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I wrote this code to animate the Lorenz attractor:

#include <iostream>
#include <GL/freeglut.h>
double x = 1, y = 1, z = 1, a = 10, b = 28, c = 8/3, precision = 0.000001, lastx = x, lasty = y, lastz = z;
int counter = 0;
void draw() {
    glBegin(GL_LINES);
        glVertex2d(lastx, lasty);
        glVertex2d(x, y);
    glEnd();
    lastx = x;
    lasty = y;
    double dxdt = a*(y-x);
    double dydt = x*(b-z)-y;
    double dzdt = x*y-c*z;
    x += precision*dxdt;
    y += precision*dydt;
    z += precision*dzdt;
    counter++;
    if (counter == 40000)
    {
        glutSwapBuffers();
        counter = 0;
    }
    glutPostRedisplay();
}
void mykey(unsigned char mychar, int x, int y) {
    if (mychar == 27) {
        exit(0);
    }
}
int main(int argc, char **argv) {
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_RGBA | GLUT_DOUBLE | GLUT_DEPTH);
    glutInitWindowSize(1000, 1000);
    glutCreateWindow("Lorenz Attractor");
    glutDisplayFunc(draw);
    glutKeyboardFunc(mykey);
    glClearColor(0, 0, 0, 1);
    glColor4d(0, 0, 1, 1);
    glViewport(0, 0, 1000, 1000);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(-30, 30, -30, 30);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    glEnable(GL_BLEND);
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    glEnable(GL_POINT_SMOOTH);
    glPointSize(1);
    glutFullScreen();
    glutMainLoop();
}

The problem is, I have no idea how to draw a continuous graph other than stitching thousands of lines with 2 points. It starts to flicker after a while and I think this is the reason. Any ideas on how to fix? Thanks!

P.S. Also, if you're feeling extra generous, drop me some hints on how to implement zooming and a white dot at the moving end of the graph. If you see anything that could be improved, please let me know. Thanks again!

EDIT:

Using a single buffer with glFlush() fixed the problem, double buffers would only work if there was a way to copy front buffer to back to buffer before swapping. If there is such a way, let me know.

DDomjosa
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    You need to call `glutSwapBuffers()` unconditionally at the end of the `draw()` function. Otherwise, 39999 out of 40000 frames you render are never shown. – Reto Koradi Dec 28 '15 at 01:38
  • If I swap buffers every time, it draws very very slowly since my precision is 0.000001. If I use a higher number, the graph becomes inaccurate. – DDomjosa Dec 28 '15 at 10:43

1 Answers1

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You are using a double-buffered opengl window. When you want to do animations in this case, you have to

  • draw the complete frame which is to be displayed to the back buffer
  • swap buffers to display the frame
  • draw the next frame from start

The key point here is that after a SwapBuffers call, the contents of the back buffer become undefined. Your implementation happens to switch between some previous framebuffers. This means that you draw 40000 lines, swap the buffers so that they are shown, draw the next 40000 lines to another buffer not containing the 40000 previous lines, and so on, so you get the flickering.

If you want your drawing to "extend" every frame, you have several options:

  1. actually store your lines in some data structure and redraw the whole image every frame
  2. copy the result of your last frame over to the new back buffer after the swap. (A more efficient way would be rendering to texture).
  3. Use single buffering (replace the GLUT_DOUBLE flag by GLUT_SINGLE and use glFlush instead of glutSwapBuffers)

The third option might seem like the best (and easiest) solution to you, but conceptually, it is the worst. You should rethink the way you implement animations in OpenGL.

derhass
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  • Thanks for answering. The second option sounds good, is there a function that would copy the frame over to the other buffer? I wasn't able to find anything in Google. – DDomjosa Dec 28 '15 at 16:03
  • I'm not sure, but `glBlitFramebuffer` _might_ be even capable of copying the contents of the front buffer into the back buffer, if you set the `glReadBuffer` and `glDrawBuffer` accordingly. – derhass Dec 28 '15 at 16:37
  • It says glBlitFramebuffer isn't declared... I'll just stick with single buffer for now, this is only for a school project. – DDomjosa Dec 29 '15 at 13:22
  • `glBlitFrameBuffer` is a GL3 function. Like anything beyoind GL1.1, you'll have to use the extension mechanism to grab the function pointers. The easiest way is using an [extension loader library](https://www.opengl.org/wiki/OpenGL_Loading_Library). – derhass Dec 29 '15 at 16:30