Issues with Octagons

I created an octagon that has a height and width of 55.70 mm

I burned the octagon and the height and width were not 55.70 mm, they was smaller.

I went back to the shape in Lightburn and used the measuring tool. I selected one edge of the octagon and the height and width show as 51.46 mm (that’s the actual size of the octagon that was burned)

Why does Lightburn create a shape that has a height and width of one size (55.70 mm) but when using the measuring tool in Lightburn to measure that same shape, the height and width are a different size (51.46 mm)?

How do I create an octagon that has a height and width of 55.70 mm so that Lightburn will burn that octagon with a 55.70 mm height and width?

I suspect what’s happening is that the octagon’s width/height is showing you the width and height of the octagon at it’s widest and highest points.

Another way to think about this is if you created an ellipse around the shape where each point of the octagon is coincident to the ellipse, the resulting width and height of the ellipse would be what you see in shape properties.

In experimenting with this note also that it appears that the width and heigh in shape properties does not change for the octagon even if you deform the shape. The dimension on the numeric edits toolbar does appear to change. I’m not sure if this is a bug or a feature but I suspect it’s simply reflecting the way the shape is represented internally within the program. This would allow for the number of sides to be changed without changing the width/height of the shape.

I found this page

I want an octagon with all sides having the same side length (a) and equal medium diagonal (m) of 55.70 mm.

Using that calculator, I wonder if I have to set the height and width in Lightburn to 60.29 mm? (Not near my laser atm, will try that later)

It looks like that’s what’s happening. Lightburn is using longest diagonal for height and width. If I put 21.32 mm for side length (a) into that calculator, I get 51.47 mm for medium diagonal (m) that I saw burned.

There is definitely an inconsistency in Lightburn in terms of the program reporting 2 different heights and widths.

In shape creation they are using longest diagonal (l) for height and width and in measure tool, they are using medium diagonal (m) for height and width.

Call it a bug or feature, I call it confusing.

Definitely a bit counterintuitive. I’m typically okay with stuff like this as long as it’s consistent and reliable once learned. I’m wondering if in this case it would make sense to have 2 different concepts shown: a parametric size and a geometric size. I suspect this might be more confusing for people but perhaps not as confusing as what you discovered.

Perhaps a feature request is in order if you’re so inclined.

None of the above. The only thing you enter in shape properties is number of sides, on the top left of screen enter desired width. Width and height in shape properties is overall diameter.

If I do it your way, the size in the upper left doesn’t match the shape properties size, nor burn size.

If I change the size from the shape properties so the numbers there match, it makes an octagon with equal angles like I want, but the numbers in the upper left don’t match.

The answer for me was to enter a height and width of 60.29 mm in the shape properties dialog and it burned the exact size octagon I wanted.

The shape properties show 60.29 for my height and width, but the burn size and the measure size is the 55.70 mm I wanted.

You’re showing a hexagon which has a different shape vertically vs horizontally. For an octagon the numeric edits approach should work.

Thanks edited. Same results though. Height and width is not consistent in various places that use the label ‘height’ and ‘width’.

The output is what matters, and if that’s what you see with shape measure, then that’s what they should use in both shape properties height and width and the upper left height and width.

I see numeric edits for a different purpose than waht’s shown in shape properties. The numeric edits view is more for layout within the workspace. So showing the geometric width and height are important so that layouts can be done accurately.

The potential drawback is that orientation of the shape changes the geometric width and height which is different than what you’re looking for but the alternative would be worse from a layout perspective.

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