Paratrooper (playable DOS game in a shader)

Paratrooper, a playable DOS game in a fragment shader.

You can find (the full source of) the fragment shader here: https://www.shadertoy.com/view/XsyfD3.

Run the shader fullscreen or on Shadertoy.

            *Your Mission*

Do not allow enemy  paratroopers to land
on either side of your gun base. If four
paratroopers  land on one  side of  your
base,  they will overpower your defenses
and blow  up your  gun.  After  you have
survived the first round of helicopters,
watch out for the jet bombers. Every jet
pilot has a deadly aim!
The numeric  key pad  controls  your gun
and the firing of your bullets. Two keys
start the gun moving:
    < and 4 counterclockwise 
    > or 6     clockwise
Using the ^ or 8 key stops  the movement
of your gun and fires your bullets.

                 *Scoring*
    HELICOPTER or JET  .  .  10 points
    ENEMY PARATROOPER  .  .   5 points
    BOMB.  .  .  .  .  .  .  30 points

Each bullet you fire costs you one point

   PRESS space bar FOR KEYBOARD PLAY