third_party/r2d2/nets/ap_loss.py

# Copyright 2019-present NAVER Corp.
# CC BY-NC-SA 3.0
# Available only for non-commercial use

import pdb
import numpy as np
import torch
import torch.nn as nn


class APLoss(nn.Module):
    """differentiable AP loss, through quantization.

    Input: (N, M)   values in [min, max]
    label: (N, M)   values in {0, 1}

    Returns: list of query AP (for each n in {1..N})
             Note: typically, you want to minimize 1 - mean(AP)
    """

    def __init__(self, nq=25, min=0, max=1, euc=False):
        nn.Module.__init__(self)
        assert isinstance(nq, int) and 2 <= nq <= 100
        self.nq = nq
        self.min = min
        self.max = max
        self.euc = euc
        gap = max - min
        assert gap > 0

        # init quantizer = non-learnable (fixed) convolution
        self.quantizer = q = nn.Conv1d(1, 2 * nq, kernel_size=1, bias=True)
        a = (nq - 1) / gap
        # 1st half = lines passing to (min+x,1) and (min+x+1/a,0) with x = {nq-1..0}*gap/(nq-1)
        q.weight.data[:nq] = -a
        q.bias.data[:nq] = torch.from_numpy(
            a * min + np.arange(nq, 0, -1)
        )  # b = 1 + a*(min+x)
        # 2nd half = lines passing to (min+x,1) and (min+x-1/a,0) with x = {nq-1..0}*gap/(nq-1)
        q.weight.data[nq:] = a
        q.bias.data[nq:] = torch.from_numpy(
            np.arange(2 - nq, 2, 1) - a * min
        )  # b = 1 - a*(min+x)
        # first and last one are special: just horizontal straight line
        q.weight.data[0] = q.weight.data[-1] = 0
        q.bias.data[0] = q.bias.data[-1] = 1

    def compute_AP(self, x, label):
        N, M = x.shape
        if self.euc:  # euclidean distance in same range than similarities
            x = 1 - torch.sqrt(2.001 - 2 * x)

        # quantize all predictions
        q = self.quantizer(x.unsqueeze(1))
        q = torch.min(q[:, : self.nq], q[:, self.nq :]).clamp(min=0)  # N x Q x M

        nbs = q.sum(dim=-1)  # number of samples  N x Q = c
        rec = (q * label.view(N, 1, M).float()).sum(
            dim=-1
        )  # nb of correct samples = c+ N x Q
        prec = rec.cumsum(dim=-1) / (1e-16 + nbs.cumsum(dim=-1))  # precision
        rec /= rec.sum(dim=-1).unsqueeze(1)  # norm in [0,1]

        ap = (prec * rec).sum(dim=-1)  # per-image AP
        return ap

    def forward(self, x, label):
        assert x.shape == label.shape  # N x M
        return self.compute_AP(x, label)