C# Xna Made Easy Tutorial 39 - Gravity Video Lecture | C# Game Development: Game Making made Easy (English)

hello buddy and welcome to your second
last c-sharp XNA tutorial so in this
tutorial we're going to be learning to
incorporate gravity within our programs
so for you physics junkies out there
we're not going to be doing the
gravitational formula we're not going to
be applying mass and all that stuff to
our players and stuff you can do that
but I'm going to be showing you a common
way on how they handle gravity in 2d
games that will give you a realistic
gravitational pull effect but without
getting in depth with all those formulas
so let us get right into the code so
first of all what we need to do is have
a texture 2d for the player okay so once
you set the texture 2d then we wanted
our player equal to content download
texture 2d and we're loading sprite one
I believe that's what it's called
if it's not we will get everyone to
actually I'll look for it
yeah it's called sprite one ok so we
have our player and um we're going to
draw a player relative to the player's
position so in our draw command we're
going to do spray Bachelor draw or draw
a player we're going to draw the
position and the color dot white I don't
know if I set a position but it says
there's a position ok says ok so what we
need to do when it says we need to
create a position so we created a
position right there and what we need to
do is also have a velocity so the way Oh
gravity works is that we're going to be
adding the position based on the current
velocity and you're going to be seeing
Maru the reason why we need to do this
is because if we move gravity watch I
should put right here I'll put floats
and I'll put it at UM say 100 F
so I didn't even put a name for the
variable I'm so tired
okay let me float equals okay so gravity
gravity has a constant gravities at a
constant rate well wouldn't within our
atmosphere whatever gravity is but then
I'll fall you fall at a constant rate
maybe at certain areas within our world
whatever grab the gravitational pole
isn't that strong but if we're not
getting in depth of physics the
gravitational pull is always the same
it's always constant you might have some
opportunity game that might increase the
gravitational pull or something like
that but in reality the the gravity is
the same so we said that there's a
constant so imagine that our player our
gravitational pull was at 100 and our
player moved down 100 say like we move
down 10 pixels each time okay because
we're going to be multiplying the
gravity times the game time dot a lapse
game time etc etc but say our player
moves down 10 pixels every time the
gravity is equal to 10 then if our
player moves down by the gravitational
value then our players just going to
move down 10 pixels every single update
and therefore you're just going to
follow the constant rate which is not
realistic the method we're going to use
is that we're going to use velocities so
we're going to increase the velocity
based on the gravitational value so each
time that um each time that we update
our player will fall down faster than
before
and when we jump when we do a jump speed
our the player when it jumps you will
kind of like slow down like you're kind
of like shoot up but kind of slow down
the air and then you'll eventually come
down which will give them a realistic
gravitational pull effect so we got all
our things set so for initialize so we
can set this equal to a vector to zero
and if you didn't want to initialize
your gravity there you can initialize in
the initialize method
but yeah so what we're going to do is
we're going to in our update method or
we really should have created a key
state but a keyboard state but I'm just
going to say that if they press the
right key then position X plus equals
and I should have made a move speed but
mu speed times game times float game
time elapsed game time
total seconds okay and elsif oh sorry
shouldn't be positioned it should be
velocity dot X is equal to now plus
equals is equal to so if they press they
press LS left key Keys left then we set
our velocities X equal to MU speed times
float game time lapse game time dot
total seconds else we set our velocity
to zero so that means we're not moving
free pressor if we're not practicing the
right or left key and then for jumping
we save keyboard dot get state is key
down keys up then our velocity dot Y is
equal to negative jump speed and we'll
be adding this for these variables after
times floats game time our lapse game
time out total seconds okay and we're
going to want to have a variable called
jump okay so what we need to add right
now is a move speed so that's a 500 jump
speed also at the 500 as well and for a
boolean variable called jump and I'll
set that to false
okay so jump is equal to true that means
we can jump and if jump is equal to
false that means we cannot
okay so now we set up our directional
buttons in our velocity so this is how
