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I would like to cut a shape out of a surface mesh using a polygon.

The "Knife Project" function isn't working for me, as it changes the size of the polygon. When it projects the polygon down onto the surface mesh, it increases the size of the polygon by some amount, depending on how far away my viewpoint is.

In this example, I want to project the polygon directly downwards, such that the polygon that is cut is exactly the same as the polygon that I started with.

I would also like to discard the outside of the cut, not the inside. I.e. I'm not trying to cut a hole, I'm trying to trim off edges.

The surface mesh is a topography, and I want to trim off the edges according to a polygon that defines where the edges should be. enter image description here

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    $\begingroup$ Have you tried the boolean modifier? $\endgroup$ – Legoman Jul 12 '18 at 18:43
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    $\begingroup$ It is possible that the increase in size is due to perspective projection. Try going to orthographic mode before doing it. $\endgroup$ – Omar Emara Jul 12 '18 at 18:47
  • $\begingroup$ Possible duplicate of Cut out surplus material Particularly point 4 of the answer $\endgroup$ – Duarte Farrajota Ramos Jul 12 '18 at 19:06
  • $\begingroup$ Related blender.stackexchange.com/questions/50538/… $\endgroup$ – Duarte Farrajota Ramos Jul 12 '18 at 19:09
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    $\begingroup$ Thank you! Switching the view from perspective to orthographic solved my problem. $\endgroup$ – Stu Jul 12 '18 at 21:27
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The reason why your polygon cut is larger than that of the original polygon is that you are performing the Knife Projection in perspective view. To make sure it maintains its scale, switch to orthographic view.

This can be explained as follows. If one want to Kinfe Project a circle onto a plane, the perspective projection of the circle is related to the field angle or focal length and the position of the camera as shown in the figure below. On the other hand, orthogonal projection is performed regardless of the position of the camera or any other property, also shown in the figure below. So, the scale of the perspective projection is directly proportional to the field angle while it is always constant in orthogonal projection.

Figure

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