Reflections of my thoughts….

I’d like to share a simple tip on enabling anti aliased line drawing in OpenGL. I hope you’ve basic understanding of OpenGL and how to use it.

To smooth a line, you will have to enable 3 OpenGL capabilities.

1. Enable line smooth option (GL_LINE_SMOOTH)
2. Enable Blending( GL_BLEND)
3. Specify pixel arithmetic(GL_SRC_ALPHA)

Draw the lines after enabling the above properties. See the sample below.

// Function to enable smoothing
void EnableSmoothing( bool bEnable )
{
	if( bEnable )
	{
		// Smooth out lines
		glEnable( GL_LINE_SMOOTH );
		glHint(GL_LINE_SMOOTH_HINT, GL_NICEST);
		// Enable Blending
		glEnable(GL_BLEND);
		// Specifies pixel arithmetic
		glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
		printf( "\nEnabled" );
	}
	else // if the options are disabled
	{
		glDisable(GL_LINE_SMOOTH); // Smooth out lines
		glDisable(GL_BLEND);
		printf( "\nDisabled" );
	}
}

// Render function
void RenderScene()
{
	glClear( GL_COLOR_BUFFER_BIT );

	glRotatef( z, 0, 0, 1 );

	/* Draw line for testing */
	glBegin( GL_LINES );
	{
		glColor3f( 0.0f, 0.0f, 1.0f );
		glVertex2f( -0.75f, 0.25f );
		glColor3f( 0.0f, 1.0f, 0.0f );
		glVertex2f( 0.75f,-0.25f );
	}
	glEnd();

	glutSwapBuffers();
}

See the images before and after conversion. Click on the image to see it in original size.

I’m also attaching a working sample source written using GLUT.

// LineSmoothSample.cpp : How to smooth line using OpenGL

#include "stdafx.h"
#include <windows.h>
#include <gl/glut.h>

// z rotation angle
GLfloat z = 0.0f;

// Function to enable smoothing
void EnableSmoothing( bool bEnable )
{
	if( bEnable )
	{
		// Smooth out lines
		glEnable( GL_LINE_SMOOTH );
		glHint(GL_LINE_SMOOTH_HINT, GL_NICEST);
		// Enable Blending
		glEnable(GL_BLEND);
		// Specifies pixel arithmetic
		glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
		printf( "\nEnabled" );
	}
	else // if the options are disabled
	{
		glDisable(GL_LINE_SMOOTH); // Smooth out lines
		glDisable(GL_BLEND);
		printf( "\nDisabled" );
	}
}

// Render function
void RenderScene()
{
	glClear( GL_COLOR_BUFFER_BIT );

	glRotatef( z, 0, 0, 1 );

	/* Draw line for testing */
	glBegin( GL_LINES );
	{
		glColor3f( 0.0f, 0.0f, 1.0f );
		glVertex2f( -0.75f, 0.25f );
		glColor3f( 0.0f, 1.0f, 0.0f );
		glVertex2f( 0.75f,-0.25f );
	}
	glEnd();

	glutSwapBuffers();
}

// Initial setup for rendering context
void SetupRC()
{
	glOrtho( -1,1,-1,1,-1,1);
	glClearColor( 0,0,0,1);
	glLineWidth( 5 );
	// Enable smoothing by default
	EnableSmoothing( true );
}

// Function to handle change in window size
void ChangeSize( GLsizei w, GLsizei h )
{
	glViewport(0,0,w,h);
}

// Keyboard callback
void SpecialKey(unsigned char key, int x, int y)
{
	if( ' ' == key )
	{
		static bool bEnable = true;
		bEnable = !bEnable;
		EnableSmoothing( bEnable );
	}
	else if( 'w' == key )
	{
		z += 0.01f;
		if( z >= 360.0f )
			z = 0;
	}
	else
		return;

	glutPostRedisplay();

}

int main(int argc, char* argv[])
{
	glutInit( &argc, argv );
	glutInitDisplayMode( GLUT_RGBA | GLUT_DOUBLE );
	glutInitWindowSize( 800,600 );
	glutCreateWindow( "Line Smooth Sample");
	glutReshapeFunc( ChangeSize );
	glutDisplayFunc( RenderScene);
	glutKeyboardFunc( SpecialKey );
	SetupRC();
	glutMainLoop();
	return 0;
}