[jME Tutorial 1-7] Cross Product in the jMonkeyEngine Video Lecture | jMonkeyEngine Math: An Ultimate Guide - Game Development

the cross product is used to find the
vector which is perpendicular to two
other vectors ie the normal so if I have
a straight line like this then the
vector perpendicular we face in this
direction more usually visualized if I
have a quad like this and all the
normals will be facing this way of
course they can also face the other way
depending on which order you do the
cross products and the unit normal
vector end of the little head is defined
as a times the over magnitude of a times
B and you can calculate this using
determinants but luckily for us jamie
has a method already in there for us we
shall use this no I'm going to create a
new file control n test really test
templates and a call this cross products
and create a quad instance variable
create the loop quad here control space
to get the construct arguments and I'll
just get this look three and a high of
thing create a new geometry James just
could give it a name of quad touch the
quad masterful chef died to import the
classes go space
we'll set the material to end attach the
Jory to the root I don't need to attach
a light this time could use the end
shaded material shift f6 to see it in
action and there's our quad so now I'm
going to try to find the normals of all
the vertices in the quad mesh so I'm
going to get the buffer pipe size i want
the positions of the vertices let sign
your a vertex project Joseph died nobody
to get the vector 3f array of the vertex
buffer using the buffer utils objects
get back three away clicky position
don't need to cast the vertex buffer to
a float but my first of all control
shift i 2409 heed to do to get the data
from the vertex buffer ok now I've got
the array with four vertices in it
corresponding to each point of the quad
I can now take the cross product of the
vertices so if I system elps to get a
raise in Russia
to cross products oh that's one in the
array and but well this isn't going to
work and I'll show you why
we're going to give us 0
but what you can see zero this is
because first array is a zero vector you
cannot take the cross product of a zero
vector so I have to use the first well
the second and third position in the
array but fashion second index okay and
then hopefully we should get a vector
pointing towards the camera 00 109 and
we normalize this
see that will make zero
Valente okay so that's taking the mesh
of the quad getting two points along the
mesh and then calculate the cross
product to return the vector
perpendicular between those two points
but what happens if i rotate the
geometry with the geometry now we'll
rotate it 45 degrees which was quartered
cry along the x-axis
shift of six planets you can see clearly
rotate it but I'm
going to get the exact same answer this
is because i'm doing the cross products
of the mesh itself and the mesh never
moves even when you rotate the geometry
the mesh stays in the same place so in
order to accurately get the normals of
this i need to multiply the vector by
quarter nian that we learned in the
previous lesson so I'm going to create a
court in tro shift i return for Smith
turn neutral turning and say from angles
I'm going to get asthma books quarter
pie
food i roll this down here i'll just
give this little turning these knows the
exact same ink just looks better so
rotated that and now i will take the
cross product qualitative will multiply
the killarian by the cross product
you
and then normalize it there should get a
completely defeat comes for the normal
social thigh right and there you go
let's see if that answer makes sense
because you can also get the normals of
a quat they come as a vertex buffer
already included I didn't tell you that
at the start as you wouldn't want to
have done diff tutorial you've typed up
normal the same way normal until we use
exact same procedure buffer normal get
data and systems
it can be of any of them so just pick
the one of course I also need to
multiple it'll turn in by this
my lips fro shift of sex less you can
say that assemble
class not found really could have food
what is going on how does this happen to
me
you
what's your go-to shift f11 this and try
again did not know why that happened it
happens every so often so just shift at
11 and it will fix itself
success so as you can see take them
across products and also getting using
the normals that come with the quad mesh
you can see the basic the answers are
the setting of course you can use the
cross product for a lot more uses and
you can figure them out for yourself