How is this sketch fully defined?

[Scoox]

I've had trouble with sketches not being fully defined when I expect them to, an vice versa, quite often. Here's a very bare-bones example. I draw a line from the origin to a random point in the sketch, I don't type in a dimension. Then I apply the horizontal constraint to the line, and suddenly it turns black. But I can still adjust it's length by dragging the free end point of the line with the moues. The line is not fully defined, but it's displayed as if it were.

Conversely, I had a fully defined sketch (all lines shown black) and I deleted a random construction line that to the best of my knowledge was not being used by any other sketch elements. As soon as I deleted the construction line, the entire sketch turned blue, including sketch elements that were fully defined even before the construction line was drawn.

Am I going mad? Thanks!

[mike.tessier]

Hi @Scoox,

Thanks for posting! 

I'm sorry to hear about the struggles you have had in creating fully defined sketches! The case that you had mentioned with the single line has been logged up for the development team (FUS-36972, internal ID). That line should not be flagged as fully constrained. The sketch engine has some issues like this pop up every now and then. That being said, we are always hard at work trying to improve the sketch engine - should you ever come across a case like this where the sketch is being flagged as fully constrained, feel free to create a new thread here on the forums and we will get the case logged up for the development team. The more test cases they have the more improvements they will be able to make.

Thanks for reporting this! We greatly appreciate your efforts in trying to help us improve the software!

Cheers,

[mike.tessier]

Hi @Scoox,

Thanks again for posting!

I had been chatting with @jeff_strater about this just because I wasn't 100% convinced that this is a bug and it turns out that it isn't actually a bug. It relates to what a "line" is.

The line in Fusion is the way line is coded, it is actually infinite until you define a length through the use of dimensions. The infinite line is fully constrained in this case, because its position and direction are fully constrained (coincident to the fixed global origin and the horizontal constraint). The right end point, however, is not fully constrained. If you add a dimension, it will then become fully constrained.

I hope this helps clarify things! Please let me know if you have any questions or concerns - I am always happy to lend a hand!

Cheers,

[Scoox]

Thanks, Mike 🙂 Based on my tests, lines in Fusion 360 are always infinite as far as constraints are concerned, even if the line start and end points are fully defined. For example, in Fusion 360 a fully dimensioned line can be tangential to a circle even if the two don't touch, which agrees with your explanation. This is actually very useful, and I don't wish for it to change, but the point is: it works regardless of whether the line is fully dimensioned or not.

So this is not a case of functionality, but a case of semantics: Should a straight line whose length is not fully defined be blue or should it be black? Up until this point my intuitive understanding was that a sketch where all geometry elements are black contains sufficient information for any draughtsman to produce an exact copy, and a sketch with any blue in it cannot be reproduced. I'm only a novice, but I believe most users assume this is how it works (even you weren't sure), and in practice it isn't. In my opinion, nothing should be black until it's not fully defined.

Put another way, any element should only be black at the point where applying one more constraint to it would cause the drawing to be over-constrained. This logic does not hold true for lines, since a black line whose length is undefined can be further constrained.

In complex sketches, it can be quite tricky figuring out what's causing the sketch to be blue. Users would expect the culprit to also be blue, but if the culprit is a straight line it could be black and still not be fully defined, so it can make sketches very hard to debug.

[Scoox]

Here's an example I'm sure I've lost a few hairs over:

The vertical line on the left is shown as being fully constrained, but the vertical line on the right is not. And yet, the height of the line on the left isn't fully defined because it must be equal to the line on the right, which isn't defined. Shouldn't both lines be shown as not fully constrained?

[jeff_strater · Accepted solution]

Hi @Scoox,

A lot of this is admittedly technical, and not obvious.  But, here goes:  A line is defined as a position and a direction.  The endpoints of those lines are independent entities, and are constrained to line on the line.  So, a line is "fully constrained" if its position and direction are both fully defined.  The vertical line on the left has both of those values defined.  Its position is defined by starting from another fully defined point (the end of the arc and the horizontal line).  Its direction is defined by the vertical constraint.  The vertical line on the right, however, has only its direction fully defined, by the vertical constraint.  It's position is not defined.  I believe you should be able to drag that line left and right, and its top point will slide along the circle.  If you add a dimension from some known position (for instance the vertical line on the left), then it should turn white and be fully defined.

Jeff

[Scoox]

Thank you Jeff, everything you said is correct. Again, as suggested by other users, this is very confusing. I've encountered situations, when working on my Surface Pro 4, where my computer can't handle dragging unconstrained geometry in complex sketches as it causes Fusion 360 to become unresponsive. Although dragging stuff around can be a good way to determine what needs to be pinned down, in such cases I refrain from dragging anything to prevent problems and instead mentally analyze the drawing to figure out where to apply the constraints. But in complex sketches with loads of constraints it is almost impossible (especially if you are inexperienced) to find out why the sketch isn't fully constrained when the problem might be a line whose length has not been specified.

It has been suggested that a third colour be used for lines, that doesn't sound like a totally crazy idea to me.