DCG discussion thread

auxiliary vanishing points are used when the surface (or the edge of the surface) is not parallel to the horizontal plane. Think of it like all "flat" surfaces (flat being hozontal) extending as far as the horizon, so the edge meets on it. so in fact all parallel lines have the same vanishing point.

when you want to find an auxiliary vanashing point, you need a few things:
1. if the line is on the same verical plane as a horizontal line, its aux vanishing point will be some height directly above the regular van. point.
2. you can get the height of it by two ways , one is kind of cheating, but makes it a lot less technical...so i'll explain that one first.
2.1 you take a point on each end of the line and join them...(you might not be able to
though, so this is not a certainty) then you continue the line until it is directly above
a regular van. point from a line thats on the same vertical plane.
other method
2.2 you use a view of the object (elevation most of the time) and you find the line in that. then you extend the line until it touches the horizontal plane (ground). you bring that point onto the plan and then join it to the point you already have of the plane in the perspective view giving you a line which goes back onto the vanishing point form here you find a regular vanishing point as if the point you had on the plane was horizontal and draw straight up from that. where your vertical line you've just drawn intersects the line i wrote in bold gives you the vanishing point...


thats long winded i'm sure and the second bit possibly might have some mistakes as i'm just writing it without drawing it...but it'll get the idea across

Getting auxiliary vanishing points is very straight forward.

At the spectator, you have a 90 degree angle. The horizontal line from S, where it cuts the picture plane, go straight up, vertical. With your 30 degree set square, go from S all the way back to the vertical line. Measure how high up the vertical line it cuts with your compass. This is the same height as the Auxiliary Vanishing point from the horizon line.

Give me 5 mins and ill sketch it and put it up in a post.

You need 4 AVPs for this, one above and below each vanishing point. Same height for all of them. may be a little confusing for what to join to what. Very time consuming though.

Thanks this is one of the things I did not fully understand. I should be able to answer the question if necessary now.

If you have to get the dihedral angle, when you get your first auxiliary where you look across the line of intersection, the angle the line of intersection makes with the x1,y1 axis in your auxiliary is equal to the angle of inclination. Just mark it in their instead of doing the rotate method.

Well I didn't do it portrait, so I can't get the last part, but here we go, yes the ****ing thing is upside down. But I think that's right so far. (If you download it to flip it, use paint to invert the colours, its not any easier to see, but its cool.

Oh one thing to note, I initially forgot the line of intersection was going to connect to point A, when it did I was like "**** I must have made a mistake" normally if I make a mistake its grand, but if you make one that "makes the question easier" you lose a **** load of marks. So I originally drew the line wrong, its still wrong in elevation1, but you can see its ok in plan, (although the wrong one is still in heavy, I am too lazy)

Final Edit, if you are planning on scanning anything, don't use Green pencil, its ****ing ****...