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ocbrdf/brdf_model_M02SeaDAS.py

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+import numpy as np
+import xarray as xr
+
+from brdf_utils import ADF_OCP, solve_2nd_order_poly, drop_unused_coords
+''' Morel et al. (2002) BRDF 校正
+    未包含 R gothic
+    f/Q 查找表与 NASA SeaDAS 中的一致（详见 BRDF_M02SeaDAS.nc 属性）
+    使用 OC4ME 叶绿素反演，进行一次迭代，未应用收敛标准
+'''
+
+""" M02 系数的类 """
+
+
+class Coeffs():
+    def __init__(self, foq):
+        self.foq = foq
+
+
+""" Class for M02 BRDF model """
+class M02SeaDAS:
+    """ Initialise M02 model: BRDF LUT, coeffs, OC4ME parameters, water IOPs LUT
+        Note: bands are fixed and defined at class initilization, but could be initialized in init_pixels if needed
+    """
+
+    def __init__(self, bands, adf=None):
+        if adf is None:
+            adf = ADF_OCP
+
+        # Check required bands are existing, within a 25 nm threshold
+        self.bands = bands
+        threshold = 25.
+        bands_required = [442.5, 490, 510, 560]
+        bands_ref = bands.sel(bands=bands_required, method='nearest')
+        for band_ref, band_required in zip(bands_ref, bands_required):
+            assert abs(band_ref - band_required) < threshold, 'Band %d nm missing or too far' % band_ref
+        self.b442 = float(bands_ref[0].item())
+        self.b490 = float(bands_ref[1].item())
+        self.b510 = float(bands_ref[2].item())
+        self.b560 = float(bands_ref[3].item())
+
+        # Read BRDF LUT and compute default coeffs
+        LUT_OCP = xr.open_dataset(adf % 'M02SeaDAS',engine='netcdf4')
+        self.LUT = xr.Dataset()
+
+        # Homogeneise naming convention with other methods... (PZA --> OZA transformation comes below...)
+        self.LUT['foq'] = LUT_OCP.f_over_q_LUT.rename({'SZA_FOQ':'theta_s',
+                                                       'PZA_FOQ':'theta_v',
+                                                       'RAA_FOQ':'delta_phi',
+                                                       'log_chl_FOQ':'log_chl_foq'})
+
+        # Index of refraction
+        self.n_w = float(LUT_OCP.water_refraction_index.data)
+
+        # Remove trivial aot indexation
+        self.LUT['foq'] = self.LUT['foq'].squeeze()
+
+        #
+        self.coeffs0 = self.interp_geometries(0., 0., 0.)
+        self.coeffs = Coeffs(np.nan)
+
+        # Parameters for the OC4ME chl retrieval
+        self.OC4MEcoeff = LUT_OCP.log10_coeff_LUT.values
+        self.OC4MEepsilon = LUT_OCP.oc4me_epsilon
+        self.OC4MEchl0 = float(LUT_OCP.oc4me_chl0.values)
+        self.niter = LUT_OCP.oc4me_niter
+
+    """ Initialize pixel: coefficient at current geometry and water IOP at current bands """
+
+    def init_pixels(self, theta_s, theta_v, delta_phi):
+        self.coeffs = self.interp_geometries(theta_s, theta_v, delta_phi)
+        
+        # Cache wavelength and chlorophyll bounds for fast access in forward()
+        self.min_wavelength_FOQ = float(np.min(self.coeffs.foq.wavelengths_FOQ))
+        self.max_wavelength_FOQ = float(np.max(self.coeffs.foq.wavelengths_FOQ))
+        self.min_log_chl_foq = float(np.min(self.coeffs.foq.log_chl_foq) / np.log(10))
+        self.max_log_chl_foq = float(np.max(self.coeffs.foq.log_chl_foq) / np.log(10))
+
+    """ Interpolate coefficients """
+    def interp_geometries(self, theta_s, theta_v, delta_phi):
+        # Transform PZA to VZA (Snell's refraction Law)
+        theta_v = np.rad2deg(np.arcsin(np.sin(np.deg2rad(theta_v)) / self.n_w))
+        theta_v_0 = np.clip(theta_v,
+                                   float(np.min(self.LUT.theta_v)),
+                                   float(np.max(self.LUT.theta_v)))
+
+        foq = self.LUT.foq.interp(theta_s=theta_s, theta_v=theta_v_0, delta_phi=delta_phi)
+
+        return Coeffs(foq)
+
+    """ Compute remote-sensing reflectance"""
+
+    def forward(self, ds, normalized=False):
+
+        if normalized:
+            coeffs = self.coeffs0
+            min_wl = float(np.min(coeffs.foq.wavelengths_FOQ))
+            max_wl = float(np.max(coeffs.foq.wavelengths_FOQ))
+            min_chl = float(np.min(coeffs.foq.log_chl_foq) / np.log(10))
+            max_chl = float(np.max(coeffs.foq.log_chl_foq) / np.log(10))
+        else:
+            coeffs = self.coeffs
+            min_wl = self.min_wavelength_FOQ
+            max_wl = self.max_wavelength_FOQ
+            min_chl = self.min_log_chl_foq
+            max_chl = self.max_log_chl_foq
+
+        wave_foq = np.clip(ds['bands'], min_wl, max_wl)
+        log10_chl_foq = np.clip(ds['log10_chl'], min_chl, max_chl)
+        
+        # f/Q LUT indexed with ln(CHL), i.e. log_e(CHL)
+        log_chl_foq = log10_chl_foq * np.log(10)
+
+        # Merged dual interpolation into single operation for better performance
+        forward_mod = coeffs.foq.interp(
+            wavelengths_FOQ=wave_foq,
+            log_chl_foq=log_chl_foq,
+            method='linear'
+        )
+
+        return forward_mod
+
+    """ Apply QAA to retrieve IOP (omega_b, eta_b) from Rrs """
+
+    def backward(self, ds, iter_brdf):
+
+        Rrs = ds['nrrs']
+
+        # Local renaming of bands (pre-converted to float in __init__)
+        b442 = self.b442
+        b490 = self.b490
+        b510 = self.b510
+        b560 = self.b560
+
+        # Apply upper and lower limits to Rrs(665) #TODO check if not finite or missing?
+        Rrs442 = Rrs.sel(bands=b442)
+        Rrs490 = Rrs.sel(bands=b490)
+        Rrs510 = Rrs.sel(bands=b510)
+        Rrs560 = Rrs.sel(bands=b560)
+        
+        # Drop unused coordinates to reduce overhead
+        Rrs442 = drop_unused_coords(Rrs442)
+        Rrs490 = drop_unused_coords(Rrs490)
+        Rrs510 = drop_unused_coords(Rrs510)
+        Rrs560 = drop_unused_coords(Rrs560)
+
+        # Compute the OC4ME "R"
+        ds['log10_chl_OC4ME_Ratio'] = np.log10(np.max([Rrs442, Rrs490, Rrs510], axis=0) / Rrs560)
+
+        ds['log10_chl'] = 0 * ds['log10_chl_OC4ME_Ratio']
+        # Apply OC4ME 5-degree polynomial using numpy for better performance
+        poly = np.polynomial.polynomial.polyval(
+            ds['log10_chl_OC4ME_Ratio'].values,
+            self.OC4MEcoeff
+        )
+        ds['log10_chl'].values = poly
+
+        return ds