setVertexCount API of QGeometryRenderer and its effect on ray casting results - qt

Question

I create a wireframe mesh of two lines between three points:
By these functions:
Qt3DRender::QGeometryRenderer *Utils::createWireframeMesh()
{
Qt3DRender::QGeometryRenderer *mesh = new Qt3DRender::QGeometryRenderer();
Qt3DRender::QGeometry *geometry = new Qt3DRender::QGeometry(mesh);
Qt3DRender::QBuffer *vertexDataBuffer = new Qt3DRender::QBuffer(Qt3DRender::QBuffer::VertexBuffer,
geometry);
Qt3DRender::QBuffer *indexDataBuffer = new Qt3DRender::QBuffer(Qt3DRender::QBuffer::IndexBuffer,
geometry);
QByteArray vertexBufferData;
QByteArray indexBufferData;
int vertexCount = 3; // Three vertices at (0, -1, 0) and (1, 0, 0) and (0, 1, 0)
int lineCount = 2; // Two lines between three vertices
vertexBufferData.resize(vertexCount * 3 * sizeof(float));
indexBufferData.resize(lineCount * 2 * sizeof(ushort));
// Arrow triangle is 2D and is inside XY plane
float *vPtr = reinterpret_cast<float *>(vertexBufferData.data());
vPtr[0] = 0.0f; vPtr[1] = -1.0f; vPtr[2] = 0.0f; // First vertex at (0, -1, 0)
vPtr[3] = 1.0f; vPtr[4] = 0.0f; vPtr[5] = 0.0f; // Second vertex at (1, 0, 0)
vPtr[6] = 0.0f; vPtr[7] = +1.0f; vPtr[8] = 0.0f; // Third vertex at (0, 1, 0)
ushort *iPtr = reinterpret_cast<ushort *>(indexBufferData.data());
iPtr[0] = 0; iPtr[1] = 1; // First line from index 0 to index 1
iPtr[2] = 1; iPtr[3] = 2; // Second line from index 1 to index 2
vertexDataBuffer->setData(vertexBufferData);
indexDataBuffer->setData(indexBufferData);
addPositionAttributeToGeometry(geometry, vertexDataBuffer, vertexCount);
addIndexAttributeToGeometry(geometry, indexDataBuffer, lineCount * 2);
mesh->setInstanceCount(1);
mesh->setIndexOffset(0);
mesh->setFirstInstance(0);
// How to set vertex count here?
mesh->setVertexCount(vertexCount);
mesh->setPrimitiveType(Qt3DRender::QGeometryRenderer::Lines);
mesh->setGeometry(geometry);
return mesh;
}
void Utils::addPositionAttributeToGeometry(Qt3DRender::QGeometry *geometry,
Qt3DRender::QBuffer *buffer, int count)
{
Qt3DRender::QAttribute *posAttribute = new Qt3DRender::QAttribute();
posAttribute->setAttributeType(Qt3DRender::QAttribute::VertexAttribute);
posAttribute->setBuffer(buffer);
posAttribute->setDataType(Qt3DRender::QAttribute::Float);
posAttribute->setDataSize(3);
posAttribute->setByteOffset(0);
posAttribute->setByteStride(0);
posAttribute->setCount(count);
posAttribute->setName(Qt3DRender::QAttribute::defaultPositionAttributeName());
geometry->addAttribute(posAttribute);
}
void Utils::addIndexAttributeToGeometry(Qt3DRender::QGeometry *geometry,
Qt3DRender::QBuffer *buffer, int count)
{
Qt3DRender::QAttribute *indexAttribute = new Qt3DRender::QAttribute();
indexAttribute->setAttributeType(Qt3DRender::QAttribute::IndexAttribute);
indexAttribute->setBuffer(buffer);
indexAttribute->setDataType(Qt3DRender::QAttribute::UnsignedShort);
indexAttribute->setDataSize(1);
indexAttribute->setByteOffset(0);
indexAttribute->setByteStride(0);
indexAttribute->setCount(count);
geometry->addAttribute(indexAttribute);
}
In above code, I tried three different statements at this line:
// How to set vertex count here?
mesh->setVertexCount(vertexCount);
mesh->setVertexCount(vertexCount * 2);
mesh->setVertexCount(vertexCount * 3);
With these results - I do some ray casting in my 3D scene which are surprisingly affected too:
Documentation explains vertexCount property of Qt3DRender::QGeometryRenderer as:
vertexCount : int
Holds the primitive count.
In my case, primitive count is line count, so I tried it but only one line is drawn:
I'm confused about setVertexCount API. Can anybody give me a hint?

Answer 1

vertexCount is the same value that you would pass to glDrawArrays or glDrawElements, ie it's the number of vertices involved in the drawing. Since you're using indexed rendering, that would typically be the number of indexes (assuming you're drawing all in data in the index array). So in the case above, it should be 4.
Please note we recently fixed a bug with line picking when using primitive restart, but that doesn't affect the code you included above.