[digiGen]

The construction isn't perfect, but I hope you guys will bear with me.

See the indigo square in perspective? As seen from the drawing, it has been projected from the square at the top and then traced to its vanishing points.

My question is then, assuming it is a cube I am drawing, how do I accurately fix the correct height, without eyeballing it?

This is really stomping me, so please help

btw, I have purposefully chosen this slightly more difficult construction method, instead of letting the square touch the picture plane. That way, I'm hoping to learn how to put several squares (and thus cubes) in perspective at various depths..



[ October 15, 2001: Message edited by: digiGen ]

[Sumaleth]

I'm yet to come across any method for directly determining the height of an object that isn't touching the "image plane". But it can be done indirectly.

All you need is to have one "edge" (or something) touching the plane for reference, and it's then possible to work out the height for all other objects from there.

You can do this quite simply;

. Extend the left-edge of the cube (top view) until it touches the image plane.
. Draw a blue line straight down from where the extended edge touches the image plane.
. Extend the left-most purple line until it intersects the new vertical blue line.
. Now you can use the 1:1 height of the cube, starting at the intersection of the new blue line and the intersection with the extended purple line.
. From the top of this newly measured height, draw a new purple line back to the left-side vanishing point.

This last purple line will give you the height of the cube. And you no longer need the extended part of the cube, that was purely for deducing the height.

There's probably a mathematical formula that does the same thing - if you happen to find it, post it here.

Row.

[digiGen]

Cool, thanks for the info. Kinda obvious (now, after you explained it

I've already ordered a bunch of perspective books through our library system. I'll post anything useful.

[digiGen]

Thanks to your info I finished constructing the cube. Now I'm trying to add in another cube, positioned a bit to the left and behind it.

I'll try and work out how to set up the relationship between the bases on my own. But if I end up stuck, I hope you won't mind helping out again.

[Shinja D]

quote

Quote:

There's probably a mathematical formula that does the same thing - if you happen to find it, post it here.

You most likely learned the method in school (german: "Strahlensatz" - does anyone know the english word for it?).

Here it is:

(not my illustration)
When e and f are parallel we can tell that a/b = e/f.


(plz excuse the bluriness)

IMO this way is sometimes quicker than doing it graphically. 'a' is measured once for a scene and 'f' is usually known.
So it's just one measurement per edge.

- Shinja

[ October 19, 2001: Message edited by: Shinja D ]