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Author   Topic : "Draw a perfect square having two vanishing points"
Wrek
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PostPosted: Wed Apr 25, 2012 4:04 pm     Reply with quote
Hi, I have two vanishing points and a rectangle in a perspective, now I would like to draw a square in the same perspective using the 2 vanishing points I have. How can I do?

Thank you!
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Sumaleth
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PostPosted: Wed Apr 25, 2012 5:34 pm     Reply with quote
The approach that comes to mind would be to project the rectangle up into a plan view, then add the square to the plan view, and then project the new square back down into the perspective view. Which isn't as difficult as it probably sounds.

The first part of the process is the reverse of the steps described here:

http://www.khulsey.com/drawing-2-point-perspective.html

You can ignore the "side elevation" steps, since you are working with flat shapes.

And then once you've drawn the square where you want it to be, in relation to the rectangle, you do those steps forward for the square.

Here are a couple more diagrams:

http://draftingmanuals.tpub.com/14276/css/14276_283.htm
http://alltheworstideas.blogspot.com.au/2011/06/construction-of-rectangular-prism-from.html
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med
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PostPosted: Wed Apr 25, 2012 5:44 pm     Reply with quote
Oh Sumaleth beat me to it!
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Wrek
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PostPosted: Wed Apr 25, 2012 5:53 pm     Reply with quote
Sumaleth: I have to draw the square inside the rectangle and I can use one of his corners. Your reply may work but it's not easy at all...
med: I'm not sure it will be a perfect square.


Example: http://img194.imageshack.us/img194/9821/examplegn.jpg
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whom
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PostPosted: Thu Apr 26, 2012 1:30 am     Reply with quote
Wrek wrote:

med: I'm not sure it will be a perfect square.



I think it is rather perfect, the perspective makes it bend. Are you talking about an orthographic box?


...
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Wrek
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PostPosted: Thu Apr 26, 2012 6:26 am     Reply with quote
I found a very old tobic about this but an image is missing and I can't understand how it works
http://forums.sijun.com/viewtopic.php?t=36492
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T_England
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PostPosted: Thu Apr 26, 2012 9:01 am     Reply with quote
Did not know about that ellipse trick, and very good for sketching and getting things close enough, its amazing how much information ellipses contain. Thanks for drawing that up med!
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T_England
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PostPosted: Thu Apr 26, 2012 10:46 am     Reply with quote
Wrek: I had a go at correcting your square, i could be wrong however, I believe the ellipse works for plotting squares because its a form with 2 lines of symmetry and an angle. But to be honest the more I think about the ellipse the more confused i get Shocked



did the back face by eye and from there its easy, happy cube Smile also note there is some distortion as its getting on the extreme edge of the perspective and the cube could be more accurate


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Wrek
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PostPosted: Thu Apr 26, 2012 11:38 am     Reply with quote
T_England: your square looks good, but can you please explain better how you created the ellipse? I don't get it. I have to write a software which handles this...
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Wrek
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PostPosted: Thu Apr 26, 2012 11:40 am     Reply with quote
Please note that the square doesn't have to be centered, it can be on the left/right too
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med
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PostPosted: Thu Apr 26, 2012 12:07 pm     Reply with quote
T_England - no prob!

Whom is right there is a bend in perspective happening.

Not sure if this helps but;

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Wrek
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PostPosted: Thu Apr 26, 2012 12:26 pm     Reply with quote
med: looks good but you have to know a side lenght in order to draw the ellipse. I can draw an ellipse larger as the rectangle, that would be a perfect square in another perspective too

EDIT: I don't have the original rectangle, I want to find a square in order to measure it (at least its aspect ratio)
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med
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PostPosted: Thu Apr 26, 2012 2:33 pm     Reply with quote
I'm kind of lost in your diction there.

The aspect ratio of a square cube is 1:1:1.

The length would be the same as the width, as well as height. If you could describe more of what you need to do, it might help.

Are you writing a program that draws cubes that add up to a rectangle?
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Wrek
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PostPosted: Thu Apr 26, 2012 2:44 pm     Reply with quote
Nope, in my program I have a shape in perspective, I have to find a square in the same perspective. I don't know anything about original shape sides
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Dile
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PostPosted: Fri May 18, 2012 9:02 pm     Reply with quote
I saw this thread and decided to make a quick tutorial on the subject. The jpg's can be found on my blog: http://thevfxartist.blogspot.com
And I know this thread is a few weeks old, but I thought that maybe someone could use the information Smile

This is what I will make




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Sumaleth
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PostPosted: Fri May 18, 2012 10:33 pm     Reply with quote
dile: nice one.
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