Rotating planes so that one rotationaxis is set to zero

That's exactly what my snippet finds. Did you try it?
Make a plane, rotate it randomly and run the code. Then check the result.
Note that the angle follows the usual convention, i.e. 0° points towards positive X and positive angles are CCW.

I tried your code but my results weren't right.

Nevertheless I found a solution my self. Thank you anyway.

Here is my final code:

	out_name = ((GetDir #export)+"/tectdata.txt")
	out_file = createfile out_name
	
	north = [0,1,0]

	for obje in Shapes  where classOf obje == Section do
	(
	sectionrotation  = quatToEuler2(obje.transform.rotation)	
	p = Plane pos: [obje.pos.x, obje.pos.y, obje.pos.z] width:10 length:10 widthsegments:1 lengthsegments:1 isSelected:off
	gSHsetObjectRotation p sectionrotation.x sectionrotation.y sectionrotation.z
	obj_poly = convertToPoly(p)
	
	local nv = polyOp.getFaceNormal obj_poly 1
	if (nv.z < 0) then
		(
			addModifier p (normalModifier flip:true)
			local nv = polyOp.getFaceNormal obj_poly 1
			local fv = [nv.x, nv.y, 0]
			local hv = normalize (cross [0,0,1] nv)
			local vv = normalize (cross hv nv)
			--local heading = acos (dot nv y_axis)
			local heading = acos((nv.x * north.x +nv.y*north.y) / (sqrt(nv.x^2 + nv.y^2) * sqrt(north.x^2+north.y^2)))
	
			local drop = acos (dot nv z_axis)
			if (drop > 90) then
			( drop = 180-drop)
			format "%; %; %\n" obje.name drop heading nv fv to:out_file
		) else
		(
			local fv = [nv.x, nv.y, 0]
			local hv = normalize (cross [0,0,1] nv)
			local vv = normalize (cross hv nv)
			--local heading = acos (dot nv y_axis)
			local heading = acos((nv.x * north.x +nv.y*north.y) / (sqrt(nv.x^2 + nv.y^2) * sqrt(north.x^2+north.y^2)))
	
			local drop = acos (dot nv z_axis)
			if (drop > 90) then
			( drop = 180-drop)
			format "%; %; %\n" obje.name drop heading nv fv to:out_file
		)
		
	delete p
	)
	close out_file

fn gSHsetObjectRotation obj rx ry rz =
(
    -- Reset the object's transformation matrix so that 
    -- it only includes position and scale information. 
    -- Doing this clears out any previous object rotation.
    local translateMat = transMatrix obj.transform.pos
    local scaleMat = scaleMatrix obj.transform.scale
    obj.transform = scaleMat * translateMat
   
    -- Perform each axis rotation individually
    rotate obj (angleaxis rx [1,0,0])
    rotate obj (angleaxis ry [0,1,0])
    rotate obj (angleaxis rz [0,0,1])
)

I'm not looking for the actual object heading (the sections(planes) are randomly placed by hand). I just need the heading of the drop. hopefully that helps a little understanding the problem.

this is what i've got so far:

	out_name = ((GetDir #export)+"/tectdata.txt")
	out_file = createfile out_name
	
	north = [0,1,0]

	for obje in Shapes  where classOf obje == Section do
	(
	sectionrotation  = quatToEuler2(obje.transform.rotation)	
	p = Plane pos: [obje.pos.x, obje.pos.y, obje.pos.z] width:10 length:10 widthsegments:1 lengthsegments:1 isSelected:off
	gSHsetObjectRotation p sectionrotation.x sectionrotation.y sectionrotation.z
	obj_poly = convertToPoly(p)
	
	local nv = polyOp.getFaceNormal obj_poly 1
	if (nv.z < 0) then
		(
			addModifier p (normalModifier flip:true)
			local nv = polyOp.getFaceNormal obj_poly 1
			local fv = [nv.x, nv.y, 0]
			local hv = normalize (cross [0,0,1] nv)
			local vv = normalize (cross hv nv)
			local heading = acos (dot nv y_axis)
--local heading = atan2 nv.y nv.x

			local drop = acos (dot nv z_axis)
			if (drop > 90) then
			( drop = 180-drop)
			format "name:%, drop:%, heading:%, nv:% fv:%\n" obje.name drop heading nv fv to:out_file
		) else
		(
			local fv = [nv.x, nv.y, 0]
			local hv = normalize (cross [0,0,1] nv)
			local vv = normalize (cross hv nv)
			local heading = acos (dot nv y_axis)
--local heading = atan2 nv.y nv.x

			local drop = acos (dot nv z_axis)
			if (drop > 90) then
			( drop = 180-drop)
			format "name:%, drop:%, heading:%, nv:% fv:%\n" obje.name drop heading nv fv to:out_file
		)

