Question

I am trying to draw a rectangle with 4 lines (left one). But no matter how I draw (strip, loop, plain lines) the result is the right side one – it is missing pixels at the corners.

|||||||||||     ||||||||||
|         |    |          |
|         |    |          |
|||||||||||     ||||||||||

Is there a way to get the left hand side result?

EDIT

Can’t post code since it depends a lot on the underlying framework, but all draw calls are nothing more than glDrawElements with index buffer pointing to 4 vertices, no alpha tests.

After fiddling with coordinates as suggested arrived at this interesting result (iOS simulator):

Notice only upper left corner is missing pixels for some rectangles…
All rectangles are drawn in GL_LINES mode within single draw elements call. No other things are drawn.

EDIT 2

OpenGL FAQ question 14.100 states that

OpenGL doesn’t provide a mechanism to cleanly join lines that share common vertices nor to cleanly cap the endpoints.

Answer 1

Shift your coordinate frame a bit so your lines fall on pixel centers:

#include <GL/glut.h>

void display()
{
    glClear( GL_COLOR_BUFFER_BIT );

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

    glMatrixMode( GL_MODELVIEW );
    glLoadIdentity();

    // important
    glTranslatef( 0.5, 0.5, 0 );

    float offset = 40;
    glColor3ub( 255, 0, 0 );
    glBegin(GL_LINE_LOOP);
    glVertex2f( 0+offset, 0+offset );
    glVertex2f( 0+offset, h-offset );
    glVertex2f( w-offset, h-offset );
    glVertex2f( w-offset, 0+offset );
    glEnd();

    glutSwapBuffers();
}

int main( int argc, char **argv )
{
    glutInit( &argc, argv );
    glutInitDisplayMode( GLUT_RGBA | GLUT_DOUBLE );
    glutInitWindowSize( 320,240 );
    glutCreateWindow( "Rect" );
    glutDisplayFunc( display );
    glutMainLoop();
    return 0;
}