打开gl画线错误？

Question

#include <stdio.h>
#include <stdlib.h>
#include <windows.h>
#include <GL/glut.h>

#define ROUND(a) ((int)+.5)

void display(void) {    
    int xa = 10, ya = 3, xb = 56, yb = 98;
    int dx = xb - xa, dy = yb - ya, steps, k;
    float xincre, yincre, x = xa, y = ya;
    glColor3f(0.0f, 0.0f, 1.0f);
    if (abs(dx) > abs(dy))
        steps = abs(dx);
    else
        steps = abs(dy);
    xincre = dx / (float)steps;
    yincre = dy / (float)steps;

    glBegin(GL_POINTS);
    glVertex2s(ROUND(x), ROUND(y));
    for (k = 0; k < steps; k++) {
        x += xincre;
        y += yincre;
        glVertex2s(ROUND(x), ROUND(y));
        printf("%f,%f", x, y);
    }
    glEnd();
    glFlush();
}

int main(int argc, char** argv) {
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE);
    glutInitWindowSize(500, 500);
    glutInitWindowPosition(100, 100);
    glutCreateWindow("CpViewer");
    glClearColor(1.0, 1.0, 1.0, 0.0);
    glClear(GL_COLOR_BUFFER_BIT);
    glutDisplayFunc(display);
    glutMainLoop();
    return 0;
}

i tried to implement the dda algorithm for making a line but it seems just to print a point . i am not able to understand where is the problem ? i only get a blue point as the output in the white background

Answer 1

The drawing coordinates are defined in terms of a normalized rectangle (x and y each between -1 and 1), not in terms of window coordinates. So, the left-most x coordinate for drawing is 0 in window coordinates and -1.0 in drawing coordinates, and the right-most x coordinate is the window width (500 in this case) in window coordinates, and 1.0 in drawing coordinates. The y-coordinates are the same, but the y-axis is flipped from window coordinates to drawing coordinates (-1 in drawing coordinates would be 500 in window coordinates).

Try drawing the points with glVertex2f and rescaling the points by the window size:

glBegin(GL_POINTS);
for (k = 0; k <= steps; k++) {
    glVertex2f((x - 250.f)/250.f, (250.f - y)/250.f);
    printf("%f,%f", x, y);
    x += xincre;
    y += yincre;
}
glEnd();

You'll also need to keep track of if the window gets resized, to make sure that you're scaling these coordinates correctly, but for the moment, this should work.