Cross section computation using the Discrete Integral Transform (DIT) for rapid spectral synthesis
==================================================================================================

We demonstarte the Discrete Integral Transform (DIT) method proposed by
D.C.M van den Bekerom and E.Pannier. DIT takes advantage especially for
the case that the number of the molecular line is large (typically >
10,000). We here compare the results by DIT with the direct computation
(LPF).

.. code:: ipython3

    import numpy as np
    import matplotlib.pyplot as plt

Here, we use FP64, but if you want you can use FP32 (but slightly large
errors):

.. code:: ipython3

    from jax import config
    
    config.update("jax_enable_x64", True)

.. code:: ipython3

    from exojax.spec.hitran import line_strength
    from exojax.spec.hitran import doppler_sigma
    from exojax.spec.hitran import gamma_hitran
    from exojax.spec.hitran import gamma_natural
    from exojax.utils.grids import wavenumber_grid
    from exojax.utils.constants import Tref_original
    from exojax.spec import api
    
    # Setting wavenumber bins and loading HITRAN database
    nus, wav, resolution = wavenumber_grid(
        1900.0, 2300.0, 350000, unit="cm-1", xsmode="dit"
    )
    mdbCO = api.MdbHitran(
        "CO", nus, isotope=1, gpu_transfer=True
    )  # here we use the isotope=1 (12C16O) in DIT.
    
    # set T, P and partition function
    Mmol = mdbCO.molmass
    Tfix = 1000.0  # we assume T=1000K
    Pfix = 1.0e-3  # we compute P=1.e-3 bar
    Ppart = Pfix  # partial pressure of CO. here we assume a 100% CO atmosphere.
    qt = mdbCO.qr_interp_lines(
        Tfix, Tref_original
    )  # use all isotopes as a partition function
    
    # compute Sij, gamma_L, sigmaD
    Sij = line_strength(
        Tfix, mdbCO.logsij0, mdbCO.nu_lines, mdbCO.elower, qt, Tref_original
    )
    gammaL = gamma_hitran(
        Pfix, Tfix, Ppart, mdbCO.n_air, mdbCO.gamma_air, mdbCO.gamma_self
    ) + gamma_natural(mdbCO.A)
    sigmaD = doppler_sigma(mdbCO.nu_lines, Tfix, Mmol)


.. parsed-literal::

    xsmode =  dit
    xsmode assumes ESLIN in wavenumber space: xsmode=dit
    ======================================================================
    The wavenumber grid should be in ascending order.
    The users can specify the order of the wavelength grid by themselves.
    Your wavelength grid is in ***  descending  *** order
    ======================================================================
    radis engine =  vaex


DIT uses a grid of sigmaD, gammaL, and wavenumber.
set_ditgrid.ditgrid_log_interval makes a 1D grid for sigmaD and gamma.

.. code:: ipython3

    from exojax.spec.set_ditgrid import ditgrid_log_interval
    
    sigmaD_grid = ditgrid_log_interval(sigmaD)
    gammaL_grid = ditgrid_log_interval(gammaL)
    
    # we can change the resolution using res option
    # sigmaD_grid=set_ditgrid(sigmaD,res=0.1)
    # gammaL_grid=set_ditgrid(gammaL,res=0.1)

.. code:: ipython3

    # show the grids
    plt.plot(sigmaD, gammaL, ".")
    for i in sigmaD_grid:
        plt.axvline(i, lw=1, alpha=0.5, color="C1")
    for i in gammaL_grid:
        plt.axhline(i, lw=1, alpha=0.5, color="C1")



.. image:: Cross_Section_using_Discrete_Integral_Transform_files/Cross_Section_using_Discrete_Integral_Transform_8_0.png


We need to precompute the contribution for wavenumber. Also, pmarray is
needed. These can be computed using init_dit.

.. code:: ipython3

    from exojax.spec import initspec
    
    cnu, indexnu, pmarray = initspec.init_dit(mdbCO.nu_lines, nus)

Then, let’s compute a cross section!

.. code:: ipython3

    from exojax.spec.dit import xsvector
    
    xs = xsvector(cnu, indexnu, pmarray, sigmaD, gammaL, Sij, nus, sigmaD_grid, gammaL_grid)

Also, we here try the direct computation using Direct-LPF for the
comparison purpose

.. code:: ipython3

    from exojax.spec.opacalc import OpaDirect
    opa = OpaDirect(mdbCO, nus)
    xsv = opa.xsvector(Tfix, Pfix, Ppart)

The difference is <~ 1%.

.. code:: ipython3

    fig = plt.figure(figsize=(10, 5))
    ax = fig.add_subplot(211)
    plt.plot(nus, xs, lw=1, alpha=0.5, label="DIT")
    plt.plot(nus, xsv, lw=1, alpha=0.5, label="Direct LPF")
    plt.legend(loc="upper right")
    plt.ylabel("Cross Section (cm2)")
    ax = fig.add_subplot(212)
    # plt.plot(nus,xsv-xs,lw=2,alpha=0.5,label="precomputed")
    plt.plot(nus, xsv - xs, lw=2, alpha=0.5)
    plt.ylabel("LPF - DIT (cm2)")
    plt.legend(loc="upper left")
    plt.show()


.. parsed-literal::

    /tmp/ipykernel_809841/4022811313.py:11: UserWarning: No artists with labels found to put in legend.  Note that artists whose label start with an underscore are ignored when legend() is called with no argument.
      plt.legend(loc="upper left")



.. image:: Cross_Section_using_Discrete_Integral_Transform_files/Cross_Section_using_Discrete_Integral_Transform_16_1.png