[Interest] Qt3D: Wireframe

[Sean Harmer]

Hi,

On 13/04/2017 06:15, Ch'Gans wrote:
> On 13 April 2017 at 14:38, Ch'Gans <chgans at 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.harmer at kdab.com | Managing Director UK
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