Hi,
On 13/04/2017 06:15, Ch'Gans wrote:
On 13 April 2017 at 14:38, Ch'Gans <chg...@gna.org> wrote:
Hi,
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
only.
You can do it a number of ways:
1) One approach is indeed to use polygon mode but as you've noticed this
renderstate is missing from Qt 3D but should be easy to add.
2) You could adjust the shader from 3 such that when in the fragment
shader you decide if a fragment is part of the line or part of the
shaded mesh, instead of shading it, you simply call discard() to throw
that fragment away.
3) Use custom geometry to render your wireframe however you see fit.
This may be as simple as using a different index attribute/buffer with
your existing vertex data buffer(s), or you may want to create entirely
new geometry if you want something other than lines joining the original
vertices. For, example if you want an extruded lattice type effect for
your wire frame. You may even be able to get away with simply using
lines as the primitive type instead of triangles. Depends upon your
geometry.
4) Render the same geometry but with an alpha blended material.
5) Probably a bunch of other ways I can't think of without another cup
of tea...
Reading through OpenGL materials, it looks like i need things like
glPolygonMode, reading through Qt3D leads me to
QGeometryRenderer::PrimitiveType.
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)...
No need to render something twice to see its back and front sides. Use a
QCullFace render state with the mode set to NoCulling. By default, it's
set to cull the back faces because most often meshes are closed so
there's no point in rasterizing the back faces as they will never be seen.
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) ...
To exclude some of the triangle edges you need an additional per vertex
attribute to use as a flag. Have a read through
http://strattonbrazil.blogspot.co.uk/2011/09/single-pass-wireframe-rendering_11.html
for the details.
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.
Answering myself partially:
QRenderState provides lot of these. Eg.: glCullFace() => QCullFace,
glFrontFace() => QFrontFace, ...
Bu as far as i can understand OpenGL and Qt3D, i would say that Qt3D
simply doesn't support the OpenGL polygon mode. Maybe because it's
either sort-of deprecated, or there are other (better?) ways to do
this sort of things...
Nope, it just slipped under the radar and you're the first person to ask
for it. Should be simple to add. Take a look at the other render states
if you want to contribute it.
Cheers,
Sean
--
Dr Sean Harmer | sean.har...@kdab.com | Managing Director UK
KDAB (UK) Ltd, a KDAB Group company
Tel. +44 (0)1625 809908; Sweden (HQ) +46-563-540090
Mobile: +44 (0)7545 140604
KDAB - Qt Experts
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