How to render a triangle using Qt3D

[cjsb]

Hi,

how can I render a triangle using Qt3D (C++) without loading a obj file?

[A Former User]

@cjsb Hi, welcome to the Qt forum! You can create your own Qt3D::QGeometry-derived class. There is an example for this, although I must admit, the example is quite complicated. See Qt3D: Tessellation Modes QML Example, especially tessellatedquadmesh.cpp Example File.

[cjsb]

Thanks Wieland,

I will try to create the QGeometry. With the new QGeometry done, how can I tell the application to render it? I am not using the QML based code =x

[A Former User]

Once you got that first thing complete, you need to build a class that inherits from Qt3DRender::QGeometryRenderer. Here is some code I wrote a while ago that demonstrates that:

header

#ifndef FEMMESH_H
#define FEMMESH_H

#include <Qt3DRender/qgeometryrenderer.h>

class MeshData;

class FemMesh : public Qt3DRender::QGeometryRenderer
{
    Q_OBJECT
public:
    explicit FemMesh(Qt3DCore::QNode *parent = 0);
    ~FemMesh();

public slots:
    void setFromMeshData(MeshData *meshData);
    void clear();

    void init();
};

#endif // FEMMESH_H

source

#include "femmesh.h"

#include "meshdata.h"
#include "femmeshgeometry.h"

#include "meshdataloader.h"

FemMesh::FemMesh(QNode *parent)
    : Qt3DRender::QGeometryRenderer(parent)
{
}


FemMesh::~FemMesh()
{
    QNode::cleanup();
}


void FemMesh::setFromMeshData(MeshData *meshData)
{
    clear();
    FemMeshGeometry *geom = new FemMeshGeometry(this);
    geom->setFromMeshData(meshData);
    setInstanceCount(1);
    setBaseVertex(0);
    setBaseInstance(0);
    setPrimitiveType(Qt3DRender::QGeometryRenderer::Triangles);
    // setPrimitiveCount has a misleading name.
    // It takes this: (number of triangles) * (number of vertices per triangle)
    setPrimitiveCount(meshData->triangleCount() * 3);
    setGeometry(geom);
}


void FemMesh::clear()
{
    if (geometry())
        geometry()->deleteLater();
    setGeometry(Q_NULLPTR);
}


void FemMesh::init()
{
    MeshDataLoader mdl;
    MeshData * meshData = mdl.load(QUrl(QStringLiteral("qrc:/assets/a.off")));
    setFromMeshData(meshData);
  //  meshData->dbg(); // print mesh data to debug output
    delete meshData;
}

With the two classes at hand you can write a complete Qt3D application in C++, like in the Qt 3D: Simple C++ Example.

[cjsb]

Great, I will try to create one.

Can I have access to the meshdata, femmeshgeometry and meshdataloader? Because in the end, I want to render a mesh and those code might help a lot.

[A Former User]

@cjsb I've sent you a chat message with a download link. Also, there is a thread related to that code: https://forum.qt.io/topic/64012/mesh-color

[iLya84a]

Hi @cjsb! I've created two small examples.
The first example is pure C++. It is exactly you are looking for - https://github.com/iLya84a/qt3d/tree/master/custom-mesh-cpp
The second one is a Qml-based app - https://github.com/iLya84a/qt3d/tree/master/custom-mesh-qml