Hi Bili, I'll let others respond in detail, but I wanted to say that a contribution along these lines would be very welcome. This is broadly along the route we'd like to take with SPH simulations for yt 3.4: relying less on the particle octree for as many analysis operations as possible Nathan On Saturday, September 10, 2016, Bili Dong - Gmail <qobilidop@gmail.com> wrote:

Hi all,

I came up with an idea to bring a particle approach to model Ray and AbsorptionSpectrum. But before I move on to implement it in yt, I'd like to let you know what it is and get feedbacks about it.

The situation is that I have an SPH simulation, and I want to model the Ray (in order to get the AbsorptionSpectrum) as accurately as possible. Currently when we create a Ray object, it's always created from the deposited grid. Although it is a good approximation to the true particle representation, it is still not the most accurate way. I'd like to be able to do it in the particle way (like in SPLASH). In the long term, I know that Matt and Meagan is working on a new system for particle dataset. The work I'm going to propose could be thought as lying on top of that, in that the method could be made faster utilizing Matt and Meagon's work, but the main infrastructure would stay the same.

To introduce what I plan to do, let's have a look at the first figure here <http://yt-project.org/docs/dev/analyzing/analysis_modules/light_ray_generato...>. The core concept of a Ray object is the *path length*, *`dl`*. Basically, if we combine the normal fields with the `dl` field, we get a Ray object. Now imagine instead of a ray intersecting a lot of grid cells, we have a ray intersecting a lot of SPH particles. How do we define *`dl`* then? We could define it as the *integral of the SPH kernel along the intersection*! And that's the whole trick. From this we could define a particle Ray that just looks the 'same' as the original grid Ray. Then any analysis built on top of the Ray object, AbsorptionSpectrum for example, don't need to change a lot. They will work in different ways simply when provided with different different kinds of Ray object.

The main difficulty in the implementation is the construction of the particle `dl` field. Currently I'm doing it brutal-forcedly by computing `dl` for all the particles and mask out those with zero values. Matt and Meagan's work will accelerate this by providing the neighboring information, so I could do the computation on a small set of particles then. The brutal-force method is not unbearably slow though. And the computation acceleration could be saved for future work.

I have an external implementation of the particle approach, and have used it in my current research. I have compared results using the particle method and those from Trident and they agree statistically as we expected (thanks Cameron for the help). Now that it looks mature, I'd like to implement it in yt.

If anyone has any comments, opinions and suggestions, I'd like to hear them.

Thanks for reading,

Bili

Great work! I am very excited about this development, as I've seen some of the results that Bili has created so far with this method. I'm fully behind this idea! Cameron On Sat, Sep 10, 2016 at 10:44 AM, Bili Dong - Gmail <qobilidop@gmail.com> wrote:

Hi all,

I came up with an idea to bring a particle approach to model Ray and AbsorptionSpectrum. But before I move on to implement it in yt, I'd like to let you know what it is and get feedbacks about it.

The situation is that I have an SPH simulation, and I want to model the Ray (in order to get the AbsorptionSpectrum) as accurately as possible. Currently when we create a Ray object, it's always created from the deposited grid. Although it is a good approximation to the true particle representation, it is still not the most accurate way. I'd like to be able to do it in the particle way (like in SPLASH). In the long term, I know that Matt and Meagan is working on a new system for particle dataset. The work I'm going to propose could be thought as lying on top of that, in that the method could be made faster utilizing Matt and Meagon's work, but the main infrastructure would stay the same.

To introduce what I plan to do, let's have a look at the first figure here <http://yt-project.org/docs/dev/analyzing/analysis_modules/light_ray_generato...>. The core concept of a Ray object is the *path length*, *`dl`*. Basically, if we combine the normal fields with the `dl` field, we get a Ray object. Now imagine instead of a ray intersecting a lot of grid cells, we have a ray intersecting a lot of SPH particles. How do we define *`dl`* then? We could define it as the *integral of the SPH kernel along the intersection*! And that's the whole trick. From this we could define a particle Ray that just looks the 'same' as the original grid Ray. Then any analysis built on top of the Ray object, AbsorptionSpectrum for example, don't need to change a lot. They will work in different ways simply when provided with different different kinds of Ray object.

The main difficulty in the implementation is the construction of the particle `dl` field. Currently I'm doing it brutal-forcedly by computing `dl` for all the particles and mask out those with zero values. Matt and Meagan's work will accelerate this by providing the neighboring information, so I could do the computation on a small set of particles then. The brutal-force method is not unbearably slow though. And the computation acceleration could be saved for future work.

I have an external implementation of the particle approach, and have used it in my current research. I have compared results using the particle method and those from Trident and they agree statistically as we expected (thanks Cameron for the help). Now that it looks mature, I'd like to implement it in yt.

If anyone has any comments, opinions and suggestions, I'd like to hear them.

Thanks for reading,

Bili

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