Consistent Freehand line in OpenGL

Question

I know how to draw freehand in opengl, but I'd like it to not have gaps if you move the mouse too fast. This is what I have:

void myMovedMouse(int mouseX, int mouseY)
{
    int x = mouseX;
    int y = IMAGE_Y - mouseY - 1;
    //int brushSize = 20;
    //glRecti(x, y, x + brushSize, y + brushSize);
    drawDot(x, y);
    glFlush();
}

//in main
glutDisplayFunc(myDisplay);
glutMotionFunc(myMovedMouse);

I also tried to use GL_LINE_LOOP, but of course that didn't work.

Answer 1

Append mouse positions to a std::vector & use GL_LINE_STRIP:

#include <GL/glut.h>
#include <vector>

std::vector< int > points;
void mouse( int button, int state, int x, int y )
{
    if( state == GLUT_DOWN )
        points.clear();
    points.push_back( x );
    points.push_back( y );
    glutPostRedisplay();
}

void motion( int x, int y )
{
    points.push_back( x );
    points.push_back( y );
    glutPostRedisplay();
}

void display()
{
    glClearColor( 0, 0, 0, 1 );
    glClear(GL_COLOR_BUFFER_BIT);

    glMatrixMode( GL_PROJECTION );
    glLoadIdentity();
    double w = glutGet( GLUT_WINDOW_WIDTH );
    double h = glutGet( GLUT_WINDOW_HEIGHT );
    glOrtho( 0, w, h, 0, -1, 1 );

    glMatrixMode( GL_MODELVIEW );
    glLoadIdentity();

    glBegin( GL_LINE_STRIP );
    glColor3ub( 255, 0, 0 );
    for( size_t i = 0; i < points.size(); i += 2 )
    {
        glVertex2i( points[i+0], points[i+1] );
    }
    glEnd();

    glutSwapBuffers();
}

int main( int argc, char** argv )
{
    glutInit( &argc, argv );
    glutInitDisplayMode( GLUT_RGBA | GLUT_DOUBLE );
    glutCreateWindow( "GLUT" );
    glutMouseFunc( mouse );
    glutMotionFunc( motion );
    glutDisplayFunc( display );
    glutMainLoop();
    return 0;
}