Simulating a Photon

[Ghiraa]

Hi, I would like to start by mentioning I am a complete beginner at Mitsuba. I am using it for my final-year project. What I am trying to do is to have multiple sensors along a cylinder and check if I can capture a photon on only one sensor. Currently, I created a cylinder, and placed along its edge sensors equally distributed with their height equal to the one of the cylinder. To simulate a photon, I am using a spotlight with a narrow but powerful beam which points to one part of the cylinder. Although light is captured by only one sensor(which is good), I feel like something is wrong. Why is that square that big? Also, what's going on with the green bit? Btw, I am using 'pixel_format': 'luminance'. I guess somehow that's some light that's not as intense as the yellow bit, but if I do the beam narrower I can't capture it no matter how much is the intensity.

This is what each sensor captures:

And this is how they look like concatenated:

Thank you!

[njroussel]

Hi @Ghiraa

I'm not totally sure what is going on either. My guess would be that the spot is indeed too wide and currently occupies two pixels.
Also, make sure you're using a box reconstruction filter on your film.

I'm assuming you're using the ptracer integrator?
It shouldn't be too hard to replicate some of its logic to debug what is happening here - or you can directly add prints into the integrator if you're compiling the project yourself and using scalar_rgb'. Particularly, this snippet: https://github.com/mitsuba-renderer/mitsuba3/blob/master/src/integrators/ptracer.cpp#L143-L159. In short, it picks a point on the emitter (in your case the spot` point), and tries to connect it to the sensor.

Currently, I created a cylinder, and placed along its edge sensors equally distributed with their height equal to the one of the cylinder

Just to double-check, what do you mean by sensor's height? Its field of view?

[Ghiraa]

I was not using ptracer :). I solve the issue thank you!