opengGL drawing a line - c++

Question

So far this is my code:
#include <iostream>
#include <cstdlib>
#include <GL/glut.h>
#include <cmath>
void keyboard(unsigned char key, int x, int y);
void display(void);
void timer(int);
static float x=0.0f,y=0.0f;
int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitWindowPosition(200,200);
glutInitWindowSize(640,480);
glutCreateWindow("draw a line");
glutKeyboardFunc(&keyboard);
glutDisplayFunc(&display);
glutTimerFunc(10,timer,0);
glutMainLoop();
return EXIT_SUCCESS;
}
void keyboard(unsigned char key, int x, int y)
{
switch (key)
{
case '\x1B':
exit(EXIT_SUCCESS);
break;
}
}
void timer(int value){
x+=0.001;
y+=0.0005;
glutPostRedisplay();
glutTimerFunc(10,timer,0);
}
void display()
{
glClear(GL_COLOR_BUFFER_BIT);
glLoadIdentity ();
glColor3f(0.0f, 1.0f, 0.0f);
glBegin(GL_POINTS);
glVertex2f(x,y);
glEnd();
glFlush();
}
What this does is that it lights up a pixel every 10 msecs from the point (0,0) to (1,0.5). What I want is that when a pixel lights up it stays in that state, so eventually you will see a line. How can I achieve this?

Answer 1

I am not familiar with glut but I am guessing display is the function that is called on each redraw. This function starts with glClear(GL_COLOR_BUFFER_BIT). This function clears your color buffer on each redraw.
You might find that removing glClear does not entirely fix your problem. This could well be because your graphics context may be double buffered and to make things more efficient, the front buffer is not copied to the back buffer on each animation run.
You best bet to get the desired effect will probably be to draw a line that grows on each animation run.

Answer 2

why do things the hard way? although the older OGL API is deprecated, you can use GL_LINES to do this:
glBegin(GL_LINES);
glVertex2f(x_start,y_start);
glVertex2f(x_end,y_end);
glEnd();
This will draw the line fully in one go, which is easier and a bit more efficient (it also allows you to benefit from the line anti-aliasing hint).

Answer 3

The display function is being called every time you call glutPostRedisplay(). And every time the display function is called, you clear the screen.
You need to write a function that will iterate through all of the points you want to display. Each iteration will call something like a drawPoint() function.
Perhaps something like this:
void display()
{
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(0.0f, 1.0f, 0.0f);
for (int i = 0; i < num_points; i++)
{
drawPoint(points(i));
}
glFlush();
}
Alternatively, you could skip writing a new function and do this:
glBegin(GL_POINTS);
for (int i = 0; i < num_points; i++)
{
glVertex2f(points(i).getX(), points(i).getY());
}
glEnd();

Answer 4

If your using, GL_LINES. A way to do it is to have an array of points you want to draw EG: p[0], p[1], p[2], p[3], p[4], p[5] ... then draw it using some for loop
glBegin(GL_LINES);
for(...) {
glVertex2f(p[i ]->x,p[i ]->y);
glVertex2f(p[i+1]->x,p[i+1]->y);
}
glEnd();
So using this method you can replace p[] with a function allowing you to make pretty shapes
glVertex2f(functionx(i ),functiony(i ));
glVertex2f(functionx(i+1),functiony(i+1));
note: lines work weirdly they dont link up you need to have a start point to end point kind of thing hence the i+1
I guess your trying to draw stuff like cardioid's using polar coordinates?