Focal lengthで画角を指定したV-Ray Physical Cameraからfovを取得するテストスクリプト。

Mayaのカメラなら以下のようにfilmWidthとfocalLengthから計算できますが、MaxのV-Ray Cameraでは被写体までの距離が必要です。

def angleOfView(filmWidth,focalLength):
    angle = 2 * math.atan(float(filmWidth) / (2*float(focalLength)))
    angle = math.degrees(angle)
    return angle

Angle of viewの「Derivation of the angle-of-view formula」が参考になります。

import MaxPlus
import math
 
def angleOfView(filmWidth,focalLength):
    angle = 2 * math.atan(float(filmWidth) / (2*float(focalLength)))
    angle = math.degrees(angle)
    return angle
 
def angleOfViewVrayCamera(filmWidth,focalLength,distance):
	focalLengthWithMagFactor = distance * focalLength / (distance - focalLength)
	angle = angleOfView(filmWidth,focalLengthWithMagFactor)
	return angle
 
def getVRayCameraFocusDistance(node):
	objPBlock = node.Object.ParameterBlock
	distance = 0.0
	if objPBlock.targeted.Value and (not objPBlock.specify_focus.Value):
		targetNode = node.GetTarget()
		vec = targetNode.GetWorldPosition() - node.GetWorldPosition()
		distance = vec.GetLength()
	elif (not objPBlock.targeted.Value) and (not objPBlock.specify_focus.Value):
		distance = objPBlock.target_distance.Value
	elif (not objPBlock.targeted.Value) and objPBlock.specify_focus.Value:
		distance = objPBlock.focus_distance.Value
	return distance
 
def getVRayCameraFOV(node):
	objPBlock = node.Object.ParameterBlock
	fov = 0.0
	if objPBlock.specify_fov.Value == True:
		fov = objPBlock.fov.Value
	else:
		fov = angleOfViewVrayCamera(
			objPBlock.film_width.Value,
			objPBlock.focal_length.Value,
			getVRayCameraFocusDistance(node))
	return fov
	
if __name__ == '__main__':
	if MaxPlus.SelectionManager.GetCount() != 0:
		selNode = MaxPlus.SelectionManager.GetNode(0)
		if selNode.Object.GetClassName() == 'VRayPhysicalCamera':
			print getVRayCameraFOV(selNode)
		else:
			print "Selection is not V-Ray Camera."
	else:
		print "Select V-Ray Camera."

はじめてPythonAPIを使ってみました、Maxの中にPythonエディタが搭載されてるわけでは無いんですね。APIリファレンスの情報量も少ないなー。