How to sample random points in a scene and generate rays from them

[ioissss]

I'm doing an experiment where I want to randomly sample N points in a scene, which cannot be inside a closed object, and generate random rays by randomly sampling directions on the sampled points.

I've tried using the mitsuba.shape.sample_position() function to get the sampled point P, but I'm not quite sure if this point P is located in the local coordinates of the shape, and if it is how do I translate it to world coordinates, I can't get the to_world transform of the shape.

    samples = []
    bsdfs = []
    num_samples = int(max(num_samples/len(scene.shapes()),1))
    for shape in scene.shapes():
        for _ in range(num_samples):
            sample = shape.sample_position(time=0,sample=sampler.next_2d())
            to_world = shape.
            si = mi.SurfaceInteraction3f()
            si.p = sample.p
            si.sh_frame = mi.Frame3f(sample.n)
            si.uv = sample.uv
            specular_reflectance = shape.eval_attribute(name="specular_reflectance",si=si)
            diffuse_reflectance = shape.eval_attribute(name="diffuse_reflectance",si = si)
            samples.append(sample)
            bsdfs.append(shape.bsdf())

Also, does every type of bsdf have specular_reflectance and diffuse_reflectance？ and how should I get the corresponding value at the sampling point?

[njroussel]

Hi @ioissss

I'm assuimg you want to sample points on surfaces. I'd recommend a two-level scheme, where you first sample a shape, and then sample a point on the shape using shape.sample_position().

To sample a shape, you can build a 1D disrete distibution from the shapes' surface areas.

Once a shape is sampled, the sample_position() will return a point in world-space.

No, not every BSDF has those two components. If you keep track of the shape you sampled from, you can request its BSDF, with shape.bsdf().

[ioissss]

thank you for your reply, i have solved this problem now