selection tool confusion in AP

[befehr]

Sorry if this has been discussed before. As a "traditional" technical illustrator, i.e. I learned on a drawing board, I am confused about how the isometric tools are supposed to work. I understand the basics but am confused with the results. I am using a 15° x 15° dimetric here.

Traditional illustration states that with an ON AXIS ellipse the minor axis should be at a 15° angle (parallel with the drawing plane) with the major axis perpendicular to the minor axis. When I use the isometric tool this does not happen. (see the attachment please). There is some skewing or somewhat perspective type of adjustments being made here that is not making sense to me.

Perhaps there is someone out there with a similar background as myself that could please explain to me the expectation of this tool?

Note: In the attached example the image on the right started as a 2D image, I then used the Fit to plane command to create the final position.

[befehr]

No one? Nothing?

[Ben]

That's because we do not currently use true ellipse representations.  That will come at some point.  What you have is your original circle shape with an applied SSR transform.

 

The SSR effect does just that - it takes your 2D geometry and applies the Shear, Scale and Rotation to put the object into to projected system.  The ellipse method you are describing attempts to put a non-sheared ellipse in 2D space directly into position to appear as a axonometric projected circle, by applying a rule that produces a good enough projection of a circle (incidentally that approach is also not very accurate, but that's another discussion).  It's not the same approach.

 

Our approach attempts to maintain the original objects editablilty in terms of the original object.  If you take a shape and project it, you'll still be able to adjust the shear and rotation as a continuation of the initial SSR transformation.  Our approach also applies to any axonometric axis set.  The isometric approach you are describing only really works well for isometric.  You'd need to get into a lot more maths to apply it to arbitrary axes.

 

[Ben]

Sorry - another thing to note.  The Fit To Plane method works best when you adopt a Cube based grid.  That allows us to calculate the correct scaling we need to apply. That is because the axis lengths are determined from a single scaling factor applied to the cube.

 

If your grid is made using the Advanced Grid option (with Two Axis Custom), then it will generate an incorrect scale in one axis - leading to a shear effect like you are seeing.