WMesh face shading artifacts in OpenCV - c++

Question

I'm using OpenCV v4.4.0 with gcc v10.1 on Ubuntu 18.04.5. This code snippet:
using namespace std;
using namespace cv;
using namespace cv::viz;
......
vector<Vec3f> points{{0, 0, 0}, {1, 0, 0}, {1, 1, 0}, {0, 1, 0}, {2, 0, 0}, {2, 1, 0}};
vector<int> faces{4, 0, 1, 2, 0, 4, 0, 2, 3, 0, 5, 1, 4, 5, 2, 1};
Viz3d window("Mesh");
WMesh mesh(points, faces);
window.setBackgroundColor(Color::gray());
mesh.setColor(Color::indigo());
mesh.setRenderingProperty(OPACITY, 0.4);
mesh.setRenderingProperty(SHADING, SHADING_FLAT);
mesh.setRenderingProperty(REPRESENTATION, REPRESENTATION_SURFACE);
window.showWidget("mesh", mesh);
window.spin();
creates and displays this planar mesh:
The square on the left is defined as 2 triangles and is shaded uniformly, but the square on the right, which is defined as a single quadrilateral face is not shaded uniformly. Again, the mesh is planar. Why the non-uniformity?
It becomes even more non-uniform when I change shading from SHADING_FLAT to SHADING_GOURAUD:
Can someone explain what is going on here? I know that quadrilateral faces are converted to triangles, but why the shading is non-uniform?
EDIT
As noted by Матвей Вислоух in his answer below, I intended to use:
vector<int> faces{3, 0, 1, 2, 3, 0, 2, 3, 4, 1, 4, 5, 2};
which properly defines two triangular and one quadrilateral face. This solves the problem of artifacts in the left half, but they still remain in the right half:

Answer 1

vector<int> faces{3, 0, 1, 2, 3, 0, 2, 3, 4, 1, 4, 2, 5};
There is a specific rule how opencv unpacks indices from polygon.
if there is a polygon : (5, 1, 4, 5, 2, 0) it consists of 3 triangles: (1,4,5) , (4, 5, 2), (5, 2, 0)
OLD:
vector faces{4, 0, 1, 2, 0, 4, 0, 2, 3, 0, 5, 1, 4, 5, 2, 1};
This means you draw 3 polygons: 2 quads and 1 pentagon.
But by indices I guess that you want to draw 2 triangles, and one quad, so try this:
vector faces{3, 0, 1, 2, 3, 0, 2, 3, 4, 1, 4, 5, 2};