Art 315
Lecture 5
Dr. J. Parker
AB 606
Last time …
We wrote our first program.
We used a tool called GameMaker.
The program we wrote causes a ball to move
left when we type aleft arrow key.
That’s all.
This is a program?
The equivalent C code for a PC would
be between 500-700 lines long.
Let’s continue with the program.
There are three other directions to move
the ball, let’s do that first.
Gamemaker
Double click on the file L0401.gmk from last time and it
will open in GameMaker.
Now we can edit things, add new ones, and run the
program.
GameMaker
Add event -> Key Press -> Right
GameMaker
Add event -> Key Press -> Up
… and Down
Add event -> Key Press -> Down
Run the program
The Program
This program is now bigger and better than a
typical ‘Hello, World’, but still has very little in
the way of function.
Most programs in intro programming classes
are, in fact, pure demonstrations, and have
no other real value. They are pedagogical
tools.
Ours will ultimately become something else.
The Relative Box
This determines whether the motion/direction
specified is absolute or relative.
If Relative is unchecked,
then motion is absolute.
EG Speed = 1 (unit/turn)
If checked, the motion is
relative to the current
motion.
That is, speed=1 means
add 1 to the current
speed.
Relative
If the motions we specified are made relative,
Then each time an arrow is pressed the speed
will change. We can make the ball go faster
and slower.
Well, faster, anyhow.
Why can’t we make it go
slower?
Slower?
We could make it go slower, we just asked
incorrectly.
What we asked for was a relative change in
speed of +1 and specified a new direction.
So it will go faster with each key press, but
in a new direction, just as we asked.
How can we get it to move slower?
Slower?
The combination of speed and direction change
is called velocity.
We can change speed by adding to the speed
value.
Making the speed value negative makes the
ball go ‘backwards’, or in the opposite
direction. (going DOWN with negative speed
means going UP)
Slower?
So we could specify the DOWN motion as a
negative change in the UP speed.
We could specify RIGHT motion as a
negative change in LEFT speed.
Right is –ve left
Down is –ve up
The Result
Gravity
You may recall that in the Move menu there
is an item for gravity.
It does what you probably
think – it pulls the object
in a specified direction.
Gravity
Problem: just turning gravity on the ball will make it
fall, past the bottom of the room, and forever.
Let’s do it anyhow.
When???
We can turn gravity on when the ball is created, in
this case it will be when the program starts to
run.
Creation is an event.
Gravity
Make a new event for Create and as the action,
select the gravity item. Set gravity down, and with
intensity 1.
When does
the creation
event occur?
In this case,
when the
program starts
running (green
arrow pushed)
Falling
The ball falls. Good.
It falls quickly, and passes off of the visible part
of the screen. I guess this is to be expected (but
did you?)
Programs are often guilty of this kind of
unexpected behaviour; that which could have been
predicted, but which did not seem obvious.
How can we keep the ball on the screen?
Build a Floor
We could build a floor, an object that the ball could collide with.
This works as before: create a sprite (get a texture, set centre),
create an object using that sprite. Then place those objects in a
row along the bottom of the room.
Important: make the object solid by checking the box. (ball too)
The Room Looks Like This:
Are we Done?
Nope.
If you run this program, the ball falls
as before, passing right through the
floor.
Why?
All interactions have to be specifically
allowed and accounted for in a program.
We did not do this.
We Program A Collision With the
Floor
Double click on ‘obj_ball’ and select Add Event->Collision
Select the obj_wall as the collision target.
This will allow us to create some
code to deal with a collision
between the ball and the wall
(floor)
Collision Code
What do we want to happen?
The ball should stop falling, I guess.
I set the vertical speed to 0.
There are other ways to do this.
Tell me what they are:
Gravity, Balls, and such
In the ‘real’ world, a ball usually bounces.
Can we write a program to do that?
Of course.
When the ball collides with the floor, is should change
Direction instead of stopping.
There is a Motion item that does that. Making it
seem realistic is a bit trickier.
Bouncing
You should play with this problem too.
My solution:
- Set vertical speed to -3 relative
- Reverse vertical direction
Why -3 relative? Because I tried -1 and -2 and it
Did not seem realistic.
Why reverse vertical direction? Because that’s
what a bounce is.
The gravity setting automatically pulls the ball
down again, and so it bounces again …
Bouncing
What Have We Learned?
(about programming, that is)
We have to understand how to do it
before we can write a program to do it.
We can experiment with a program if
we don’t know exactly what to do.
Programs can do things we may not have
anticipated (usually this is our fault)
Programming can be fun. *(RH)
Now Lets Have Some Fun
It’s called ‘GameMaker’, so let’s make a
game.
We can make a ball move around, and ‘fall’
in simulated gravity. Sounds a bit like
basketball. But perhaps we can do better.
Ideas?
An Opponent or Target
Conflict is a key aspect of game design. I
suggest a target, as an opponent is harder.
Each time the ball, controlled by the
player, collides with a new object, a
target, then a point will be scored.
Make it challenging – reverse the control
directions left for right.
Game
Reverse control directions:
Pressing left key goes right, right key goes
left.
Make a Target
Target could be another ball-type object
that moves. Or a ‘goal’-like thing.
Ideas??
Goal
You will make your own game in the lab.
Mine will have a goal, and a wall protecting it. Behind
the goal is a target that moves. Quite tricky.
Scores
In a game, there must be some sort of goal
or objective to be achieved. Often this is
recognized by an increase in a value called
the score.
In a shooting game, for instance, the score
increases whenever an opponent is shot or
destroyed.
Scores
Because GameMaker was designed to make
games, there is a built-in mechanism for
keeping track of the score.
Score
Set Score
Test score
Draw score
Score is a name that
represents a value,
the number of ‘points’
earned by the player.
It can be incremented,
decremented, or displayed.
You get to decide how points are added/subtracted.
Score
Variable
Score is an example of a variable.
This will be the subject of the next class.