[Tron]

Hi everyone - I�m happy about the server being up again
Here�s my problem:

How can I construct a circle just with a pen?
That means: how can I construct the points, which lead to a circle when joining them with lines?


And now here�s the real problem how con I construct those points in perspective?


I know, it�s difficult to explain, but please make a try.
I already know how to construct geometrical objects and I already know that fact (please have a look at the picture, it�s hard to explain…



But want to know how to get some helping points in this construction, that help me to paint an exact circle in perspective.


How do you handle you handle this problem?


Thanx for your replies.

[ May 30, 2001: Message edited by: Tron ]

[Derek]

Answering in reverse... first of all, the ellipses in your diagram are terrible. Ignore them. Don't ever draw them like that! Find D'Amelio's book on perspective if you can and look at what he says about circles and ellipses (probably the best overall book on persepctive out there right now, even if the drawings are a bit hoaky)
Something to keep in mind when drawing an ellipse: if you were to divide it equally down the major and minor axes, you could then take any one of the resulting four quadrants and flip it, flop it, etc., and it would rest exactly on the other three... this is a way of saying that ellipses (aka circles in perspective) are symmetrical along both axes. Knowing this, you can freehand an ellipse better than plot one in many instances. As for building a perfect circle, I am guessing you want to know how to do this freehand? If you really need perfect circles, get a template. Will save you a lot of aggravation. If you're hell-bent on doing it by hand, you have a long road ahead of you, and you will hear many arguments as to how many points to plot and what devices to use to do so.
Ellipse templates are an option as well, but I don't have the means to show you how to make use of the degree settings on them. However, they can be useful for helping you when you understand A. why the visual 'height' of the base of the cube (you are setting these into a perfect square or rather the perspective representation of one right? GOOD.) is what dictates the center of the ellipse and not the center of the square itself as found by its diagonals, and understanding B. the true width of a circle from a given station point (hint... the widest part to the viewer isn't the diameter of a true circle).
I know... a lot of theory when all you want is a simple means of doing this, but in order to truly understand and 'get' perspective...
You'll want to put a good deal of time also learning how to put an ellipse on various planes in a drawing, not just keep them parallel to the ground plane.
Man, I don't know if you quite know the size of the can of worms you have in your hands with this. But you can catch some very big fish if you learn the rules that govern these answers really well.
I can try to get a few quick diagrams up to show you more precisely what I'm getting at, pictures being worth a thousand words and all, but it may be a day or so.

[henrik]

If you want to draw ellipses in perspective, there's no exact way of saying which angle (in degrees) the ellipse should have (correct me if i'm wrong).
But the perspective is easy. What you want to draw is a circle viewed from an angle with perspective.

1. The red line should be facing the side of the cube 90 degrees. Always.

2. Look at my picture where i've marked 2 points of the longer sides of the ellipse. They should be aligned along the red line, and not anything else like in picture 1. (NO!)

Hope this helps

[henrik]

Whoho! 100 posts in 2 years. Must be some kind of record. (101 including this...damn i'm slow) ...Just had to do that! Please forgive me.

[Derek]

Wow... a fast and furious topic... woo hoo!
Okay, Henrik these are on a better track for working on differing planes in a drawing, specifically in this case on working with specific vanishing points and surfaces. There is a way, when drawing the kind of ellipses you have, to determine the correct angle template to use. It's a bit involved and my roomies wouldn't appreciate me firing up my scanner and such right now... (Jeez it's only 2 a.m.) :-(
As for your diagrams, it may help to explain that the minor axis of the ellipse (pretty well represented by that red line) happens to recede to the same vanishing point as the sides of the cube. Always works out that way. Also, yes the axis recedes to the VP opposite the face of the cube it rests on. As evidenced in the drawing. Your ellipse is on the right 'face' of the cube and the red line and top and left side construction lines of the cube recede to the left VP.
Now, one last request... everyone please get into the habit of drawing real cubes to put these on and be sure your ellipses, for the sake of diagrams like this, contact the edges of the face they are on, or a lot of newbies are gonna get awfully confused.
Diagrams of amazement in a day or so, promise.
Thanks for posting diagrams Henrik!
Derek

[spooge demon]

You only need to do this when your vanishing points start to get pretty close. You can make the ref grid as painful as you want.

If their is not much distortion going on, you can use a guide easily. If a little, just draw each quadrant a little differently but still using the guide.

Or use a 3-d program. Works pretty well.

[Tron]

wow - thank you all for your replies!
i don�t have much time at the moment, but i�ll test all your ideas.
i read some books about perspective and i�m going to post good things that i found there.

[ May 31, 2001: Message edited by: Tron ]