models/decoder.py

"""
SIREN-based implicit decoder.

For any continuous coordinate x = (u, v) ∈ [0,1]²:
    RGB(x) = G_θ( γ(x), z_x )

where γ is Fourier positional encoding and z_x is bilinearly
interpolated from the encoder feature grid Z.
"""

import math
import torch
import torch.nn as nn
import torch.nn.functional as F


class SineLayer(nn.Module):
    """Single SIREN layer:  sin(ω₀ · (Wx + b))"""

    def __init__(self, in_features, out_features, omega_0=30.0, is_first=False):
        super().__init__()
        self.omega_0 = omega_0
        self.linear = nn.Linear(in_features, out_features)
        self._init_weights(is_first, in_features)

    def _init_weights(self, is_first, fan_in):
        with torch.no_grad():
            if is_first:
                self.linear.weight.uniform_(-1.0 / fan_in, 1.0 / fan_in)
            else:
                bound = math.sqrt(6.0 / fan_in) / self.omega_0
                self.linear.weight.uniform_(-bound, bound)

    def forward(self, x):
        return torch.sin(self.omega_0 * self.linear(x))


class FourierEncoding(nn.Module):
    """Fourier positional encoding γ(x) with L frequency bands."""

    def __init__(self, n_bands: int = 10, input_dim: int = 2):
        super().__init__()
        self.n_bands = n_bands
        self.input_dim = input_dim
        # output dim = input_dim * 2 * n_bands  (sin + cos per band per dim)
        self.out_dim = input_dim * 2 * n_bands

    def forward(self, coords: torch.Tensor) -> torch.Tensor:
        """coords: (..., input_dim) → (..., out_dim)"""
        freqs = 2.0 ** torch.arange(self.n_bands, device=coords.device, dtype=coords.dtype)
        # (n_bands,)
        # coords: (..., D) → (..., D, 1)  ×  (1, n_bands) → (..., D, n_bands)
        scaled = coords.unsqueeze(-1) * freqs * math.pi
        enc = torch.cat([torch.sin(scaled), torch.cos(scaled)], dim=-1)
        # (..., D, 2*n_bands) → (..., D*2*n_bands)
        return enc.flatten(-2)


class SIRENDecoder(nn.Module):
    """
    Implicit decoder:
        input  = Fourier(coord) ⊕ z_x
        output = RGB ∈ [-1, 1]
    """

    def __init__(
        self,
        feat_dim: int = 768,
        hidden_dim: int = 256,
        n_layers: int = 5,
        omega_0: float = 30.0,
        fourier_bands: int = 10,
        out_channels: int = 3,
    ):
        super().__init__()
        self.fourier = FourierEncoding(n_bands=fourier_bands, input_dim=2)
        coord_dim = self.fourier.out_dim          # 2 * 2 * L = 40
        in_dim = coord_dim + feat_dim

        layers = []
        layers.append(SineLayer(in_dim, hidden_dim, omega_0=omega_0, is_first=True))
        # Fix: is_first init uses fan_in=in_dim=808 which makes coord contribution
        # negligible (±0.00124). Re-init so coord columns use 1/coord_dim (±0.025)
        # and feature columns use standard SIREN hidden-layer bounds.
        with torch.no_grad():
            w = layers[0].linear.weight        # (hidden_dim, in_dim)
            w[:, :coord_dim].uniform_(-1.0 / coord_dim, 1.0 / coord_dim)
            feat_bound = math.sqrt(6.0 / feat_dim) / omega_0
            w[:, coord_dim:].uniform_(-feat_bound, feat_bound)
        for _ in range(n_layers - 2):
            layers.append(SineLayer(hidden_dim, hidden_dim, omega_0=omega_0))
        # final linear (no sine) → RGB
        final = nn.Linear(hidden_dim, out_channels)
        with torch.no_grad():
            bound = math.sqrt(6.0 / hidden_dim) / omega_0
            final.weight.uniform_(-bound, bound)
        layers.append(final)
        self.net = nn.ModuleList(layers)

    def forward(self, coords: torch.Tensor, features: torch.Tensor) -> torch.Tensor:
        """
        Args:
            coords:   (B, N, 2) continuous coordinates in [0, 1]
            features: (B, C, H_z, W_z) spatial feature grid from encoder
        Returns:
            rgb: (B, N, 3) predicted RGB in [-1, 1]
        """
        B, N, _ = coords.shape

        # 1) Fourier encode coordinates
        enc = self.fourier(coords)                 # (B, N, coord_dim)

        # 2) bilinear sample from feature grid
        #    grid_sample wants coords in [-1, 1]
        grid = coords * 2.0 - 1.0                 # [0,1] → [-1,1]
        grid = grid.unsqueeze(1)                   # (B, 1, N, 2)
        z_x = F.grid_sample(
            features, grid, mode="bilinear", align_corners=True
        )  # (B, C, 1, N)
        z_x = z_x.squeeze(2).permute(0, 2, 1)     # (B, N, C)

        # 3) concatenate and pass through SIREN
        h = torch.cat([enc, z_x], dim=-1)          # (B, N, coord_dim+C)
        for layer in self.net[:-1]:
            h = layer(h)
        rgb = self.net[-1](h)                      # (B, N, 3)
        rgb = torch.tanh(rgb)                      # clamp to [-1, 1]
        return rgb


def make_coord_grid(H: int, W: int, device: torch.device) -> torch.Tensor:
    """Create a flat coordinate grid in [0, 1]².  Returns (1, H*W, 2)."""
    ys = torch.linspace(0, 1, H, device=device)
    xs = torch.linspace(0, 1, W, device=device)
    grid_y, grid_x = torch.meshgrid(ys, xs, indexing="ij")
    coords = torch.stack([grid_x, grid_y], dim=-1)   # (H, W, 2)  x then y
    return coords.reshape(1, H * W, 2)