Godot: Vector2.reflect() result is unexpected

[cbscribe]

Operating system or device, Godot version, GPU Model and driver (if graphics related):
3.0master build 0dbec3a

When reflecting a vector by a normal, for example to bounce off a surface, the resulting vector isn't reflected as expected (it's reversed):

var v1 = Vector2(100, 100)
var n = Vector2(0, -1)
var v2 = v1.reflect(n)

Expected result: (100, -100)
Actual result: (-100, 100)

As a workaround, I've been using -v1.reflect(n)

[cbscribe]

Actually, now that I've looked at the code, 3.0 has a Vector2.bounce() method that does produce the expected result. Is that intended to replace reflect()?

[ghost]

Reflecting along y axis will flip the sign of x coordinate.

Reflect is just reflection in mathematical sense. Bounce is what you'd get for example if you had a ray that bounces off from a surface

[Boothand]

That looks unexpected to me too.
In this image: https://docs.unity3d.com/StaticFiles/ScriptRefImages/Vec3ReflectDiagram.png
The normal vector in Godot pixel units is Vector2(0, -1).
In-direction is equivalent to Vector2(1, 1), out-direction is Vector2(1, -1).

[ghost]

That image describes a bounce, as I tried to explain above

[ghost]

This is what reflection means

[Boothand]

Then it must be different from other Vector classes I've come across.

In Unity:
`Vector2 v1 = new Vector2(1, 1);

Vector2 normal = new Vector2(0, -1);

Vector2 reflectedVec = Vector2.Reflect(v1, normal);

print(reflectedVec);
`

Output: (1, -1)

[ghost]

In Godot, reflect means mathematical reflection and bounce is physical reflection. Documentation describes this behavior.

Reflect typically refers to mathematical reflection, just like glsl reflect function, because that's what reflecting a vector, an abstract mathematical entity, means. A mathematical vector doesn't move. Reflection of a light from a surface, both represented by time-dependent vectors, ray is different. Unlike light, a pure mathematical vector doesn't move in time.

We don't need to do whatever Unity does.

[groud]

This probably have to be better documented then. The lastest documentation says:

Vector2 reflect ( Vector2 n )
Reflects the vector along the given plane, specified by its normal vector.

I think in fact it use the direction vector instead of the normal.

[ghost]

I think that's an accurate description of vector reflection although could be made more clear

[groud]

Yup, but in OP's case: if reflect uses the normal of the "plane" (in fact a straight line here), with Vector2(0,-1), the plane is the X axis. So the reflection should change the Y coordinate instead of the X one.

[ghost]

Ah, I missed that part, yse I agree, that parameter is the normalized reflection line

[ghost]

You do have a plane for bouncing though,cant checwhat the docs say for that now

[groud]

For the bounce doc:

Vector2 bounce ( Vector2 n )
Bounce returns the vector “bounced off” from the given plane, specified by its normal vector.

Actually I think bounce and reflect are doing more or less the same thing in fact. A "bounced" vector is reflected relatively to the plan it's bouncing on I'd say. Like illustrated here:
http://ogldev.atspace.co.uk/www/tutorial19/reflected_light.png

[ghost]

Great we can them simply say a normal vector along reflection line direction doesn't matter it's same either way

[cbscribe]

I'm still not sure I see a use for the reflect method as written.

If this is the intended behavior then we definitely need to clarify the documentation. Especially since in 2.1 reflect did what bounce now does.

[ghost]

I'm still not sure I see a use for the reflect method as written.

Reflection is a basic affine transformation just like rotation. It allows you to do reflection without constructing a reflection matrix, similar to what Vector2.rotate does. Together they give you the functionality of rotation-reflection matrices (that is, 2D orthogonal matrices, O(2)).

You can read more about it here

[jack-vo]

In the stable release of Godot3, reflect and bounce always return wrong value

var test = Vector2(100, 100)
var reflection = Vector2(0, -1).reflect(test) # returns (0, 0)
var bouncing = Vector2(0, 1).bounce(test); # return (-0, -0)

[akien-mga]

@jack-vo No, it's just that the syntax changed: http://docs.godotengine.org/en/stable/classes/class_vector2.html#class-vector2-reflect

It's no longer normal.reflect(vector) but vector.reflect(normal), so you should do test.reflect(Vector2(0, -1)).

[aaronfranke]

Yup, but in OP's case: if reflect uses the normal of the "plane" (in fact a straight line here), with Vector2(0,-1), the plane is the X axis. So the reflection should change the Y coordinate instead of the X one.

So then the problem seems to be that we're defining one dimensional lines with normal vectors instead of a vector in the actual direction of the line? Normal vectors for planes makes sense for 3D but for one dimensional lines in 2D it makes more sense to just define the line.

So, are we aiming for simpler 2D math, or consistency between 2D and 3D math?

And obviously this would be a breaking change so it'd be best to wait until 4.0 to make changes (though we can still discuss them now)

[vnen]

So then the problem seems to be that we're defining one dimensional lines with normal vectors instead of a vector in the actual direction of the line? Normal vectors for planes makes sense for 3D but for one dimensional lines in 2D it makes more sense to just define the line.

Not really, the problem (which is not really a problem but a more of a misunderstanding) is that reflect() does a mathematical reflection (which is more like a "flip") and not a physical reflection, like a ray reflecting of a mirror (which can be accomplished by the bounce() function).

The referred comment is more about how it's written in the docs. Using a normal vector in this case is perfectly fine, even in 2D.

[Chaosus]

cc @mhilbrunner Could you fix a documentation of reflect/bounce ?

[romlok]

Can I request that any documentation clarification come with at least one diagram which visualises the difference between bounce() and reflect(), and/or some practical examples of what each is useful for? I still don't understand how they would differ when operating on a vector.

It probably doesn't help that existing documentation seems to use the terms "reflect" and "bounce" interchangeably. Eg. Using KinematicBody2D or Vector Math.

[DleanJeans]

Just ran into this problem. I suggest renaming reflect() to mirror().

[vnen]

I don't think mirror() will clarify. I would call it flip().