Inside the Light Pen Gun

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Inside the Light Pen / Gun

• cs480
• 20053259 ???

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CRT monitor /1

• A CRT monitor works by scanning across the screen
  one line at a time.
• It scans each line until it gets to the bottom of
  the screen.
• These are called raster lines.

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CRT monitor /2

• When the electron beam hits the phosphor the
  pixel lights up and fades.
• Our eyes cannot detect this since our visual
  response time is too slow.

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Light Pen /1

• A light pen is a photodiode.
• It will detect the passing of the electron beam
  (brightening of pixel).
• The video card knows exactly where it is
  drawing at any instant.

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Simple Algorithm

• Button is pressed.
• Wait until next redraw of screen.
• Start timing (t0)
• Stop timing (tT) when light pen detects light.
• Calculate position using T, VREFRESH, HREFRESH
• The vid card measures the y-position by counting
  the number of HSYNC pulses between last VSYNC
  pulse and the pulse from light pen. For
  x-position, we use the time from last HSYNC pulse
  to light pen pulse.

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Light Gun

• Light guns work on the same principle.
• There is a photodiode inside the gun.
• The photo diode inside the gun is far away from
  the CRT.
• Lenses are used to focus light.
• Resolution of the light gun used to be low so old
  computer games used different ways to detect
  where the gun is aimed.

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Method 1

• 1. Pull trigger
• 2. Screen is blacked out.
• 3. Photo diode activated
• 4. Brighten each target sequentially.
• 5. If gun detects light we know what target.
• On poorly designed games you can point the gun at
  any light and always hit first target.
• Whole process too fast for human eye.

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Method 2

• 1. Pull trigger (activate diode)
• 2. Wait for refresh
• 3. Time diode activation
• 4. Calculate position using T, Vrefresh,
  Hrefresh.
• The same principle as light pen.
• Most modern arcade games use this method instead
  of first.

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Notes

• Many guns ignore red light since red light has
  much slower decay rate than green/blue.
• Light guns/pens can only be used with CRT
  monitors.
• Pens used on LCD use different principle.

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References

• http//www.answers.com/topic/light-gun
• http//en.wikipedia.org/wiki/Light_pen
• http//www.epanorama.net/documents/joystick/lightg
  un.html
• Target acquisition of stationary and moving
  targets using light pen and joystick with
  position and velocity control, Richard Lynn
  Harner, 1980