Start a new Scratch project - "File" and then "New"
Replace the cat sprite with the "Ship" sprite in the folder "Invaders". You will find the "Invaders" folder in the usual place in the Student share. make the background black - space is black again - and give the ship some left right motion. Set up the ship at 0,-150 using a "go to" block; 0 being the x value and -150 being the y value.
Make a new sprite and create its costume. It is a very short, white vertical line. When you press the space bar it is fireed straight up toward the top of the screen; this is done by changing the y value of the position of the laser. The x value controls left and right and the y value controls up and down. When you press the space bar2. Screenshot your project so far and describe how you created the laser.
Add another sprite and give it the alien costume. The aliens have a strange but predictable motion, they will march to the right and when the first one gets to the edge of the screen they all come down a little and then march to the left and when the first one gets to the left edge of the screen, march down a bit and then march to the right and then repeat till they get to the bottom of the screen. To make life more interesting there are groups of them.
The thread that you write must check that the right most alien is near the edge of the screen and then make all of the aliens simultaneously march down a short space and then march to the left. This is done with a broadcast instruction.
Now duplicate the alien and change the "go to" block x values to be -200, -150, -100. -50, 0 and 50 for the 6 aliens.
When you press the space bar they should all move together.
When you shoot an alien with the laser the alien vanishes (or hides).3. Screenshot your project so far and describe how you created the first set of aliens.
Add another row of aliens by duplicating each of the existing aliens and change the y value from 120 to 80
Now add two threads to each alien, one to detect being struck by a missile and then hide or vanish and then another to collide with the ship and say "Game over".
Check to see that this works correctly4. Screenshot your project so far and describe how you created the first set of aliens.
Create a row of 4 barriers to protect the ship using the "barrier" costume from the "Invaders" folder. If the aliens hit the barriers the game is over and the ship cannot fire through the barriers at the aliens.
You can duplicate the barrier and change the x value of the "go to" block.to be -150, -50, 50 and 150. This will space the barriers out neatly.
Add the largest asteroid from the folder and give it the motion as shown in hte image. The idea of the game is that when the asteroid passes out of view it will continue its journey in the same direction from the opposite side of the screen4. Screenshot your project so far and describe how you added At least one of the improvements given above.
Add the other smaller asteroids as costumes to the larger asteroid.
This is the working game although there are a few little issues to clear up. First, when the missile hits an asteroid it continues still hitting the asteroid so one shot is usually enough. Change this so that the asteroids has to be hit by different missiles to be destroyed. Second, in the real game when the asteroid is struck it splits in to two so there will be two middle sizes asteroids and then each if these will divide into two. Thirdly the asteroids get faster as they get smaller.4. Screenshot your project so far and describe how you added At least one of the improvements given above.
The arcade game also had an interesting way of moving the spaceship. As there is no gravity it behaves like a free body, i.e. if there is no acceleration then it continues in the same direction at the same velocity. It has the same limitation as an asteroid in that if it goes off the screen then it reappears on the opposite side.5. Screenshot your project so far and describe how you added all of the improvements given above in both extensions