Turning off each head's attention maps of Decoder in DETR : Focusing on generic attention model explainability

sjinu·2021년 9월 30일
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아래 그림들은 generic attention model explainability.. 연구의 '예측 타당성 유지 방법'을 적용해 DETR 내 Transformer Decoder의 6개 layer를 거치면서 시각화한 것 입니다.
특정한 Average 방법을 토대로 8개의 attention heads를 평균내기 때문에 각각의 head에 대한 insight는 존재하지 않습니다).

DETR 내부 Decoder의 Multiheadattn에서 특정 head를 off

위 그림에서와 같이, 첫 번째 head에서 마지막 head(총 8개)까지 누적해가며 모든 Decoder layer(총 6개)의 attention weights를 zero로 만듭니다.

즉, 자세히 보면 아래와 같은 과정을 반복해 진행합니다.

Modified Code

in DETR.modules.layers

class MultiheadAttention(RelProp): # in layers.py
    ...

    def __init__(self, embed_dim, num_heads, dropout=0., no_weight:'[num_heads] idx vector'=False): #수정-0927:Deocder attention off
        ...

        self.attn_gradients = None
        
        # 수정-0927:Deocder attention off
        self.no_weight = no_weight 

    def forward(self, query, key, value, key_padding_mask=None,
                need_weights=True, attn_mask=None):

       ...
        
        # 추가-0927:Deocder attention off
        if self.no_weight: # self.no_weight : length : multiheads. ex) [0,1,0,1,1,0,0,0] --> 2,4,5th head off
            mask = torch.stack([torch.zeros_like(a[0]) if (idx==1) 
                                                                 else torch.ones_like(a[0]) for idx in self.no_weight])
            attn_output_weights=attn_output_weights * mask
     
    
        self.save_attn(attn_output_weights)
        attn_output_weights.register_hook(self.save_attn_gradients)

        ...

in DETR.models.transformer_jsp(new

class Transformer(nn.Module):)


    def __init__(self, d_model=512, nhead=8, num_encoder_layers=6,
                 num_decoder_layers=6, dim_feedforward=2048, dropout=0.1,
                 activation="relu", normalize_before=False,
                 return_intermediate_dec=False, off_decoder_head=False):
        ...
        self.encoder = TransformerEncoder(encoder_layer, num_encoder_layers, encoder_norm)
        
	#추가-0927 : add decoder head	
        decoder_layer = TransformerDecoderLayer(d_model, nhead, dim_feedforward,
                                                dropout, activation, normalize_before, off_decoder_head=off_decoder_head)
        decoder_norm = LayerNorm(d_model)
        self.decoder = TransformerDecoder(decoder_layer, num_decoder_layers, decoder_norm,
                                          return_intermediate=return_intermediate_dec)

       ...
        
class TransformerDecoderLayer(nn.Module):

    def __init__(self, d_model, nhead, dim_feedforward=2048, dropout=0.1,
                 activation="relu", normalize_before=False, off_decoder_head=False): #추가:0927-off_decoder_head : Off.
        super().__init__()
        self.self_attn = MultiheadAttention(d_model, nhead, dropout=dropout)
        ...
        

def build_transformer_jsp(args): 
    return Transformer(
        ...
        
        return_intermediate_dec=True,
        off_head = False #수정(1) : Out heads(input : [layers x heads ] matrix) 
    )

in DETR.detr

from DETR.models.transformer_jsp import build_transformer_jsp #추가-0927: add off decoder

...

...

def build(args):
 
    ...

