Blue line, frame sequential stereo

Written by Paul Bourke
August 2003


Introduction

So called "blue line" stereo is also often called the poor mans stereo is a technique for supporting frame sequential stereo on cards without specific stereo support. Cards designed for stereo normally have an additional plug on the card that provides signals for synchronising shutter glasses, emitters, or other hardware. Cards that don't have this synchronisation signal can be still be used if the driver manufacturer supports blue line stereo (or one of the many variants). The most common implementation is for the content creator to draw a blue line on the last row of the display, the driver for the card (it can be done at other places on some systems) generates the syncing voltages on one of the unused pins on the DVI or VGA cable depending on whether the blue line exists or not. A special cable is used to extract the sync signal from the VGA cable.

The most common sync signal is a simple square wave at around 3 volts peak to peak and running at the same frequency as the frame sequential stereo, usually 120Hz for a flicker free effect. NVidia were perhaps the first card supplier to incorporate this into their drivers, they use pin 12 on the VGA connector for the sync signal.

 

Source

/* 
   Draw a blue line on the last pixel row for poor mans stereo syncing
*/ 
void DrawBlueLine(int width,int height)
{     
   GLint i;
   unsigned long buffer;
      
   glPushAttrib(GL_ALL_ATTRIB_BITS);

   glDisable(GL_ALPHA_TEST);
   glDisable(GL_BLEND);
   for (i=0;i<6;i++) 
      glDisable(GL_CLIP_PLANE0 + i);
   glDisable(GL_COLOR_LOGIC_OP);
   glDisable(GL_COLOR_MATERIAL);
   glDisable(GL_DEPTH_TEST);
   glDisable(GL_DITHER);
   glDisable(GL_FOG);
   glDisable(GL_LIGHTING);
   glDisable(GL_LINE_SMOOTH);
   glDisable(GL_LINE_STIPPLE);
   glDisable(GL_SCISSOR_TEST);
   glDisable(GL_STENCIL_TEST);
   glDisable(GL_TEXTURE_1D);
   glDisable(GL_TEXTURE_2D);
   glDisable(GL_TEXTURE_3D);
   glDisable(GL_TEXTURE_CUBE_MAP);

   for (buffer=GL_BACK_LEFT; buffer<=GL_BACK_RIGHT; buffer++) {

      /* Will affect the last row on each buffer, left and right */
      glDrawBuffer(buffer);

      /* Set the viewport to the entire window */
      glViewport(0,0,width,height);

      glMatrixMode(GL_PROJECTION);
      glPushMatrix();
      glLoadIdentity();
      glOrtho(0,glutGet(GLUT_WINDOW_WIDTH),0,glutGet(GLUT_WINDOW_HEIGHT),-1,1);
      glMatrixMode(GL_MODELVIEW);
      glPushMatrix();
      glLoadIdentity();

      /* Erase the last pixel row to black */
      glColor3d(0.0,0.0,0.0);
      glBegin(GL_LINES);
      glVertex2i(0,0);
      glVertex2i(width,0);
      glEnd();

      /*
         Draw a line of the correct length
         the cross over is about 40% across the screen from the left
      */
      glColor3d(0.0,0.0,1.0);
      glBegin(GL_LINES);
      glVertex2i(0,0);
      if (buffer == GL_BACK_LEFT)
         glVertex2i(width/4,0);
      else
         glVertex2i(3*width/4,0);
      glEnd();

      glPopMatrix();
      glMatrixMode(GL_PROJECTION);
      glPopMatrix();
   }

   glPopAttrib();
}