Cylinder Challenge : Circle or Oval Cylinder

12:55 PM, Friday September 3rd 2021

This is kind of similar to the previous question. The 2nd part of the cylinder challenge is close to breaking me. I have been playing on procreate with the perspective guide to try and understand things better. My main questions are.

  1. When you have constructed your cylinder in a box can the final result be either a circle or an oval ended cylinder in the box depending on interpretation? Therefore the end of the box be either square or rectangle depending upon interpretation?

  2. A key factor in determination a valid cylinder is whether the both minor axes are aligned and that the near minor/major axis ratiois smaller than the one further away?

I think I understand a lot of the theory but have less confidence of my application of it.

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1:59 PM, Friday September 3rd 2021

Hi! To try to answer your questions:

  1. I believe that only a cylinder by itself is open to interpretation in that way. In a box, if the contact points of the ellipses at the end of the cylinder (along with its minor axis) go towards the respective vanishing points of the box, then the cylinder has circular ends, and the box has square ends. In all other cases, the cylinder does not have circular ends. I think the box in this case is technically open to interpretation, but since creating these cases is not the goal of the exercise, I don't think it helps much.

  2. While cylinders do have this property, I do believe it is a property of perspective in general. For example, this note on the 250 box challenge states that the further box side has a greater proportional width compared to the closer side. The corresponding property in ellipses would be a wider degree, or a larger minor to major axis ratio. On a more personal note, I didn't really use this property to construct my cylinders in boxes.

If I may offer some advice, I think one major thing that helped me in this challenge is realizing that while it's impossible to draw a circle that touches all 4 sides of a rectangle, it's perfectly possible to draw an oval that touches all 4 sides of a square. Therefore I found that figuring out "ideal" contact points after drawing the box (and later on, while plotting out points for the box), and trying to align the ellipses to those points, helped me create circular ended cylinders. It's still not going to work if the boxes don't have square ends, but the error shown in the line extensions were more useful to me.

Hope this helps!

2:57 PM, Friday September 3rd 2021

Thanks for your quick reply.

  1. This was part of Uncomfortable's reply on this subject to a previous question on this.

"That's actually the purpose of the exercise. To draw boxes, then to use the ellipse on either end of the cylinder to test how far off the two opposite ends were from being proportionally square."

So given this I think the boxes are very much part of the purpose. I think my brain has a problem holding all the visualisation clues at one time. I agree that I think the contacts points ( and their need to be at the midpoint of the edge ) and their correct alignment to perspective are critical> I think I haven't been visualising them as much as I should before drawing the second square.

  1. I have drawn some boxes where the major and minor axes swap directions between the two end planes. I know it's wrong but it keeps happening. I think I need to stick with obvious perspective before becoming too nuanced that my errors make them flip.

Thanks for your help. I think my brain struggles because there are so many variables now in this exercise and it is all freehand.

4:21 PM, Friday September 3rd 2021
edited at 4:27 PM, Sep 3rd 2021

Sorry, I didn't mean to imply that boxes or their square faces are not part of the purpose of the challenge. I meant that in the case in which a "not circle in 3d space" ellipse is drawn in a box face, while technically the box face in some cases can be interpreted as square, such a case is not really important to the challenge, since it's not really possible to tell without further context.

If you're running into the case where the minor axis of one (or both) of the cylinder's faces are going off in a completely different direction, like towards one of the other vanishing points instead of the correct one, I believe this is due to the proportions of the face of the box being significantly off (as in, much wider than it is long, or vice versa). It's kind of hard to tell without an example though. I've run into a few situations like this, and it's usually because my box face was very rectangular.

EDIT: A second case for the minor/major axis switching is due to the cylinder face being very close to circular (in 2d), so a small change can cause the minor axis to change significantly. Don't think there's much to do in this case other than to try to be more accurate with the ellipses or boxes.

edited at 4:27 PM, Sep 3rd 2021
5:05 PM, Friday September 3rd 2021
edited at 5:22 PM, Sep 3rd 2021

No worries. The lack of context around what is being drawn is one of the challenges. There is little to tell you you are wrong until you have run the ruler over it. And even then.....

PS I cut out a cardboard square and put the circle and axes on it and pushed a brush through the centre which identifies the minor axis. It is helpful for visualisation.

edited at 5:22 PM, Sep 3rd 2021
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2:12 PM, Friday September 3rd 2021


If the endcaps of the box that the ellipses sit in are perfectly square in perspective then the cylinder will objectively be circular in that same perspective. However there can be some other perspective grid/camera lens that could make a different oval-ended cylindar look exactly like that first circular cylinder. In the same way that a lot of optical illusions like playing tricks with skewed forms that appear like boxes to the viewer and immediately collapse when you view it from a different angle.


The difference in ellipse degress (minor/major axis ratio) comes from behavior of perspective and the dimensions of the box remaining constant. So, yes that is one indication that you got it correct.

That is where I am at with my understanding anyway. I hope that answers your questions.

3:01 PM, Friday September 3rd 2021

Thanks for the reply.

I gave answer fuller above. I think consideration of contact points, their centering and perspective alignment before drawing the second plane are part of the problem. So much to both keep in my head and visualise at the same time.

I feel I am getting closer but it is more painfully slow than any previous exercise.

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