    
    #추가-0927 : add decoder off
    if args.off_decoder_head: 
        transformer = build_transformer_jsp(args) #추가-0927 : add decoder off
    else : 
        transformer = build_transformer(args)

DETR

Non-off

plt.figure(figsize=(16,16))
scores, bboxes = detect(im, model, confidence=0.5)
plot_results(im, scores, bboxes, line_width=5, font_size=15)

1~4 Heads Off

plt.figure(figsize=(20,20))
for idx, head in enumerate(range(4)):
    for layer in model.transformer.decoder.layers:
        layer.multihead_attn.no_weight = [1 if j<=(head) else 0 for j in range(8)]
    plt.subplot(2,2,idx+1)
    scores, bboxes = detect(im, model, confidence=0.5)
    plot_results(im, scores, bboxes, line_width=5, font_size=15)
    plt.title(f'first {head+1} heads off')

5~8 Heads Off

plt.figure(figsize=(20,20))
for idx, head in enumerate(range(4,8)):
    for layer in model.transformer.decoder.layers:
        layer.multihead_attn.no_weight = [1 if j<=(head) else 0 for j in range(8)]
    plt.subplot(2,2,idx+1)
    scores, bboxes = detect(im, model, confidence=0.5)
    plot_results(im, scores, bboxes, line_width=5, font_size=15)
    plt.title(f'first {head+1} heads off')

Generic Attention-model Explainability

Non-off


for layer in model.transformer.decoder.layers:
    layer.multihead_attn.no_weight = False
    
gen=Generator(model)
evaluate(model, gen, im, 'cuda', show_all_layers=True, confidence=0.5)

1~4 Heads Off

for idx, head in enumerate(range(4)):
    for layer in model.transformer.decoder.layers:
        layer.multihead_attn.no_weight = [1 if j<=(head) else 0 for j in range(8)]
    
    gen=Generator(model)
    evaluate(model, gen, im, 'cuda', show_all_layers=True, confidence=0.5)




5~8 Heads Off

for idx, head in enumerate(range(4,8)):
    for layer in model.transformer.decoder.layers:
        layer.multihead_attn.no_weight = [1 if j<=(head) else 0 for j in range(8)]
    
    gen=Generator(model)
    evaluate(model, gen, im, 'cuda', show_all_layers=True, confidence=0.5)


6, 8 heads off는 detect한 object가 2개 미만이라 시각화하지 않았습니다.

Turn Off head attentioing for Specific Object

Implementation Code

Object Detection

def detect(im, model, confidence=0.5):
    img=transform(im).unsqueeze(0).to(device)

    outputs=model(img)
    
    assert img.shape[-2] <= 1600 and img.shape[-1] <= 1600

    probas = outputs['pred_logits'].softmax(-1)[0, :, :-1]
    keep = probas.max(-1).values > confidence

    # 0과 1사이의 boxes 값을 image scale로 확대합니다.
    bboxes_scaled = rescale_bboxes(outputs['pred_boxes'].cpu()[0, keep], im.size)
    scores, boxes=probas[keep], bboxes_scaled
    
    return scores, boxes

Bounding box Visualization

def plot_results(pil_img, prob, boxes, line_width=8, font_size=20):
#     plt.figure(figsize=(16,10))
    plt.imshow(pil_img)
    ax = plt.gca()
    colors = COLORS * 100
    for p, (xmin, ymin, xmax, ymax), c in zip(prob, boxes.tolist(), colors):
        ax.add_patch(plt.Rectangle((xmin, ymin), xmax - xmin, ymax - ymin,
                                   fill=False, color=c, linewidth=line_width))
        cl = p.argmax()
        text = f'{CLASSES[cl]}: {p[cl]:0.2f}'
        ax.text(xmin, ymin, text, fontsize=font_size,
                bbox=dict(facecolor='yellow', alpha=0.4))
    plt.axis('off')
#     plt.show()

Evaluation & Visualization

def evaluate(model, gen, im, device, image_id=None, show_all_layers=False, show_raw_attn=False, confidence=0.7):

    # 평균-분산 정규화 (train dataset의 통계량을 (test) input image에 사용
    img=transform(im).unsqueeze(0).to(device)
    
    # model 통과
    outputs =model(img)
    
    # 정확도 70% 이상의 예측만 사용
    probas = outputs['pred_logits'].softmax(-1)[0, :, :-1] # background 제외
    keep = probas.max(-1).values > confidence
    
    if keep.nonzero().shape[0] <=1 : # detect된 object
        print('detected object is under 2')
        return
    
