Gravity and jumping in
platformers
Phizzy
is here to talk about proper gravity stuffs. Now, I've seen
people use a couple of different things that they dare to
call 'gravity'. Here are some of the more retarded-looking
methods I've seen...
The
'Running up a Triangle' method.
The
'Top o' the Trapezium' method.
The
'OH NO RHE SKY GRAVITY HAS ME OH WAIT NEVER MIND IT'S
STOPPING KTHXBAI' method.
And here's something
that it should look at least a little like:
The...
'Actual gravity-looking' method.
Let's
take a look at how these methods would work.
The
'Running up a Triangle' method.
This is one of
the more common 'ideas' of gravity. A likely option for
n00bs and their allies.
It is often done by using a flag
and a counter. User presses jump button, flag goes on. When
the flag is on, the player is moved up constantly the same
amount, and a counter is added to. When this timer reaches a
certain amount, the flag turns off, and the user stops
rising. This, combined with a 'flag is off' + 'player is not
overlapping obstacle' - move player down event gives the
crappy illusion of triangular gravity.
Now, try
jumping.
Done it? Yeah? Did your speed stay consistent?
Yeah? No? Explosion? No. You probably smoothly decelerated
as you went up, then accelerated towards the ground. Unless
you were wearing those awesome anti-gravity jumping boots I
know you have. Those are so cool. Can I borrow them? Yes?
No? Explosion!? Aww...
The
'Top o' the Trapezium' method.
This is rather
bizarre, but I have seen it used on more than one
occasion.
What happens is the player moves up with a
consistent speed, then gets suspended in mid-air for a
moment, then continues to move down with a consistent speed.
Insane, no?
It's done by using two flags and a counter.
When the user presses the jump button, the first flag is set
on, and the second is set off. While the first flag is on
and the second is off, the character is moved up and the
counter is added to. When the counter reaches some arbitrary
number or the character hits an obstacle above it, both
flags are turned on, and the counter is reset. While both
flags are on, the character is not moved vertically at all,
and the counter is added to again. This time, when the
counter reaches the end number, both flags are turned off.
While both flags are off and the character is not on the
ground, it should be moved down.
I think it's fair to say
that it's obvious why this is not a veritable gravity
effect... That's a lot of writing for some superfluous
gravity method, though. ^_^
Onto some 'kinda almost
right' method...
The
'OH NO THE SKY GRAVITY HAS ME OH WAIT NEVER MIND IT'S
STOPPING KTHXBAI' method.
Okay, this is...
Quite retarded, really, but it’s the most common method of
gravity I’ve seen besides triangular gravity.
It works
with two counters and a flag. When the player presses the
jump button, turn the flag on. Then, make the
events:
Flag is on --> Add 1 to
Counter1.
Flag is on & Counter1 >= 3 & Counter2 > -5
--> Subtract 1 from Counter2 & Set Counter1 to
0.
Counter2 <> 0 --> Set Ypos to Ypos -
Value(Counter2)
That’s clearly not proper syntax, by
the way... Don’t try to copy and paste it or anything. You
can change the Counter1 >=3 to a higher or lower number to
change the ‘gravity’ strength.
Okay. Now, when the
character is moving up far enough, he needs to invert his
velocity to move back down, or some other silly words like
that.
Flag is on & Counter1 >=3 & Counter2 = 5
--> Set flag off & Set Counter1 to 0
So now,
you get sucked upwards until your velocity reaches a number,
and then it is stopped, ready for the gravity to kick
in...
Flag is off & Not overlapping backdrop
--> Add 1 to Counter1
Flag is off & Counter1 >= 3
& Counter2 < 15 --> Add 1 to Counter2 & Set Counter1
to 0.
Okay! That does it alls.
As you can see, you
get a weird Right-->Up curve, then an
Up-->Right curve, followed by a Right-->Down
curve with this method. If you try jumping again, you should
be able to see that that’s not actually what happens to you
under gravity.
That’s quite possibly the most complicated
and least useful method... Now, onto the real thing!
:O
The...
‘Actual Gravity-Looking’ method.
Finally, I
get to write about something that doesn’t offend my
eyes.
Right. Normal gravity should create a single curve
trajectory when used this way. So... What will be needed for
this is... Just the two counters.
To start with this,
when the player presses the jump button, Set Counter1 to -9
& Set Counter2 to 0. Here, Counter1 will represent the
vertical velocity variable, and Counter2 the delay in action
of the gravity. Next, make an event that goes... Not
overlapping backdrop --> Add 1 to Counter2. Now, for
the gravity effect...
Not overlapping backdrop &
Counter2 >= 3 & Counter1 < 15 --> Add 1 to Counter1 &
Set Counter2 to 0.
This will make the vertical
velocity rise when the gravity variable reaches a certain
amount, and will not rise above 15 pixels/frame (terminal
velocity). If you change these, you can alter the gravity
strength and the terminal velocity of the
character.
Finally, to make this code take effect,
you need to make one more event...
Counter1 <> 0
--> Set Ypos to Ypos + Counter1
All of that
should make a smooth, easily tweakable gravity effect.
And that’s all I need to
say!
If you didn't understand any of that,
here's a link showing all the
different methods and how sucky some of them
are:
Includes an example of a playable level with
changeable gravity methods (can be used as a full platform
movement tutorial), and a more theoretical frame that
clearly shows the exact path taken with each
method.
Example controls:
1 - 'Running
up a Triangle' gravity method.
2 - 'Top o' the Trapezium'
gravity method.
3 - 'OH NO THE SKY GRAVITY HAS ME OH WAIT
NEVER MIND IT'S STOPPING KTHXBAI' gravity method.
4 -
‘Actual Gravity-Looking’ gravity method.
Left/Right -
Walk (Game mode only).
Shift - Jump.