[Interest] Qt3D: Wireframe

Ch'Gans chgans at gna.org
Thu Apr 13 04:38:48 CEST 2017


I would like to be able to select at run-time how my entities are
rendered (ideally on a per entity basis).
I would like to have 3 choices:
- 1. wireframe
- 2. shaded
- 3. shaded + wireframe

I had a look at the wire frame QML example, and translated it to C++.
The code is working. I have simplified the fragment shader, as i don't
need fancy light effect, other than a shaded color per face.
These shaders allow me to achieve 2 & 3. But my feeling is that i need
something completely different to render a 'transparent' wireframe
Reading through OpenGL materials, it looks like i need things like
glPolygonMode, reading through Qt3D leads me to

But i am having problems connecting the dots.
PrimitiveType is a property of QGeometryRenderer, which means that it
is closely related with my buffer data and how the data are arranged
within the buffer.
So If i want to switch from 'shaded' to wireframe I need to change my
data (and my shader code too), surely there is to be a way to use the
same data buffer to achieve the 3 rendering modes. Should I use a 2
passes rendering? The first one renders (or not) the face shades and
the second one renders (or not) the wireframes? Actually this should
even be a 3 passes: back of wireframe (or not), shaded faces (or not)
and front of wireframe (or not)...
Is this the right approach? Or should I instead define 3 materials and
simply switch the material as i want a different drawing style?

Last thing is that my input data are triangulated polygon with holes,
but i have as well access to the original polygon with holes alone.
And i actually don't want to render the internal (triangulation) edges
of my polygonal faces (up and down) or rectangular faces (side ones).
So basically in shaded mode, i only need to calculate the color per
geometric face (as oppose to triangular face) ...

PS: I've spent serious time yesterday reading through
https://learnopengl.com, although I think i now understand (way
better) how OpenGL works and how to use it, Qt3D seems to be way more
than a convenience wrapper around the OpenGL API.
I couldn't find a place in Qt3D where you would manipulate OpenGL
directly (eg calling glFooBar()). I am not even sure if it was design
too do this sort of things.


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