    
    # 원래 cuda에 적재되어있던 좌표들
    outputs['pred_boxes'] = outputs['pred_boxes'].cpu()
    
    # [0,1]의 상대 좌표를 원래의 좌표로 복구
    bboxes_scaled = rescale_bboxes(outputs['pred_boxes'][0, keep], im.size)


    #attention weight 저장
    
    hooks=[]
    conv_features_out, enc_attn_out, dec_attn_out = [], [], []

      
    for layer_name in model.backbone[-2].body:
        hook=model.backbone[-2].body[layer_name].register_forward_hook(
        lambda self, input, output : conv_features_out.append(output)
        )
        hooks.append(hook)
    
    model(img)
    

    # hook 제거
    for hook in hooks:
        hook.remove()
    
    
    #  get the shape of feature map
    h, w = conv_features_out[-1].shape[-2:] # Nested tensor -> tensors
    
    #######################
    ######## Modified Code
    
    if not show_all_layers == True:
        fig, axs = plt.subplots(ncols=len(bboxes_scaled), nrows=2, figsize=(22,7))
    else:
        n_layers=len(model.transformer.encoder.layers)
        if not show_raw_attn:
            fig, axs = plt.subplots(ncols=len(bboxes_scaled), nrows=n_layers+1, figsize=(22, 4*n_layers))
        else:
            fig, axs = plt.subplots(ncols=len(bboxes_scaled), nrows=model.transformer.nhead+1,
                                    figsize=(22, 4*model.transformer.nhead))
    # object queries는 100차원(default)이기 때문에 그 중에 
    # 0.7(default) 이상의 신뢰도를 보이는 query만을 사용해야 한다. 
    
    for idx, ax_i, (xmin, ymin, xmax, ymax),p in zip(keep.nonzero(), axs.T, bboxes_scaled, probas[keep]):
        
        ax = ax_i[0]
        ax.imshow(im)
        ax.add_patch(plt.Rectangle((xmin.detach(), ymin.detach()), 
                                  xmax.detach() - xmin.detach(),
                                   ymax.detach() - ymin.detach(), 
                                   fill=False, color='blue', linewidth=3))
        
        # 0929, 확률추가
        cl = p.argmax()
        text = f'{CLASSES[cl]}: {p[cl]:0.2f}'
        ax.text(xmin, ymin, text, fontsize=10,
                bbox=dict(facecolor='yellow', alpha=0.4))
        ax.axis('off')
        ax.set_title(CLASSES[probas[idx].argmax()])
        
      
        
        if not show_all_layers == True:
            ax = ax_i[1]
                            
            
            cam = gen.generate_ours(img, idx, use_lrp=False)
            cam = (cam - cam.min()) / (cam.max() - cam.min()) # 점수 정규화
            cmap = plt.cm.get_cmap('Blues').reversed()

            ax.imshow(cam.view(h, w).data.cpu().numpy(), cmap=cmap)
            ax.axis('off')
            ax.set_title(f'query id: {idx.item()}')
        else:
            
            if not show_raw_attn:    
                cams = gen.generate_ours(img, idx, use_lrp=False, use_all_layers=True)
            else:
                cams = gen.generate_raw_attn(img, idx, use_all_layers=True)
            
            num_layer=n_layers
            if show_raw_attn:
                num_layer=model.transformer.nhead
            for n, cam in zip(range(num_layer), cams):
                ax = ax_i[1+n]
                cam = (cam - cam.min()) / (cam.max() - cam.min()) # 점수 정규화
                cmap = plt.cm.get_cmap('Blues').reversed()

                ax.imshow(cam.view(h, w).data.cpu().numpy(), cmap=cmap)
                ax.axis('off')
                ax.set_title(f'query id: {idx.item()}, layer:{n}', size=12)
        #######################
profile
Jin Su, Park. Department of Mathematics, University of Seoul: https://github.com/sjinu96

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