[jME Tutorial 1-9] Reflections in the jMonkeyEngine (Part 1/3) Video Lecture | jMonkeyEngine Math: An Ultimate Guide - Game Development

reflections are very useful they can be
used to emulate mirrors for instance
reflections in water and lakes
deflecting objects of a plane such as
like pond and I will be discussing two
types of reflections so 5 a straight
line like this and i have an object
moving in this direction i can have it
reflect what kind of deflects when it
hits the surface or i can have it
reflect along the same axis and these
two are basically just negative
directions of each other now going to
show you how to calculate the directions
of both of these reflections and then i
use two methods to do so i'm going to
show you with quarter nians and without
creating page if i have a straight line
like this but no it doesn't have to be
straight horizontal line like this along
the x axis it can be rotated in anywhere
you want but for demonstration purposes
I'm just going to use a horizontal line
along the x axis so first of all I need
to know the normal vector more
specifically I'm going to use in the
unit normal vector and so you can get
these UW's on the cross product like
I've shown you earlier or you can use a
quads or a model which already has the
normals calculated for you so if using a
ray you can use collision results to get
the normals from them so but you need to
know the normal that's a very important
point so if i have a 10
rection back today but I need to do it
in this direction I don't need to but is
a lot easier to demonstrate if I have
the direction pointing away first not
towards the plane so i know that i can
calculate this angle here using the dot
product and but first of all i'm going
to draw the reflection so i'm going to
have it reflect i was too pale going to
have it reflect like this and we know
that angle is going to be the same as
this nose nuts it's going to be the same
as this angle get this
will draw the other normal going down
with that okay so with each of these are
unit vectors then the both going to have
lengths of one and if I take the cross
product which if you remember is a dot B
equals cos theta assuming a and B are
unit vectors so i'll be taking the cross
product between the normal and call this
a so we didn't say that B is the normal
and then this will give us cos theta and
if you remember earlier coz theta equals
adjacent over the hypotenuse triangle so
if I draw oops if I draw a straight line
draw a straight line and we know the
hypotenuse is one so we can say that coz
theta equals the adjacent length which
is this length here
should have used a and they going to get
confused so this is vector a and this a
means adjacent so a dot B or in our case
big a dot n equals the adjacent length
and if i multiply the adjacent length by
the unit normal vector i get the length
in the normal direction which in our
case is directly in the y-axis if i take
two of these so that the lengths are
this magnitude then taking two dot
products x the direction of the normal
and take that away from the a vector
then I get this direction here which is
the reflection direction so you didn't
quite get that so I take vector a and i
subtract it by this vector here which is
the dot product of a den which gives me
this this distance here and i'm going to
multiply that by the unit normal to give
me the distance in the unit normal
direction i'm going to multiply that by
two and that gives me the reflection
direction here if I wanted on don't
forget to normalize it afterwards so
that this direction is a unit vector as
well and if I wanted to find the
reflection like this old simply negate
the direction is facing it and Jamie as
a method for
I'll now show you how to do this using
called Tony ins going to create a new
page going to draw a line again dro we
still need a normal vector going to name
this n again the quick primer if you
remember if i have a bad day to have a
quarter nyan to find for a rotation if i
multiply the quarter nyan by the vector
I get the gut I get the new vector in
that core training rotation
I'm going to be using that to create
this reflection here Philip get rid of
those ok so if I have a direction vector
again this direction I will pull this
this little a this time and I have an
angle theta then the first thing I'm
going to do is sucking away before that
i'm going to be doing the rotations
deflecting this time because it's easier
to demonstrate by quarter nians so if i
can find the quarter nyan of this angle
like they need to find the axis at which
i rotate and then i can rotate this
vector all the way to this site so if
you imagine it's not a straight line
along the x axis at x axis but it's for
instance like a quad like this at a very
obscure angle I've got a direction
hitting Direction vector hitting this
side I need to reflect it in direction
like this so I need to know what access
to
rotate the new vector in our case is
very simple it's going to be the
z axis and if you remember the cross
product can be used to find the vector
perpendicular to two vectors which in
our case will be the normal and the x
axis and so that will give us the z axis
and that is all can also be used for any
types of different rotations of the
plane so first of all I need to find the
axis to rotate around that can be found
simply by doing a
dots knows a cross product of n and in
our case it's going to be the z-axis
and I then need to find that angle and
we're going to use the dot product for
that so if i have a dot B equals cos
theta and I'm going to assume that both
the normal and vector a are both unit
vectors so they both have length 1 so
that this is valid I then need to find
theta so I'll get theatre equals
30 degrees whatever that is in radians
no use I'll show you how to do this in
Jamie in a minute and then all I need to
do is rotate this angle of what I've got
creative core turning out of it first
and I will use the from angle axis
method in jme using the cross-product
axis now rotate it to get this vector
here so to recap do the cross product to
find the accessing it rotate use the dot
product to find the angle create a
quaternion out of it and then rotate the
normal vector by that angle simples
let's go do it in jme okay going to
create a new file control in my test
template and pull this riff collection
we're going to create a geometry class
variable I'm first going to
click create a quad fast actually record
let's give it a width of 3 and just
quite big say two and two patrol shift I
and to create a new geometry out of the
quad give it a name of quality of the
quad mesh know what get you I know this
is cute sign the material to the
geometry attached ability to all load
but my dog I've control shift effects to
see it in action okay also I'm going to
rotate it a little bit low B X and
y-axis quarter Prime and click apply the
y axis then going to set up some inputs
weapons but you a matured a good
laughing go to discuss G say new key
trigger takes a key input kicker finish
just key any put G then need to add a
key listener look at what action
listener I'm going to touch the G
mapping are they need to create the
action listener you know action
yep we're going to create the
declaration for me remember if you say
if the name equals gene then I'm going
to first of all create a mean way I'm
going to point it from the cam direction
of isolated cam location to face the
camera direction will shift I