int error := deltax / 2 
int ystep; // int ystep

int y = y0;  //int y := y0

    if (y0<y1){  //if y0 < y1 then ystep := 1 else ystep := -1 

        ystep=1;

    }

    else {ystep=-1;}

for (int x=x0; x<=x1; x++){ //for x from x0 to x1
  if (steep){  // if steep then plot(y,x) else plot(x,y)

       renderPoint(y,x);
   }
   else {

    renderPoint(x,y);
   }

error = error - deltay; //error := error - deltay

if (error<0) {  //if error < 0 then 
  y = y + ystep; // y := y + ystep
  error = error + deltax; //error := error + deltax

} // end if (error<0)


}// end for from x0 to x1


}// end bresenham










void renderAll (void)
{
    glutSetWindow(mainWindow);
    glutPostRedisplay();
    glutSetWindow(subWindow);
    glutPostRedisplay();
}

void movimiento(int boton, int estado, int x, int y)
{
    if((estado == GLUT_DOWN) && (boton == GLUT_LEFT_BUTTON))//mouse down
    {
        X1 = x; Y1 = y;
        //printf("Down| x:%d, y:%d\n",x,y);

        renderPoint(x,y);
    }
    if((estado == GLUT_UP) && (boton == GLUT_LEFT_BUTTON))//mouse up
    {
        //printf("  Up|x:%d, y:%d\n",x,y);
        X2 = x; Y2 = y;
        //renderLine();
        bresenham1(X1,X2,Y1,Y2);
        //renderPoint(x, y);
    }
}

void MouseMove(int x, int y)
{
    //printf("x:%d  | y:%d\n", x,y);
    X2 = x; Y2 = y;
    //renderLine();
    //bresenham1(X1, Y1, X2, Y2);
}

void teclado(unsigned char key, int x, int y)
{
    if(key == 27)exit(0);
}

void especiales(int key, int x, int y)
{
    if(key == GLUT_KEY_F1) exit(0);
}

static void
key(unsigned char k, int x, int y)
{
  switch (k) {
  case 27:  /* Escape */
    exit(0);
    break;
  default:
    return;
  }
  glutPostRedisplay();
}

int main (int argc, char *argv []) 
{
    i = 0;
    //inicializa las operaciones de OpenGL/GLUT, db cr antes de usar funciones GLUT
    glutInit (&argc, argv);

    glutInitDisplayMode (GLUT_SINGLE | GLUT_RGBA);
    glutInitWindowPosition (100, 100);
    glutInitWindowSize (W, H);
    //Crea una ventana  de Opengl
    glutCreateWindow ("tarea");
    glutDisplayFunc (renderPoint);
    glutMouseFunc(movimiento);
    glutKeyboardFunc(teclado);//teclas ASCII
    glutSpecialFunc(especiales);//captura las teclas [f1..f12]

    //glutPassiveMotionFunc(pasivo);
    glutKeyboardFunc(key);
    glutMotionFunc(MouseMove);
    crearMenu();
    glutMainLoop ();


}

Answer 1

The issue is on bresenham1 being called wrong:

bresenham1(X1,X2,Y1,Y2);

should be:

bresenham1(X1,Y1,X2,Y2);