How to make a perfect cube in perspective -- does this make sense?

[nickbernstein]

I found a thread on how to create a perfect cube in perspective here: http://conceptart.org/forums/showthr...ng-perspective
The solution proposed in the thread assuredly would work, but seemed really complicated. I figured there had to be an easier way, and it occurred to me that by measuring 1/2 the original vertical line, you could find the point at which the diagonals from the top and bottom of line to their opposite corners would intersect.

This seems to make sense to me, and I can't think of any problems with it, but I wanted to see what other people thought and if anyone could find a hole in my logic. I've included an image to hopefully help clarify the solution:



Ps: sorry if this is in the wrong place, I'm fairly new here.

[Benedikt]

How do you determine the angle of the lines you use to locate point C in step 5? Seems to me that depending on the angle of those lines you would get different rectangles and there's no way to determine which angle is the "correct" one that way.

[arenhaus]

The flaw is in placement of C. There is no way to ensure that the distance from it to AB is correct.

You should simply use the "architect's method" to draw the square base of the cube from its top view. If you search through the forum, you'll find a couple threads dealing with that; or you could get "Perspective Made Easy" by Norling, which also contains an explanation.

[Elwell]

A counterexample points out the flaw in your reasoning:

[bill618]

Use this 2pt perspective method:

[nickbernstein]

You're absolutely right, in retrospect that's very obvious. At least it's still somewhat useful, as it will always be a multiple of a perfect cube.

[nickbernstein]

Ah, this is a much better approach. Thanks!

[Elwell]

You're absolutely right, in retrospect that's very obvious. At least it's still somewhat useful, as it will always be a multiple of a perfect cube.

Again, not necessarily (at least not if you mean an integer multiple).

[Velocity Kendall]

the lines cross at the center of any rectangular face not just squares, as elwell demonstrated

[sytrex]

Dont discard the OP's method. If you continue to draw the line cross the mid point of the next vertical line, you will get the relative/realistic equal distance between each line (1,2,3,4,5,6,7)

Useful for drawing light posts on streets or like a fences where the distance between each poll is equal.

Here, C is half way point from A to B



EDIT:
Or any other things that are same size

[bill618]

Dont discard the OP's method.

The OP is trying to draw a proper cube in 2-pt perspective, not any randomly sized rectanglular block. The OP’s C-point is the center point of any arbitrarily sized rectangle (including a square if you're lucky), as Elwell points out in his example. You're describing an equal spacing technique.

In order to represent a proper cube (essential for drawing proper squares which are essential for drawing proper ellipses for disk or cylindrical things like wheels, building arches, airplane fuselages, coffee cups, etc.), you need to determine the location of the two vertical edges on either side of vertical line AB of the cube as well as the accurate length of AB. AB (in the OP's example) is a line segment on the picture plane (in 2-pt perspective), so it can be measured 1:1 with the cube reference, as in my example (top and left side reference squares).

I know this doesn’t explain things in full, but that’s what all of those books and online resources are for, to walk one through the entire process from beginning to end.

[Erayo]

Just as a side note, placing VP1 and VP2 is also something not too obvious and should be done thoughtfully.

[Kamber Parrk]

Just as a side note, placing VP1 and VP2 is also something not too obvious and should be done thoughtfully.

Their placement is governed by rigid principles of geometry.

[Beeston]

Or you could refer to that tutorial that never gets mentioned anymore that Carl Dobsky did.

[Cortes]

What exactly is meant by 'perfect cube'? Google's not my friend for this apparently. For months after my perspective class finished I believed it meant a cube where all the dimensions were equal (apologizes for lack of proper terminology). That is, a perfect Platonic equally propotioned cube.

If it isn't that, then I don't know what it is.

[[email protected]]

What exactly is meant by 'perfect square'? Google's not my friend for this apparently. For months after my perspective class finished I believed it meant a square where all the dimensions were equal (apologizes for lack of proper terminology). That is, a perfect Platonic equally propotioned cube.

If it isn't that, then I don't know what it is.

You want me to explain the difference between a cube and a square?

[Cortes]

I don't think you understood my question, sorry. By 'perfect square', I meant cube. My apologies. I guess that a perfect square is just one plane of the whole cube though, like the first side that you plot from the VP's as in here: http://thevfxartist.blogspot.com/201...utorial-1.html. I suppose my question could be shortened to, what is meant by 'perfect'?

[[email protected]]

A perfect square, cube, chick with boobs and everything, is well-defined in mathematics, and perspective theory teaches you how to project these on a 2 dimensional plane. If you want to know how a perfect chick with boobs and everything looks, study mathematics, if you want to know how to project these, study perspective.

[thepace]

Well, I am going to throw my hat in the ring here...

Knowing how to do this in 2pt perspective is very useful for keeping things in scale, when translating a diagram, or just trying to come up with units across the image that look correct; thus, I made a tutorial video on how to make a perfect cube in 2pt perspective, that only assumes very basic knowledge, and hopefully should help.

http://www.youtube.com/watch?v=OB3UEpxFlj8