it's going to work so we're going to say
that velocity dot y plus equals gravity
times gametime times floats game town
the lab scheme times our total seconds
and now we're going to say position X +
equals now we'll just say position +
equals velocity ok so then just saying
that's basically saying position X plus
equals velocity X and position y +
equals velocity y okay um I don't know
why I don't know if I made the gravity I
don't know if I spelled gravity wrong up
there let me go in check yes I did like
an idiot anyways so we got our we got a
gravity set so every single time we loop
so what we're going to do is once a jump
is false so if not jump then we decrease
the velocity so that means the players
in the air that means we decrease the
velocity and else we just add velocity
dot y equals zero okay so that means
we're I player won't move down if it's
jumping so they press up and jump then
we just want to set jump equal to false
if they press the jump okay so we add
our velocity to that and what we're
going to do is we're going to say that
if position dot y is greater than or
equal to
what's up ground height let's say our
ground height is um 500 okay so if
position naught Y is greater than equal
to 500 or actually we can do this we can
say if position that we can say that
jump is equal to true and we could play
else jump equals to false but I was
wasting a lot of code if you notice in
our if statements these are conditional
operators this statement can only
rather what this statement can only be
true or false right it can't be any of
the value the position y is less than
500 then it will return false and their
position Y is greater than or equal to
500 this statement really is true so
instead of doing all of this all we have
to do is say jump is equal to position
dot y dot y is greater than equal to 500
and then this value of return equals the
true or false so if this is greater than
equal to 500 then jump is going to be
hooda true and if it's less than the or
you go to 500 then jump is going to be
equal to false okay
so simple enough so we got our player
plus equals velocity we got our jump and
we got everything set so have we run
this program let's see what we get so
far so so far we went into a problem so
then we say we have to say that if jump
is equal to true then we want to reset
our players Y position equal to 500 so
if we run this one more time I don't
know what's going on let's set this to
400 yeah because I didn't really know
the screens height so my bad so then
when it touches the when he when the
player touches the ground then it will
stay on the ground in term will be equal
to true okay so one problem is that one
thing I forgot to note is that for
velocity I'm supposed to set it that
they press the left key velocity X is
equal to negative movespeed times R the
game time so then if we press this right
here so we move towards the right if you
press right and we move towards the left
V person left key so input interview so
and so on and so forth so if we try to
jump you notice that we're kind of
jumping we have gravitational pole
excite or etc but our jump speed isn't
really high enough so I'm going to be
doing as I'm gonna set the drum speed to
say a thousand
so if we jump you notice that our player
can jump and then it will fall down at a
con and a nice smooth rate and if you
don't want the gravitational pull to be
that that great that strong and you can
set to like a lower value like 50 or
something and then your player will kind
of like float in the air bit before it
falls down depending on what type of how
much gravitational pull you want you can
adjust that but that is it for this
tutorial so I'm just going to kind of
skim through it quickly just to show you
what I did
so we have our velocity so our velocity
excellent velocity Y okay and if they
press right we increase the velocity X
if they press left we decrease the
velocity X and it's not pressing
anything we set it to zero if they press
the up key and jump is equal to true
then we set velocity with a negative
drum speed times the game time to our
laps game time we do negative John speed
because remember up is to our negative
whenever you go the smaller the value
the higher up on the screen a day so we
do a negative value to go towards the
top of the screen and we set jump to
false when jump is equal to false then
we do we increase velocity Y by gravity
so save velocity Y is equal to negative
one hundred and say gravity is equal to
10 or whatever so it's going to do
negative 100 plus 10 it's going to be
negative 90 etc etc we add the position
we add the velocity to the position and
therefore that gives us the change in
position we calculate this you've jumps
equal to true or false
and if jumps equal to true then we set
it to the grounds position so that is it
for this tutorial I hope you enjoyed it
and have fun with gravity so that's it
for this tutorial thanks for watching
and bye