Dot vs cross

I just write this thread because there still have questions about dot and cross products here, and I remeber that I wrote some other threads about them (links at the end).

Some reminders about differences between cross product and dot product.

The first one is important, the cross product creates a vector, the dot product creates a number !

Graphically, the dot can be represented like this :

The same way for the cross :

The problem about the cross product with PC-DMIS, is that the result is always a unit vector (the result is divided by its own length).
So you can't use it directly to calculate angles or distances.

A little more about it on those threads, old or not, giving explanations or examples...
https://www.pcdmisforum.com/forum/pc...xamples-of-use
https://www.pcdmisforum.com/forum/pc...sing-variables
https://www.pcdmisforum.com/forum/pc...of-dot-product
https://www.pcdmisforum.com/forum/pc...-about-vectors
https://www.pcdmisforum.com/forum/pc...on-from-points
https://www.pcdmisforum.com/forum/pc...point-to-plane

dph51 over 5 years ago

Thanks for sharing the knowledge.
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JEFMAN over 5 years ago in reply to dph51

Thx, dph51 !
This thread is not very new, but it allows finding some answers to those who use "search button" !
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Kp61dude! over 5 years ago

Great stuff!
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mbatten over 4 years ago

I just ran into the cross product returning the unit vector after beating my head against the wall for an hour trying to figure out why my point-to-line code wasn't working. Ugh... It would be helpful if that defect was at least stated in the documentation.
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JEFMAN over 4 years ago
There's a very old thread (or maybe it was a phone answer) in which I asked the question, the answer was "there's a cross_only function inside the soft, but it's not accessible to users", or something like this.
In the thread above, "cross_only" function should be "P_Vect" ("produit vectoriel" in french )

ASSIGN/V1=F5.XYZ-F4.XYZ ASSIGN/V2=F6.XYZ-F4.XYZ ASSIGN/P_VECT=CROSS(V1,V2)*SQRT(DOT(V1,V1))*SQRT(DOT(V2,V 2))*SIN(DEG2RAD(ANGLEBETWEEN(V1,V2)))

https://www.pcdmisforum.com/forum/pc...xamples-of-use
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mbatten over 4 years ago in reply to JEFMAN

Thank you! That should help. Do you know if computing it from the angle is better than using the XYZ coordinates? It's dumb we have to do this...

Interestingly, if you type out CROSS_ONLY in an expression, PC-DMIS takes it, but it returns zero. Weird...
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mbatten over 4 years ago in reply to JEFMAN

Also, do you know if this has been submitted as a bug?
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JEFMAN over 4 years ago

mbatten : I'm sorry, I'm not sure to understand the first question... If you want an accurate result, you have to use the max decimal places.
If you calculate a lonfg vector with only 3 decimal places, I'm not sure that the result is good (I will check it, asap )
For the second, I remember now, it has been "reported" long time before I registered on the forum. As usual, the answer was : " it works as it's concepted"...
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mbatten over 4 years ago in reply to JEFMAN

There's a somewhat messy formula to compute the cross product using the X, Y, and Z coordinates of each vector. I was just wondering if it would give a more accurate result in some situations.
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JEFMAN over 4 years ago

mbatten : here at #25 ?
I think it's the same, except if you don't use decimal places enough...
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