GeometryRenderer Why render lines, but not Triangles - qt

Question

I am trying to proceduraly generate Cube with a GeometryRenderer. Why when GeometryRenderer.primitiveType is set to LineLoop, Lines or TriangleFan everything works, but with Triangles or TrianglesStrip draws absolutely nothing?
PhongMaterial { id: material; }
GeometryRenderer {
id: renderer
primitiveType: GeometryRenderer.LineLoop
instanceCount: 1
geometry: Geometry {
attributes: [
Attribute {
name: defaultPositionAttributeName
attributeType: Attribute.VertexAttribute
vertexBaseType: Attribute.Float
vertexSize: 3
byteOffset: 0
byteStride: 3 * 4
count: 6
buffer: Buffer {
type: Buffer.VertexBuffer
data: new Float32Array([
// #1 triangle
-0.9, -0.5, 0.0,
-0.0, -0.5, 0.0,
-0.45, 0.5, 0.0,
// #2 triangle
0.0, -0.5, 0.0,
0.9, -0.5, 0.0,
0.45, 0.5, 0.0,
])
}
}
]
}
}
Entity {
components: [
renderer,
material
]
}

Answer 1

If I adapt your QML the following way then I can see two triangles:
import QtQuick 2.2
import Qt3D.Core 2.0
import Qt3D.Render 2.0
import Qt3D.Extras 2.0
Entity {
id: root
PhongMaterial { id: material; diffuse: Qt.rgba(0.8, 0., 0., 1.0) }
GeometryRenderer {
id: renderer
primitiveType: GeometryRenderer.Triangles
instanceCount: 1
geometry: Geometry {
Attribute {
name: defaultPositionAttributeName
attributeType: Attribute.VertexAttribute
vertexBaseType: Attribute.Float
vertexSize: 3
byteOffset: 0
byteStride: 3 * 4
count: 6
buffer: Buffer {
type: Buffer.VertexBuffer
data: new Float32Array([
// #1 triangle
-0.9, -0.5, 0.0,
-0.0, -0.5, 0.0,
-0.45, 0.5, 0.0,
// #2 triangle
0.0, -0.5, 0.0,
0.9, -0.5, 0.0,
0.45, 0.5, 0.0,
])
}
}
}
}
components: [ renderer, material ]
}
I suppose the error is related to the wrong hierarchy of GeometryRenderer and Entity.
To get correct lighting you need to give the GeometryRenderer the vertexNormals too.
Edit:
As it is a common question whether a GeometryRenderer always needs an indexBuffer for primitiveType: Triangles, TriangleStrip, and TriangleFan, I'll come up with a somewhat more detailed answer.
A GeometryRenderer of primitiveType: Triangles does not necessarily need an index array (I checked the source code of Qt3D as I was unsure).
The reason you are not seeing your triangles is: You are defining the vertices in the wrong order! Change the order of the vertices in the vertexBuffer so that every three consecutive vertices form a triangle when going counterclockwise around the triangle. The triangle normal will then point at you.
Or, have a look onto your Entity from the opposite direction: you'll see the two triangles.
When using huge buffers and you don't want to repeat large amounts of vertices for memory/efficiency reasons you'll definitely should consider using an indexBuffer.