Units for scene files about pbrt-v4

[rainbow-app]

I'm no substitute for mmp, but here are my 5 cents.

Often it doesn't make huge sense to specify/require "units". Suppose you have algorithm that computes distance travelled by a point with const velosity. The "algorithm" will just spit out the product v*t, and the units will be determined by whatever units have your input data.

I think you can generally assume it's all in SI.

As to light intensity, if it has same RGB (=gray), and if light is point, you can specify it in candellas (cd=lm/sr; it will be luminous intensity then), thus factoring the eye-sensitivity curve into the intensity. The output will be luminance (cd/m^2; for exposure=1s). 100lux is good illumination, at distance 10m you'll need 10000cd. The proper way is to specify intensity in physics-based units (like Watt instead of lumen), over wavelengths, but I never did it. "Point and gray" was my only use case in v3.

Disclaimer: I don't know v4.

[mmp]

@rainbow-app is right: it's all SI. Meters for distance, W/(m^2 sr) for radiance, power and radiant intensity per SI, etc. Time is seconds. I agree that it would be good to be clear about that in the documentation and will make a note to do so.

[mmp]

You seem to be assuming that pixel values will be between [0,1] for display. Strictly speaking (and with pbrt's RealisticCamera), a camera sensor is measuring energy in Joules over each pixel area; see https://www.pbr-book.org/4ed/Cameras_and_Film/Film_and_Imaging#TheCameraMeasurementEquation and equation 5.5. So in general you will need to remap those measurements to your display's range (e.g. by scaling them).

[lobneroO]

@mmp Time is seconds. That probably doesn't hold true for the shutter open/close time, right?

[mmp]

Nope, it does. (Why wouldn't it?)

[lobneroO]

I increased the shutter close to 200 for a scene of mine to get quickly visible results for debugging purposes. This worked well, but in a real camera I would expect a 200 second shutter time to lead to practically just a white image.

Since I am not a photographer, I just did a sanity check on wikipedia, the shutter speeds listed are not really exceeding 1 second and a lot of them are well below that.