Pytorch model stuck at 0.5 though loss decreases consistently
This is using PyTorch

I have been trying to implement UNet model on my images, however, my model accuracy is always exact 0.5. Loss does decrease.

I have also checked for class imbalance. I have also tried playing with learning rate. Learning rate affects loss but not the accuracy.

My architecture below ( from here )

""" `UNet` class is based on https://arxiv.org/abs/1505.04597

The U-Net is a convolutional encoder-decoder neural network.
Contextual spatial information (from the decoding,
expansive pathway) about an input tensor is merged with
information representing the localization of details
(from the encoding, compressive pathway).

Modifications to the original paper:
(1) padding is used in 3x3 convolutions to prevent loss
    of border pixels
(2) merging outputs does not require cropping due to (1)
(3) residual connections can be used by specifying
    UNet(merge_mode='add')
(4) if non-parametric upsampling is used in the decoder
    pathway (specified by upmode='upsample'), then an
    additional 1x1 2d convolution occurs after upsampling
    to reduce channel dimensionality by a factor of 2.
    This channel halving happens with the convolution in
    the tranpose convolution (specified by upmode='transpose')


    Arguments:
        in_channels: int, number of channels in the input tensor.
                     Default is 3 for RGB images. Our SPARCS dataset is 13 channel.
              depth: int, number of MaxPools in the U-Net. During training, input size needs to be 
                     (depth-1) times divisible by 2
        start_filts: int, number of convolutional filters for the first conv.
            up_mode: string, type of upconvolution. Choices: 'transpose' for transpose convolution 

"""

class UNet(nn.Module):

    def __init__(self, num_classes, depth, in_channels, start_filts=16, up_mode='transpose', merge_mode='concat'):

        super(UNet, self).__init__()

        if up_mode in ('transpose', 'upsample'):
            self.up_mode = up_mode
        else:
            raise ValueError("\"{}\" is not a valid mode for upsampling. Only \"transpose\" and \"upsample\" are allowed.".format(up_mode))

        if merge_mode in ('concat', 'add'):
            self.merge_mode = merge_mode
        else:
            raise ValueError("\"{}\" is not a valid mode for merging up and down paths.Only \"concat\" and \"add\" are allowed.".format(up_mode))

        # NOTE: up_mode 'upsample' is incompatible with merge_mode 'add'
        if self.up_mode == 'upsample' and self.merge_mode == 'add':
            raise ValueError("up_mode \"upsample\" is incompatible with merge_mode \"add\" at the moment "
                             "because it doesn't make sense to use nearest neighbour to reduce depth channels (by half).")

        self.num_classes = num_classes
        self.in_channels = in_channels
        self.start_filts = start_filts
        self.depth = depth

        self.down_convs = []
        self.up_convs = []

        # create the encoder pathway and add to a list
        for i in range(depth):
            ins = self.in_channels if i == 0 else outs
            outs = self.start_filts*(2**i)
            pooling = True if i &lt; depth-1 else False

            down_conv = DownConv(ins, outs, pooling=pooling)
            self.down_convs.append(down_conv)

        # create the decoder pathway and add to a list
        # - careful! decoding only requires depth-1 blocks
        for i in range(depth-1):
            ins = outs
            outs = ins // 2
            up_conv = UpConv(ins, outs, up_mode=up_mode, merge_mode=merge_mode)
            self.up_convs.append(up_conv)


        self.conv_final = conv1x1(outs, self.num_classes)

        # add the list of modules to current module
        self.down_convs = nn.ModuleList(self.down_convs)
        self.up_convs = nn.ModuleList(self.up_convs)

        self.reset_params()

    @staticmethod
    def weight_init(m):
        if isinstance(m, nn.Conv2d):

            #https://prateekvjoshi.com/2016/03/29/understanding-xavier-initialization-in-deep-neural-networks/ 
            ##Doc: https://pytorch.org/docs/stable/nn.init.html?highlight=xavier#torch.nn.init.xavier_normal_ 
            init.xavier_normal_(m.weight)
            init.constant_(m.bias, 0)



    def reset_params(self):
        for i, m in enumerate(self.modules()):
            self.weight_init(m)


    def forward(self, x):
        encoder_outs = []

