03-09-2016, 05:24 PM
I've rewritten my answer several times but I can't seem to write it properly without either repeating myself or getting too convoluted with basic details, so I might just write a step by step.
But I'll try and summarize - the reason why you use a second station point is because the SP is always used as the 90 degree intersection of the orthogonal that determine your vanishing points. SPa determines VP1 and VP2, SPb determines VP3 and VLHL. The reason you must run it through VLHL is that the 3rd vanishing point by necessity is at 90 degree angles to VP1 and VP2. The reason you use the COV triangle to do all this is because the COV originates from SPa, SPb is in the same physical location as SPa, but it's rotated 90 degrees to give us orthogonals that we can draw to the VL.
The angle of the object is determined by the VP's, and the VP's are determined by the SP's. You can move the VL, HL, change the angle of COV, rotate the Orthogonals, but you can't change the orthogonal angle and you can't change the SP.
I hope I make sense. If not I'll have to do a step by step I guess.
EDIT - To further illustrate how there is only 1 physical location for the station point, I've used SPa and SPb to hope clarify that it is actually the same physical location in space redrawn twice.
But I'll try and summarize - the reason why you use a second station point is because the SP is always used as the 90 degree intersection of the orthogonal that determine your vanishing points. SPa determines VP1 and VP2, SPb determines VP3 and VLHL. The reason you must run it through VLHL is that the 3rd vanishing point by necessity is at 90 degree angles to VP1 and VP2. The reason you use the COV triangle to do all this is because the COV originates from SPa, SPb is in the same physical location as SPa, but it's rotated 90 degrees to give us orthogonals that we can draw to the VL.
The angle of the object is determined by the VP's, and the VP's are determined by the SP's. You can move the VL, HL, change the angle of COV, rotate the Orthogonals, but you can't change the orthogonal angle and you can't change the SP.
I hope I make sense. If not I'll have to do a step by step I guess.
EDIT - To further illustrate how there is only 1 physical location for the station point, I've used SPa and SPb to hope clarify that it is actually the same physical location in space redrawn twice.