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copengllinked-listglutopengl-compat

Drawing a linestrip with opengl1 by looping through a linked list of vectors

i have previously asked a question about appending coordinates of clicks to a singly linked list, in order to loop through the linked list ,put vertices at the positions of the clicks and connect them a linestrip with 6 vertices

The answer given solved my issues with overflow of the linked list but afterwards i had problems with the display function, such that when i loop through the linked list and add those coordinates to glVertex2i();, it doesn't draw a linestrip when i click multiple times on the graphical window.

I tried to see what would happen when i removed the while loop, but that would cause segmentation fault.

These are the structs.

typedef struct vector{int x;int y;}Vector;
typedef struct VectorList{Vector X; struct VectorList*next; }node_v;

And i had declared these

Vector P;
node_v * prev;

And with the help of the answer to my previous question, initialized Vector P with the coordinates of the clicks and appended to the linked list of vectors node_v * prev;

static void _display_CB()
{
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    glOrtho(0,window_width,window_height,0,-1,1);

    glClear(GL_COLOR_BUFFER_BIT);

    glColor3ub(0,0,0);
    glBegin(GL_LINE_STRIP);
    while(prev){
        glVertex2i( prev->X.x, prev->X.y);
        prev=prev->next;
    }

    glEnd();
    glFlush();
  glutSwapBuffers();
glutPostRedisplay();
}

What changes should be done in _display_CB() for the program to be able to draw linestrips like in the picture?

Solution

  • prev is the head of the list. You actually change the head of the list to the tail of the list when you do elem = elem->next;. Keep the head of the list and use a local variable elem to iterate through the list.
    Furthermore the orthographic projection matrix glOrtho should be set to the current projection matrix (GL_PROJECTION):

    void _display_CB() {

    node_v * elem = prev;

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(0, window_width, window_height, 0, -1, 1);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    glClear(GL_COLOR_BUFFER_BIT);

    glColor3ub(0, 0, 0);
    glBegin(GL_LINE_STRIP);
    while(elem){
        glVertex2i(elem->X.x, elem->X.y);
        elem = elem->next;
    }
    glEnd();

    glutSwapBuffers();
    glutPostRedisplay();
}