        # encoder pathway, save outputs for merging
        for i, module in enumerate(self.down_convs):
            x, before_pool = module(x)
            encoder_outs.append(before_pool)

        for i, module in enumerate(self.up_convs):
            before_pool = encoder_outs[-(i+2)]
            x = module(before_pool, x)

        # No softmax is used. This means we need to use
        # nn.CrossEntropyLoss is your training script,
        # as this module includes a softmax already.
        x = self.conv_final(x)
        return x


Parameters are : 

device = torch.device('cuda' if torch.cuda.is_available() else 'cpu')
x,y = train_sequence[0] ; batch_size = x.shape[0]
model = UNet(num_classes = 2, depth=5, in_channels=5, merge_mode='concat').to(device)
optim = torch.optim.Adam(model.parameters(),lr=0.01, weight_decay=1e-3)
criterion = nn.BCEWithLogitsLoss() #has sigmoid internally
epochs = 1000


The function for training is :

import torch.nn.functional as f


def train_model(epoch,train_sequence):
    """Train the model and report validation error with training error
    Args:
        model: the model to be trained
        criterion: loss function
        data_train (DataLoader): training dataset
    """
    model.train()
    for idx in range(len(train_sequence)):        
        X, y = train_sequence[idx]             
        images = Variable(torch.from_numpy(X)).to(device) # [batch, channel, H, W]
        masks = Variable(torch.from_numpy(y)).to(device) 

        outputs = model(images)
        print(masks.shape, outputs.shape)
        loss = criterion(outputs, masks)
        optim.zero_grad()
        loss.backward()
        # Update weights
        optim.step()
    # total_loss = get_loss_train(model, data_train, criterion)


My function for calculating loss and accuracy is below:

def get_loss_train(model, train_sequence):
    """
        Calculate loss over train set
    """
    model.eval()
    total_acc = 0
    total_loss = 0
    for idx in range(len(train_sequence)):        
        with torch.no_grad():
            X, y = train_sequence[idx]             
            images = Variable(torch.from_numpy(X)).to(device) # [batch, channel, H, W]
            masks = Variable(torch.from_numpy(y)).to(device) 

            outputs = model(images)
            loss = criterion(outputs, masks)
            preds = torch.argmax(outputs, dim=1).float()
            acc = accuracy_check_for_batch(masks.cpu(), preds.cpu(), images.size()[0])
            total_acc = total_acc + acc
            total_loss = total_loss + loss.cpu().item()
    return total_acc/(len(train_sequence)), total_loss/(len(train_sequence))



Edit : Code which runs (calls) the functions:

for epoch in range(epochs):
    train_model(epoch, train_sequence)
    train_acc, train_loss = get_loss_train(model,train_sequence)
    print("Train Acc:", train_acc)
    print("Train loss:", train_loss)



Can someone help me identify as why is accuracy always exact 0.5?

Edit-2:

As asked accuracy_check_for_batch function is here:

def accuracy_check_for_batch(masks, predictions, batch_size):
    total_acc = 0
    for index in range(batch_size):
        total_acc += accuracy_check(masks[index], predictions[index])
    return total_acc/batch_size


and 

def accuracy_check(mask, prediction):
    ims = [mask, prediction]
    np_ims = []
    for item in ims:
        if 'str' in str(type(item)):
            item = np.array(Image.open(item))
        elif 'PIL' in str(type(item)):
            item = np.array(item)
        elif 'torch' in str(type(item)):
            item = item.numpy()
        np_ims.append(item)

    compare = np.equal(np_ims[0], np_ims[1])
    accuracy = np.sum(compare)

    return accuracy/len(np_ims[0].flatten())


I found the mistake.

model = UNet(num_classes = 2, depth=5, in_channels=5, merge_mode='concat').to(device)


should be

model = UNet(num_classes = 1, depth=5, in_channels=5, merge_mode='concat').to(device)


because I am using BCELosswithLogits.
