[Charles]

I am working on two seperate pieces. Each use single point perspective. The only difference in each piece is the vanishing point.

My question is: how do I accuarately represent the height of the objects in each picture.

For example one box could be exactly 5cms tall. How do I work out how long the lines are when the vanishing point is really close, and then far away.

The viewer needs to be able to tell that they are looking at the same set of boxes, just from a different perspective.

I assume there is some sort of formula to work this stuff out, but not even my lecturers have been able to help me.

This picture should help to illustrate my point.



Where should box [2] be cut off[?] when vanishing at VPb, assuming it has exactly the same dimensions as box [1].

Thank you very much

Charles

[/img]

[ColdKodiak]

If it's the same object in two different perspectives, the object has the same vanishing point... thus a and b really are the same point.


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[Charles]

ColdKodiak,

Thank you for your reply.

But I'm not quite sure that I understand what you are saying.

For my pieces, the object stays in the same place for the two pictures. The vanishing point is in a different location. As if the spectator has moved their position.

With the picture you drew, how did you work out where to cut off the 'top' box? What is the cross on the picture, it seems important but I can't understand how it works.

The closer a box is to a vanishing point, the smaller the sides become, you can see less. In single point perspective how do I work out how much?

I hope that makes sense..

Thanks

Charles

[Probustion]

i'm not sure if i fully understand your question, but maybe you're looking for a way to make sure the cube is really a cube, with all sides in accurate proportions? this can be done in 2 points perspective, but i'm not sure if it can be done in 1. what i'm thinking of uses the angle at which both vanishing points stand in relation to each other, and i only know a way how to do that with 45+45 and 30+60 degrees angles.
so maybe that explains why a 1 point persp wouldn't work.

if this is what you're looking for i'll explain it to you.
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[Charles]

Probustion,

I think if you could explain how to draw a cube in accurate proportions in single-point-perspective that would answer my question.

Perhaps it isn't possible in SPP? Does anywone know?

Thanks!

Charles

[Probustion]

sorry, edited my previous post to explain it a little further.
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[spooge demon]

I might be wrong, but I don't think there is an answer. You get distortion the farther out from the center of vision you go, so there are no absolutes. You can infer dimensions if you know the dimensions of one object in the scene, but that is it. You can measure around to get the height of anything else that is seen. Engineers who need to show exact dimensions use isometric perspective, or one without vanishing points.

In the first drawing, you can measure accurately what is on the picture plane (that is the bottom horizontal line.) You then set up a measuring point, at upper left, and project back to get consistent recession. But you can set up the measuring point anywhere; it can float around due to the angle of vision (or the lens length). There might be a rather involved method if you know the angle of vision, but it has been a while since I learned any of this.

The second drawing shows a little about this problem. The top is a square with a circle tangent to the edges. We know that in �correct perspective� with our fancy ellipse guides, that the circle in perspective forms an ellipse that is symmetrical around two axes. But take that square that enclose the circle and lay it down into �correct� one point perspective and it is not symmetrical anymore. Your ellipse guides are now broken. The wider the angle of vision (or lens) the worse this distortion gets. But it is always there, even slightly. But in isometric perspective, your ellipse guides are accurate.

So, eyeball it, or throw out a measuring point to make the two cubes consistent.

[AndyT]

Ok here's the answer to my interpretation of the question



The line from the corner of the object to the viewer shows where the edge is on the picture plane.
It'll show where you have to cut off the object in the perspective view.

You ensure that the object is the same by drawing it in the top view and front view.
You get the correct height from the front view if the object touches the picture plane.

Then you construct it the same way you'd do it in two point perspective.
The difference is that the only VP is always directly under the station point.


The only thing you change for the other versions is the position of the viewer or the horizon.
The horizon has something to do with the position of the viewer too btw.

http://www.teamgt.com/forum/cubes01.htm
http://andyart.de/forum/index.php?showtopic=32

Does that help? That's how it works in theory.
Maybe if you understand that you can get what spooge wrote about.
I don't get it yet
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[Drew]

As far as I can tell, it's a question of ratios. For instance, if the depth of the box went all the way to the vanishing point, that would be 100% depth. If the box is actually a square, with no depth at all, it's depth would be 0% of the distance to the vanishing point. No matter how you moved the viewer's position, the square will still have a depth of 0%.

I couldn't figure out how to do what you wanted in Photoshop, but it should be pretty easy with a ruler. Measure the depth of the first box. Let's say that 100% depth (all the way to the vanishing point) is 10 inches, and then the depth of the box itself is 5 inches. That's 50% of the total depth to the vanishing point. So, to draw the same box in the same perspective, but a different view, give it a depth of 50% of the distance to the vanishing point.

So, in the new drawing, if the total distance to the vanishing point (100%) is 6 inches, then the depth of the box will be 50%, or three inches. Of course, you have to make sure you are measuring the appropriate lines. I hope I've made myself clear, and also I hope that this is a workable and correct solution. Let me know if I need to clarify.

[Charles]

Hi!

Thank you all for you replies. Very helpful and interesting. I love this sort of thing.

Drew, I ended up using your idea in the end. I think it worked out pretty good.

Working out the ratios of each building was such a logical thing to do; it was staring me in the face, but I just couldn't see it. Oh well.

Thanks again!

Charles