How do I draw a simple line in Qt3D?

From the video you linked, I came up with the code below (also posted in Qt forums: https://forum.qt.io/topic/66808/qt3d-draw-grid-axis-lines/3).

First, you need to create your QGeometry. As it is a simple line, it is only composed of 2 vertices (start point, end point), and 2 indices (that links the vertices). To do so, you need to create 2 QByteArray and store them into a QBuffer. In the first one, you store the 2 vertices (x, y, and z coordinates for each). In the second, you just say that you want to link your first vertex to the second. As we use Qt3DRender::QGeometryRenderer::Lines on the renderer, only 2 indices are required.

Once it is done, you just have to put your QGeometry in a QGeometryRenderer to have a mesh, and put the mesh in a QEntity so it appears in the tree and it is rendered.

#include <Qt3DCore/QEntity>
#include <Qt3DCore/QTransform>
#include <Qt3DExtras/QPhongMaterial>
#include <Qt3DRender/QAttribute>
#include <Qt3DRender/QBuffer>
#include <Qt3DRender/QGeometry>

void drawLine(const QVector3D& start, const QVector3D& end, const QColor& color, Qt3DCore::QEntity *_rootEntity)
{
    auto *geometry = new Qt3DRender::QGeometry(_rootEntity);

    // position vertices (start and end)
    QByteArray bufferBytes;
    bufferBytes.resize(3 * 2 * sizeof(float)); // start.x, start.y, start.end + end.x, end.y, end.z
    float *positions = reinterpret_cast<float*>(bufferBytes.data());
    *positions++ = start.x();
    *positions++ = start.y();
    *positions++ = start.z();
    *positions++ = end.x();
    *positions++ = end.y();
    *positions++ = end.z();

    auto *buf = new Qt3DRender::QBuffer(geometry);
    buf->setData(bufferBytes);

    auto *positionAttribute = new Qt3DRender::QAttribute(geometry);
    positionAttribute->setName(Qt3DRender::QAttribute::defaultPositionAttributeName());
    positionAttribute->setVertexBaseType(Qt3DRender::QAttribute::Float);
    positionAttribute->setVertexSize(3);
    positionAttribute->setAttributeType(Qt3DRender::QAttribute::VertexAttribute);
    positionAttribute->setBuffer(buf);
    positionAttribute->setByteStride(3 * sizeof(float));
    positionAttribute->setCount(2);
    geometry->addAttribute(positionAttribute); // We add the vertices in the geometry

    // connectivity between vertices
    QByteArray indexBytes;
    indexBytes.resize(2 * sizeof(unsigned int)); // start to end
    unsigned int *indices = reinterpret_cast<unsigned int*>(indexBytes.data());
    *indices++ = 0;
    *indices++ = 1;

    auto *indexBuffer = new Qt3DRender::QBuffer(geometry);
    indexBuffer->setData(indexBytes);

    auto *indexAttribute = new Qt3DRender::QAttribute(geometry);
    indexAttribute->setVertexBaseType(Qt3DRender::QAttribute::UnsignedInt);
    indexAttribute->setAttributeType(Qt3DRender::QAttribute::IndexAttribute);
    indexAttribute->setBuffer(indexBuffer);
    indexAttribute->setCount(2);
    geometry->addAttribute(indexAttribute); // We add the indices linking the points in the geometry

    // mesh
    auto *line = new Qt3DRender::QGeometryRenderer(_rootEntity);
    line->setGeometry(geometry);
    line->setPrimitiveType(Qt3DRender::QGeometryRenderer::Lines);
    auto *material = new Qt3DExtras::QPhongMaterial(_rootEntity);
    material->setAmbient(color);

    // entity
    auto *lineEntity = new Qt3DCore::QEntity(_rootEntity);
    lineEntity->addComponent(line);
    lineEntity->addComponent(material);
}