# Strange particle direction behavior

Do I understand the Direction-Pin of the Particle Plugin wrong (XYZ coordinates), or is there a wrong behavior indeed (see patch)?
The Particles should move to the green quad (Direction), but the coordinate mapping seems wrong.

particle q.v4p (17.7 kB)

hi phl,
problem was: you put the second point into the direction pin. but you would need to put the difference vector (from source to destination) into the direction pin.
direction = destination - source

particle q.v4p (18.3 kB)

ah, ok. I don’t get why, but i bet the answer is in basic vector maths ;)
thanks gregsn

yes, the answer is also hidden somewhere in basic vector math…

however, the basic concept can be understood easily without vector math at all! lets call it “the theorie of relativity” :P

no i really think these basic math operations can be understood without being a math genius. i try to explain…

we want to describe the direction to some destination (relative to some starting point). this is like showing someone the way. so the emphasis by describing a direction is put onto the destination, not the source. but still the way to go is relative to the starting point.

so we have:

• a direction is dependant of both source and destination. so both have to show up in the “formula”
• emphasis is on destination. so destination gets the pole position.
• finally: ways add up. but since you are already at some source position, the way from greenwich to our starting point needs to be substracted (like - is opposite of +)…

so it only can be:
direction = destination - source

to get something relative to another thing you need substraction or division. in most cases it is easy to find out what operation you need.

so A relative to B can be:
A/B
or
A-B
never B/A or B-A.
emphasis is on A. so A gets the unsigned uncompromised pole position. B gets into the “modifier” position.

never A+B or A*B.
the substraction is the relativational operation for addition.
the division is the relativational operation for multiplication.

another example would be: you want to know how much you earned this year relative to the last year. this year gets the pole position, last year the relativational position. if you want to know it in terms of percentage (how many times the amount of the last year) the operation would be /. if you want to know how much money more or less the op is -.

at least this is how i think of these things. it helps most of the time…