Add a QTextureMaterial to a custom mesh - qt

Question

I have a custom mesh (created in blender) that I insert into Qt3D using the following code:
QMesh *mesh = new QMesh(rootEntity);
mesh->setSource(QUrl::fromLocalFile(baseUrl+"mesh.obj"));
This works fine; I can add it to an entity with a material and everything.
Then I create a custom material using a texture loaded from a .png. I do this using the following code:
Qt3DRender::QTextureLoader *loader = new Qt3DRender::QTextureLoader(rootEntity);
Qt3DExtras::QTextureMaterial *material = new Qt3DExtras::QTextureMaterial(rootEntity);
loader->setSource(QUrl::fromLocalFile(baseUrl+"pattern.jpg"));
material->setTexture(loader);
This also works fine. When I add this material to a built-in Qt mesh (e.g. QPlaneMesh or QSphereMesh) it shows perfectly on the surface as one would expect.
However - now comes the problem - if I add it with the QMesh specified above, the mesh just gets one homogeneous color which seems to be the average over the colors in the pattern. Here you can see what I mean: both objects have the same material. The top one is inserted externally while the bottom one is a QPlaneMesh.
Can someone explain me why that is the case? And is there a way to successfully add textures to custom meshes?
Note: I have tried this with 2D and 3D meshes and it is the same outcome.
Note 2: I have also tried it with diferent images and it still just gets one homogeneous average color.
UPDATE: I tried (following the suggestion in the answer) to add a texture attribute to the geometry of my imported mesh like the following:
Qt3DCore::QEntity *entity = new Qt3DCore::QEntity(rootEntity);
QMesh *mesh = new QMesh(entity);
mesh->setSource(QUrl::fromLocalFile(baseUrl+"mesh.obj"));
const int stride = (3 + 2 + 3 + 4) * sizeof(float);
QSize resolution = QSize(2,2);
const int nVerts = resolution.width() * resolution.height();
QAttribute *texCoordAttr = new QAttribute(mesh->geometry());
Qt3DRender::QBuffer *vertexBuffer = new Qt3DRender::QBuffer(mesh->geometry());
texCoordAttr->setName(QAttribute::defaultTextureCoordinate1AttributeName());
texCoordAttr->setVertexBaseType(QAttribute::Float);
texCoordAttr->setVertexSize(2);
texCoordAttr->setAttributeType(QAttribute::VertexAttribute);
texCoordAttr->setBuffer(vertexBuffer);
texCoordAttr->setByteStride(stride);
texCoordAttr->setByteOffset(3*sizeof(float));
texCoordAttr->setCount(nVerts);
vertexBuffer->setDataGenerator(QSharedPointer<PlaneVertexBufferFunctor>::create(1.0f,1.0f,resolution, false)); //these input values (width, height, resolution, mirrored) are probably the cause of the problem
mesh->geometry()->addAttribute(texCoordAttr); //it crashes here
entity->addComponent(mesh);
entity->addComponent(transform);
entity->addComponent(material);
I created the functor for setDataGenerator like in the QPlaneMesh code. Now I am suspecting the segmentation fault is because of sizing mismatch. So how can I get the correct width and height of an external mesh from its QGeometry? And what else might be wrong here?

Answer 1

It looks like the mesh is missing the texture coordinates. When you open the file with a text editor, do you see the key vt somewhere? Those are the texture coordinates. You can read about the format here.
If you still want the obj file that you have, you have to add texture coordinates if it doesn't have any. It's probably best to open the file in Blender and use its texture mapper - at least for more complex meshes. Guessing which vertex needs which texture coordinate is not really feasible.
The texture coordinates work as follows:
If you have an image of, say 500 by 400 pixels, the texture coordinate (0.7, 0.3) is (500 * 0.7, 400 * 0.3) = (350, 120), meaning that the vertex which has that texture coordinate will receive the color value of the pixel at (350, 120). Values inside a triangle will get interpolated.
If your obj file comes along with a mtl file then it probably already has texture coordinates. If you want to load this mtl file use the QSceneLoader and add it to its parent QEntity to display everything.