)
close out_file

fn gSHsetObjectRotation obj rx ry rz =
(
    -- Reset the object's transformation matrix so that 
    -- it only includes position and scale information. 
    -- Doing this clears out any previous object rotation.
    local translateMat = transMatrix obj.transform.pos
    local scaleMat = scaleMatrix obj.transform.scale
    obj.transform = scaleMat * translateMat
   
    -- Perform each axis rotation individually
    rotate obj (angleaxis rx [1,0,0])
    rotate obj (angleaxis ry [0,1,0])
    rotate obj (angleaxis rz [0,0,1])
)

and I'm using the following script to generate my test enviroment:

for xr = 0 to 360 by 30 do
	(
		for yr = 0 to 360 by 30 do
		(
			
				p = section pos: [xr,yr,0] 
				gSHsetObjectRotation p xr yr 0
				
		)
	)

fn gSHsetObjectRotation obj rx ry rz =
(
    -- Reset the object's transformation matrix so that 
    -- it only includes position and scale information. 
    -- Doing this clears out any previous object rotation.
    local translateMat = transMatrix obj.transform.pos
    local scaleMat = scaleMatrix obj.transform.scale
    obj.transform = scaleMat * translateMat
   
    -- Perform each axis rotation individually
    rotate obj (angleaxis rx [1,0,0])
    rotate obj (angleaxis ry [0,1,0])
    rotate obj (angleaxis rz [0,0,1])
)

As you can see I implemented your samples and now all my drop angles are correct. Thank you very much for that! But ;-) ...my heading is still messed up. It seems if it is giving me less than half the time correct results and the rest is just screwed up.

What am I missing here guys?

Both the atan2 and the acos(dot v1 v2) solutions aren't working correctly.

Thanks in advance for all your help.

hi fexed , the garp 's solution is right and
work with face normal as well ( just have to convert negative angle in positive value )

i just want to give you a small advice
when you work with maxscript, dont hesitate to create small interface to check
your resutt in real time

quick new script a couple of output field
a timer then you could see really well what s happen in your test case
cut and paste when everithing s fine

obviously your calculation was bad :)

sorry

example :

north = [0,1,0]
rollout unnamedRollout "Untitled" width:388 height:265
(
Timer tmr1 "Timer" pos:[328,220] width:24 height:24 interval:100
edittext drop "" pos:[72,30] width:125 height:22
edittext dir "" pos:[72,75] width:125 height:22

on tmr1 tick do
(
if $ == undefined do return false
nv = polyOp.getFaceNormal $ 1
heading = (atan2 nv.y nv.x )
dropt = acos ( dot nv z_axis )
drop.text = ( dropt as string )
dir.text= ( heading as string )
)
)

createdialog unnamedRollout

theFile = createFile "C:\\testdata.txt"
for obj in geometry do
(
  local nv = obj.dir
  local drop = acos (dot nv z_axis)
  local heading = atan2 nv.y nv.x
  format "name: %, drop: %, heading: %\n" obj.name drop heading to:theFile
)
close theFile

You don't need to convert to anything.
These angles correspond to the pivot, not the geometry. If the pivot has been rotated independently of the object, you need a face normal.

Hi again,

two things that I've come up with so far.

First I simplified my code a lot:

	out_name = ((GetDir #export)+"/testdata.txt")
	out_file = createfile out_name
	
	north = [0,1,0]
	--only editable Poly in the Scene
	for obj in Geometry do
	(
	--NormalVector
		break()
	nv = polyOp.getFaceNormal obj 1
	--HorizontalVector
		break()
	hv = normalize (cross north nv)
	--VerticalVector
		break()
	vv = normalize (cross hv nv)
	--FrontVector
		break()
	fv = normalize ([nv[1], nv[2], 0])
	drop = acos(dot vv fv)
		break()
	heading = acos(dot fv [0,1,0])
		break()
	format "name:%, drop:%, heading:%, nv:%\n" obj.name drop heading nv to:out_file
		break()
	)
	close out_file

And second I created a testing setup here:

for xr = 0 to 360 by 45 do
	(
		for yr = 0 to 360 by 45 do
		(
			
				p = plane pos: [xr,yr,0] lengthsegments:1 widthsegments:1
				gSHsetObjectRotation p xr yr 0
				p = convertToPoly(p)
				
		)
	)

fn gSHsetObjectRotation obj rx ry rz =
(
    -- Reset the object's transformation matrix so that 
    -- it only includes position and scale information. 
    -- Doing this clears out any previous object rotation.
    local translateMat = transMatrix obj.transform.pos
    local scaleMat = scaleMatrix obj.transform.scale
    obj.transform = scaleMat * translateMat
   
    -- Perform each axis rotation individually
    rotate obj (angleaxis rx [1,0,0])
    rotate obj (angleaxis ry [0,1,0])
    rotate obj (angleaxis rz [0,0,1])
)

So what I need now are drop angles between 0-90 degrees and heading angles between 0 and 360 degrees. But what I get are angles between 0 and 180 degrees that are obviously not correct.

Could anybody help me with that? I'm really stuck right now.