Godot create an extra vertex?

[Torguen]

Godot version:

3.3.2 stable mono

OS/device including version:

win10

Issue description:

When I convert a sprite to polygon2D godot creates 5 vertices: 0-1-2-3-4
I think it should create 4 vertices.

Steps to reproduce:
Convert a sprite to polygon2d with the default values.

Minimal reproduction project:

[clayjohn]

When you convert a Sprite to a Polygon2D Godot tries to fit the Polygon2D around the opaque parts of the image. Because the corners of the sprite are transparent, the resulting polygon is not simply a quad with 4 corners.

You can see this very clearly in the docs for converting a sprite into a mesh

[Torguen]

When you convert a Sprite to a Polygon2D Godot tries to fit the Polygon2D around the opaque parts of the image. Because the corners of the sprite are transparent, the resulting polygon is not simply a quad with 4 corners.

You can see this very clearly in the docs for converting a sprite into a mesh

Ok, now I understand the transparency and leaving that aside I can see the following:
I saw a youtube video and now I was able to get a polygon of 4 vertices correctly but the deformation is not symmetric.
Notice the texture - shouldn't it be the same on both sides?

[Torguen]

[Zylann]

This is a common issue with triangles and interpolation (graphics cards work with triangles). The only way to workaround this is to use a shader that performs quadrilateral interpolation instead https://www.reedbeta.com/blog/quadrilateral-interpolation-part-1/

[Torguen]

So there is no way to get this (krita) in godot?
without using that shader.

[Zireael07]

Make a square polygon instead of using convert sprite to polygon?

[Torguen]

Make a square polygon instead of using convert sprite to polygon?

It does not work, they have the same behavior as the previous case.

This is the image in case anyone wants to try

[clayjohn]

The only way to workaround this is to use a shader that performs quadrilateral interpolation instead https://www.reedbeta.com/blog/quadrilateral-interpolation-part-1/

The other way to work around it is to subdivide the mesh to use more internal vertices. If your mesh looks like a grid with 4 squares it will also interpolate correctly (for this texture, more detailed textures may need more subdivisions)

[Torguen]

Why isn't there a node to 2D (or some kind of polygon) that does the same thing krita does in the gif?
Krita uses four points or vertices and you get the result that (I think) most are looking for when using this.

[Zylann]

So there is no way to get this (krita) in godot?

There is, what I told you, it's just not the default in realtime rendering.
You can also subdivide it, which doesnt eliminate the issue but makes it way less noticeable, at the cost of polycount.

[Torguen]

But I don't know how to create that shader, and I will also have to add the shader to all the nodes that need it, I mean you say there is "there is" but there isn't if you don't create the shader you indicated before.