Radiancemeter sensor plugin

[leroyvn]

This commit ports Mitsuba 1's radiance meter sensor.

Co-authored-by: @schunkes

[Speierers]

Hi @leroyvn,

Thanks a lot for opening this PR. Really happy to see people porting/implementing more plugins in Mitsuba 2!

The code looks great to me, and the tests as well. Just one minor comment (see above).

I am curious to know if you have encountered any bugs when rendering 1x1 images with this plugin. Not sure this is something we have ever tested.

[leroyvn]

I had similar behaviour as in #82 with both the perspective and radiancemeter plugins, but I don't think it's due to the film size.

Forcing the use of a box reconstruction filter could be good. Maybe I should change something to implement this behaviour?

[Speierers]

Forcing the use of a box reconstruction filter could be good. Maybe I should change something to implement this behaviour?

Indeed it would be nice to have a warning in the constructor if the film->reconstruction_filter() isn't a box filter. Or maybe it is enough to check whether the radius of that filter is <= 0.5f?

One other thing: could we add a test where we construct a simple scence (e.g. no shapes, just a constant emitter) with this sensor, and call scene.integrator().render()?

[leroyvn]

Sure, I will do do that.

[leroyvn]

There it is! I used numpy to check the results, is that okay?

[Speierers]

There it is! I used numpy to check the results, is that okay?

Yes, numpy is fine when dealing with Bitmap.

One minor comment regarding the filter radius check (see above) and it would be great to squash those commits into one after that.

Then this should be ready to launch! 🚀

Thanks again @leroyvn