shadowslair

Hi, I have some issues working with path constrained bones and I assume it could be a bug in the way the path lengths are calculated. In particular - the last line segment.

Repro steps:
1) Open Spine and create close-to-fully symmetrical closed path
2) Export to .json
3) Explore the "lengths" json object in the corresponding path attachment
4) Notice there could be a problem

I attached a minimal repro case file:
paths.spine
as well as an image of the shape:
spline.jpg

In the file the line:
"lengths": [ 182.59, 468.34, 749.23, 840.52 ],
should be:
"lengths": [ 182.59, 468.34, 749.23, 930.26 ],
which would mean that the last segment length will be around 181. Right now Spine exports its length as 91 (840.52 - 749.23 = 91) which is almost two times shorter. Looking at the image the last segment length is very close to the first segment one.

I have a question also related to the way the animations rely on path lengths during runtime.
Looks like all animated path constraint calculations rely on path segment lengths (and overall path length) as they are in the setup pose and I don`t see them being recalculated in the Spine CPP Runtime.
As I stretch the path with bones, also apply deformation the whole path shape shriks, expands etc. and with that all segment lengths do change (also the overall path length). When I have animated parameters that rely on current path lengths the observed behavior is strange. I understand that as long as both the Editor and the Runtime rely on everything being animated according to setup pose lengths the animation will look the same in the application as it looks in the Editor, also recalculating the segment lengths in runtime would need some extra calculations, but to me this is not the way it should be done.

Do you think it would be more correct if the path constraint calculations are done with the current (recalculated after deformation) segment and path lengths both in Editor and in Runtime?
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shadowslair
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Nate

You're right, there was a bug that the last curve was using the wrong values. We've fixed it in 3.9, great catch!

As you found, currently paths at runtime use the lengths calculated for the setup pose in the editor. This is often good enough when either the path is not deformed much from its setup pose, or the path is deformed in a way that keeps the ratio of the curve lengths (eg scaling the whole path), or constant speed along the path is not required. Otherwise you are right that constant speed along the path can suffer when the path is deformed.

We could fix this by recalculating the lengths, but this would need to be done every frame and is a relatively expensive operation, which is unfortunate when many scenarios don't require it. We could have a setting so it is recalculated only for paths that need it, or possibly a method to manually trigger it at runtime.

There is some further complexity in that attachments are stateless. They can be shared across skeletons and so cannot store the recalculated lengths for the pose of one particular skeleton. In other cases we store additional data on the slot that has the attachment (eg Slot deform). If the recalculated lengths are only needed by PathConstraint, they could be calculated and used there. If we want to store the recalculated lengths for general use in addition to PathConstraint, possibly we could jam it on the end of the slot's deform vertices.
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Nate

Nate
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shadowslair

Thanks for the reply, Nate. I`m glad this bug will be fixed soon. I spent 3 hours debugging stuff trying to figure what I did wrong because of that.

In my opinion the path should contain two arrays:
Vector<float> _setupLengths;
Vector<float> _recalcLengths;
one for the setup pose lengths and one for temporary storage. When the path is deformed by bone or by deform animation, it will be marked as dirty, so before being used, '_recalcLengths' will be filled with the current values and they will be used. Recalculating them will not be that expensive IMO and will not be made unless the animation needs it. Not to mention that paths are rarely shared. From my tests it looks like the lengths are calculated via naive approach and 4 linear samples per segment. Which if true for my example shape recalculating the lengths will be 16 2d bezier curve samples. At least this is how I was able to get close to the exported values. The 'real' lengths are a bit longer actually.

Adding a setting for this would bloat the interface and can cause some extra confusion.
Putting this in slot will bloat the code.
Putting them at the end of the vertices array will bloat them the most, as IMO they are bloated enough at the time being.

I think it`s best to just automatically recalculate the correct lengths and use those, otherwise working with non percent values will make no much sense and will be incorrect. In this case we should not sacrifice correctness for tiny performance gain.

The biggest problem with how it is now is that the path 'position', bone lengths, spaces values and on do not make much sense when the path is changed. So it is a matter of just tweaking some values visually and hoping it`ll 'look nice'.
Also, if user wants to manually modify the 'position' parameter from the game with actual values calculated from e.g. character speed and the path passed since last frame, the results he will observe will be incorrect, as the path length will no longer be consistent.

Thanks for your time.
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shadowslair
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Nate

shadowslair писал(а):In my opinion the path should contain two arrays:
The path can't store state. Consider two instances of the same skeleton, they will both use the same path, but the path can be deformed differently for each skeleton and therefore have different lengths.
shadowslair писал(а):Recalculating them will not be that expensive IMO and will not be made unless the animation needs it.
How would we know if the lengths need to be recalculated? Paths lengths can be changed through deform keys or bone weights. If a path does need recalculation, it almost certainly needs recalculation every frame. Computations for a "good enough" length estimate require ~60 adds/subtracts/multiplies and 4 sqrts per curve in the path. It's enough that it isn't insignificant. We could do a worse estimate for cheaper, eg we know the length is less than the sum of the distance from the vertex to the handle, to the next handle, to the next vertex.
shadowslair писал(а):The biggest problem with how it is now is that the path 'position', bone lengths, spaces values and on do not make much sense when the path is changed.
True, but in many cases the additional accuracy is unnecessary. I would lean toward a setting that causes PathConstraint to recompute the lengths and use them without storing them. Users would only enable it if they find movement along their paths isn't accurate enough when the path is deformed.
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shadowslair

Yes, having them recalculated each time will be simple and will do the job. The option can activate/deactivate the recalculation and it`ll be fine for all needs. =)
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shadowslair
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Nate

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