Implementing a Custom Repeated Sampler

[juhyeonkim95]

Hi,

I want to implement a new sampler in C++ code, but I'm having difficulties.

What I want to implement is a repeated sampler.

For example, if spp=8, next_1d() of independent sampler creates [r1, r2, r3, r4, r5, r6, r7, r8] where r1-r8 are independent random numbers in [0,1].

I want to implement a repeated sampler that creates [r1, r2, r3, r4, r1, r2, r3, r4] or [r1, r1, r2, r2, r3, r3, r4, r4] (both are okay).

I tried to use dr::repeat or dr::gather, but was unsuccessful.

Any help would be appreciated.

[njroussel]

Hi @juhyeonkim95

Nothing immediately comes to mind as to why this wouldn't be feasible.

I tried to use dr::repeat or dr::gather, but was unsuccessful.

This should have worked, what exactly did you try? The naive implementation would be to just wrap the independent sampler and every next_1d() would simply gather the first half from it with a repeating index.

[juhyeonkim95]

I tried something like this, but got an error message with RuntimeError: jit_var_gather(): cannot gather from a placeholder variable!

Float next_1d(Mask active = true){
        Assert(seeded());
        Float r = m_rng.template next_float<Float>(active);
        if constexpr(dr::is_array_v<Float>){
            UInt32 sample_indices = current_sample_index();
            r = dr::gather<Float>(r, sample_indices / 2);
        }
        return r;
}

[njroussel]

Ah, indeed. In short, we can't use gather inside of a recorded loop (basically the main integrator loop).

The solution would then be to construct the sampler to have the repetition.
The inc variables defines multiples RNS "streams" in the PCG32 sampler. It is initialized here. You should be able to modify it such that is is repeating (it is a public member).

[juhyeonkim95]

It worked well! Thank you so much.