# Arch in forced perspective.

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I am trying to draw a forced perspective archway on a flat. I can't find an accurate way of doing this.

The ellipse needs to be tilted so that it's axis is not horizontal to match the perspective needed. When I take an ellipse, then rotate it, I can only snap to the "center left" or right, of the ellipse, and not the tangent of the ellipse that is now the furthest right or left point of the ellipse.

thanks for all help.

Vectorworks 2010

A quick and dirty method ? convert the rotated ellipse to a polygon (Modify>Convet>To Polygons). You should be able to draw a vertical line and then eyeball which point is closest to the tangent.

-Josh

Thanks, I've tried things like that but I'm really trying to find a way that is not "eyeballing" it. Seems like there has got to be something more accurate.

The easiest way to get a snap point that is on the ellipse is to cross it with a horizontal line through the furthest right or left point. With the ellipse and the line crossing you can use the Snap to Intersection to find your snap point.

Keep in mind that the point is only as accurate as your placement of the horizontal through the extreme point.... As far as I know there is no exact method for doing what you want... and the rotated ellipse is really only an approximation of the perspective. You may get a better arc shape by creating a custom polyline.

Kevin

OK, thanks, as long as I know not to look for a better method anymore! That's pretty much what I was doing.

I played with it a little just to understand what was going on. Shouldn't the point your looking for be easily defined by the initial side point, vanishing point and vertical edge of the door? I have attached an example. The dotted red ellipse represents my understanding of your method. The green arch represents how I would approach it as a polyline using fillet points as the corners.

(This example is scenic only ie. the centre of the arch is at the centre of the door in my example, which would not be accurate in true force perspective.)

Kevin

The red line was my method - and notice how your red ellipse extends outside the boundary of the vertical edge of the door on the left? That was the problem I was having. I could then not find that left-most edge.

But I believe that that red ellipse is now actually true perspective as the center point of the top has shifted to the left, but it's hard to get an accurate width once the ellipse is tilted.

A different approach:

Draw the arch in front view normally.

Convert to Nurbs

Top/Plan view

Set up camera - Display Camera View

Select the Nurbs

Modify>Convert to Polygon

Thanks!

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