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# SPDX-FileCopyrightText: Copyright (c) 2021-2022 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
# SPDX-License-Identifier: LicenseRef-NvidiaProprietary
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# without an express license agreement from NVIDIA CORPORATION or
# its affiliates is strictly prohibited.
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# Modified by Zexin He
# The modifications are subject to the same license as the original.


"""
The ray marcher takes the raw output of the implicit representation and uses the volume rendering equation to produce composited colors and depths.
Based off of the implementation in MipNeRF (this one doesn't do any cone tracing though!)
"""

import torch
import torch.nn as nn


class MipRayMarcher2(nn.Module):
    def __init__(self, activation_factory):
        super().__init__()
        self.activation_factory = activation_factory

    def run_forward(self, colors, densities, depths, rendering_options):
        
        deltas = depths[:, :, 1:] - depths[:, :, :-1] 
        colors_mid = (colors[:, :, :-1] + colors[:, :, 1:]) / 2
        densities_mid = (densities[:, :, :-1] + densities[:, :, 1:]) / 2
        depths_mid = (depths[:, :, :-1] + depths[:, :, 1:]) / 2

    

        # using factory mode for better usability
        densities_mid = self.activation_factory(rendering_options)(densities_mid)

        density_delta = densities_mid * deltas

        alpha = 1 - torch.exp(-density_delta)

        alpha_shifted = torch.cat([torch.ones_like(alpha[:, :, :1]), 1-alpha + 1e-10], -2)
        weights = alpha * torch.cumprod(alpha_shifted, -2)[:, :, :-1]

        composite_rgb = torch.sum(weights * colors_mid, -2)
        weight_total = weights.sum(2)
        composite_depth = torch.sum(weights * depths_mid, -2) / weight_total

        # clip the composite to min/max range of depths
        composite_depth = torch.nan_to_num(composite_depth, float('inf'))
        composite_depth = torch.clamp(composite_depth, torch.min(depths), torch.max(depths))

        if rendering_options.get('white_back', False):
            composite_rgb = composite_rgb + 1 - weight_total

        # rendered value scale is 0-1, comment out original mipnerf scaling
        # composite_rgb = composite_rgb * 2 - 1 # Scale to (-1, 1)

        return composite_rgb, composite_depth, weights


    def forward(self, colors, densities, depths, rendering_options):
        composite_rgb, composite_depth, weights = self.run_forward(colors, densities, depths, rendering_options)

        return composite_rgb, composite_depth, weights