sample_baked_with_rotation gives wrong transform

[botero-dev]

Godot version

4.0.3.stable

System information

Windows 10

Issue description

evaluating a curve's sample_baked_with_rotation gives wrong Transform2D:

55374fd468146e8611b46b327faa6843.mp4

The texture should read A B C D correctly, you can see it flips the texture vertically

It is trying to match textures Positive Y to the path direction, and then positive x to the right of the path.

I think the two correct options would be:

• map negative y (which would be up) to the path direction
• map positive x to the path direction (which I would like better, but would like to know your opinion)

I found this while working on this PR #77819
I could give a shot at fixing this during the weekend

Steps to reproduce

Just run the example project

Minimal reproduction project

PathSampleBug.zip

[AThousandShips]

Will look into it, yes, it seems to flip the axes

It does work correctly with a PathFollow2D

[AThousandShips]

It appears the transform returned from the Curve2D is not a real transform, as the documentation shows to extract origin and rotation, but should be clearer, it works if you do:

target.position = tx.get_origin()
target.rotation = tx.get_rotation()

I'm unsure why it is using this unconventional format, but changing it would break compatibility so best to just clarify in the documentation.

Will fix the documentation

[capnm]

It appears the transform returned from the Curve2D is not a real transform, as the documentation shows to extract origin and rotation, but should be clearer, it works if you do:

I'm not sure what you exactly mean. The negative y-scale in transform seems to be the culprit or bug here...

print(tx.get_scale())
(1, -1)

I.e. this works as expected:

target.transform = curve.sample_baked_with_rotation(distance)
target.scale = Vector2(1,1)

[AThousandShips]

I mean no that gives a different result than the behaviour of PathFollow2D, it's not a negative y-axis, it's that the x and y axes are swapped, it only works if you treat the curve as having the y axis along the curve

The returned transform is: Transform2D(side, forward, pos), where forward is along the curve, i.e. the x-axis in a normal transform, based on the code, and how that code is used, it's not intended to be treated as a conventional transform

If you create a curve of the points [(0, 0), (1, 0)] it will return [(0, 1), (1, 0), (0, 0)] if sampled at 0.0 for example

"Forward" isn't a thing in the 2D coordinates but one would expect it to be the X-axis, otherwise a curve would be very strange to use no? If you want a character to move along a circular curve shouldn't it be facing normally at the top of that curve?

Considering how this code was directly moved from PathFollow2D the assumption should really be that the axes, and use of it from there, is the way to go

One can argue that this convention is confusing and harmful, but changing it would not be possible at this time, so documenting it and clarifying how it's made up is the best way

[capnm]

I haven't had time to read/decipher the corresponding code. Other practical question, why it here manipulates the scale of the target transform at all?

[AThousandShips]

This is as I said because it swaps the X and Y axes, meaning that it no longer follows the left hand convention, by default the Y axis is 90 degrees clockwise of the X axis, but in this case it is counter clockwise, because of the swapped axes, the scale is extracted in a way that assumes the left hand convention

Remember that scale from a transform is calculated not stored, it makes some assumptions, like certain correctness or rather conventions

[capnm]

Thanks a lot. I'm starting to not know what my left and right hands are in the matrix. 🔃 🔁 🔄 Godot obviously prefers 3D and 2D objects from behind :) Personally, I would rather break whatever compatibility and fix it once forever ...

[AThousandShips]

No problems! I had a hard time getting my head around it for a while, and my right wrist is all sore from trying to work out the left and right hand rules, note that 3D in Godot is right hand, but 2D is left hand, very confusing

Unfortunately we can't do that, at least not for some time, I suspect not until 5.0 as it's an easy thing to resolve with documentation clarification, and doesn't break anything, as long as you compensate for it, you don't really need the whole transform anyway as it doesn't scale or skew, we could just return a Vector3 with a rotation alone

See if the documentation update in the linked PR is clear to you if you wouldn't mind

[botero-dev]

I just tried the PathFollow2D Node and it indeed does what I expected to do. I guess I didn't try it because I was focused on the text transform effect PR.

I think we should just make this function just return a good transform and match the behavior of PathFollow2D. I get the idea of using a Transform2D and then reading Y and X axis as forward and right, but the convenience of just using the transform to slap it into some visual element is too good to let it pass.

I like that PathFollow2D chooses X+ for forward. It is my preferred way of solving the problem. It makes it seamless to integrate with the work I did in the linked PR for flowing the text over a path.

I think that, given the use case is currently broken, and the docs previously had no suggestion on how to use this. I feel we should do an appropiate fix now instead of documenting the quirkiness of current implementation.

I can do a PR later today or tomorrow with my suggested fix.