[LSC] LDraw coplanarity check

[Michael Heidemann]

Thanks for your quick answer.

My mathematics are not so good that I fully understand what you are 
writing. But if we measure an angle, what is about huge parts? Same 
angle (that is good for us) but big gap in the part. I think we need 
another way to measure this. At present I have no Idea about that, but I 
will think of it the next days.My problem then would be to put that 
thought into a formula with which a programm can work.

cu
mikeheide

Philo schrieb:

> (resent with my LSC email address...)

>

> Hello Michael,

>

> I bring this discussion to the LSC as others could also shed some 

> light...

>

> Thanks for the link to Lars explainations, I never got deep enough into

> that issue. Clearly a figure of merit should be a dimentionless value,

> which is not the case of -dist (equivalent to a distance) or even worse

> of -det (homogeneous to a volume !!!).

>

> The -dist with variable threshold works rather well for me, but that

> could be improved... (I never used -det)

>

> Here is the algorithm I would try:

> - Compute vector N1 = (ab)X(ad) and N2 = (cb)X(cd) (X means cross

> product) N1 and N2 are orthogonal to planes abd and dbc

> - calculate s1 = N1 X N2 / ||N1||.||N2|| . s1 is the sine of the angle

> of the two planes.

> - Calculate s2 similarly for planes adc and abc

> - Max(s1, s2) is an dimentionless angle (for small angles sine is about

> equal to angle) that should be a good measure of coplanarity.

>

> Thoughts?

>

> Philo

>

>

> Michael Heidemann wrote:

>

>> Hello Philo,

>>

>> I have now checked the file 

>> http://www.ldraw.org/cgi-bin/ptdetail.cgi?f=parts/6942.dat

>>

>> As you are an engineer maybe you can solve the problem with the 

>> different values for the det and dist options.

>>

>> I have also noted in the comment of my last vote a link to the 

>> discussion of a previous occurence of this problem.

>>

>> cu

>> mikeheide

>>

>

>

>

>