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Is there a way to draw a perfect circle in a 2-point perspective drawing in Inkscape? I'm looking for a way that I don't need to estimate.

For example, I want to draw the largest circle that fits inside this rectangle at its center. enter image description here

I couldn't find answer to this. If that's the case for Inkscape, what software can I use to achieve that?

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3 Answers 3

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Unfortunately having only a projected rectangle does not define how a circle should be projected. A projected rectangle can present infinitely different rectangles seen in different projections. To remove the ambiguity you must have a square seen in the wanted projection.

Let's assume you have a properly projected square. It can be made using a descriptive geometry construction, calculated, copied from already existing image or simply drawn using artistic intuition "This is how a square is seen from my wiewpoint, no matter if it's a little tilted"

Fitting a circle to the square:

enter image description here

  • draw a circle, select it
  • insert path effect Perspective Envelope
  • select the envelope
  • use the node tool, drag the corners to the corners of the projected square

Be sure you have enough snaps to points ON (crossings, nodes)

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To draw this accurately use Sketchup ("Make" is free)

and export as pdf. svg here: SVG

enter image description here

if you want it like this enter image description here

just do this make sure both are pathsenter image description here

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enter image description here

  1. you copy your perspective rectangle
  2. you rotate the copy to sit horizontally on the ground
  3. you skew that copy, so that the bounding box is a rectangle
  4. you draw a circle which fits at the bottom
  5. convert to path, move the control points left, top and right to their place at the border and adjust the control tangentiales to the shape
  6. you group shape and circle
  7. skew the rectangle to the figure 2
  8. rotate it to fit to figure 1, ungroup

It is easy to find the correct angle for rotation and skewing in step 7 and 8, if you align both objects by their center, first.

Maybe you get similar results, if you apply the procedure 4, 5 directly to figure 1.

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  • I tried this, but first of all, my rectangle doesn't look like what you have there, so it doesn't align horizontally when I rotate it, so i'm stuck at Step 2.
    – CookieEater
    Dec 10, 2017 at 11:54
  • Ah, now I realize. Didn't look precise enough. So you can bring it with these steps to something close to form 4, but then the vertical lines will not be vertical, but asymmetrical run to the inside. Then you could manually put a circle inside, convert to path, and put the 4 control points to the (assumed) middle of the lines of the rectangle and align the tangential curve controls to the shape of the rectangle. Then transform as shown in 8-11. I reworked my answer.
    – user unknown
    Dec 12, 2017 at 14:50

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