因为yolov8是free anchor的，所以prediction的元素仅仅是[xywh, cls]，没有conf
'''
假设predict: shape:torch.Size([bs, 6, 4410])
6 = xyxy(4) + cls(2)
4410: num_boxes
'''
def non_max_suppression(
prediction,
conf_thres=0.25,
iou_thres=0.45,
classes=None,
agnostic=False,
multi_label=False,
labels=(),
max_det=300,
nc=0, # number of classes (optional)
max_time_img=0.05,
max_nms=30000,
max_wh=7680,
):
"""
Perform non-maximum suppression (NMS) on a set of boxes, with support for masks and multiple labels per box.
Arguments:
prediction (torch.Tensor): A tensor of shape (batch_size, num_classes + 4 + num_masks, num_boxes)
containing the predicted boxes, classes, and masks. The tensor should be in the format
output by a model, such as YOLO.
conf_thres (float): The confidence threshold below which boxes will be filtered out.
Valid values are between 0.0 and 1.0.
iou_thres (float): The IoU threshold below which boxes will be filtered out during NMS.
Valid values are between 0.0 and 1.0.
classes (List[int]): A list of class indices to consider. If None, all classes will be considered.
agnostic (bool): If True, the model is agnostic to the number of classes, and all
classes will be considered as one.
multi_label (bool): If True, each box may have multiple labels.
labels (List[List[Union[int, float, torch.Tensor]]]): A list of lists, where each inner
list contains the apriori labels for a given image. The list should be in the format
output by a dataloader, with each label being a tuple of (class_index, x1, y1, x2, y2).
max_det (int): The maximum number of boxes to keep after NMS.
nc (int): (optional) The number of classes output by the model. Any indices after this will be considered masks.
max_time_img (float): The maximum time (seconds) for processing one image.
max_nms (int): The maximum number of boxes into torchvision.ops.nms().
max_wh (int): The maximum box width and height in pixels
Returns:
(List[torch.Tensor]): A list of length batch_size, where each element is a tensor of
shape (num_boxes, 6 + num_masks) containing the kept boxes, with columns
(x1, y1, x2, y2, confidence, class, mask1, mask2, ...).
"""
# Checks
assert 0 <= conf_thres <= 1, f'Invalid Confidence threshold {conf_thres}, valid values are between 0.0 and 1.0'
assert 0 <= iou_thres <= 1, f'Invalid IoU {iou_thres}, valid values are between 0.0 and 1.0'
if isinstance(prediction, (list, tuple)): # YOLOv8 model in validation model, output = (inference_out, loss_out)
prediction = prediction[0] # select only inference output
device = prediction.device
mps = 'mps' in device.type # Apple MPS
if mps: # MPS not fully supported yet, convert tensors to CPU before NMS
prediction = prediction.cpu()
bs = prediction.shape[0] # batch size
nc = nc or (prediction.shape[1] - 4) # number of classes
nm = prediction.shape[1] - nc - 4
mi = 4 + nc # mask start index
# xc: tensor([[False, False, False, ..., False, False, False]], device='cuda:0')
# xc: torch.Size([bs, 4410])
# 判断num_boxes中有哪些boxes的cls是大于conf_thres
xc = prediction[:, 4:mi].amax(1) > conf_thres # candidates
# Settings
# min_wh = 2 # (pixels) minimum box width and height
time_limit = 0.5 + max_time_img * bs # seconds to quit after
redundant = True # require redundant detections
multi_label &= nc > 1 # multiple labels per box (adds 0.5ms/img)
merge = False # use merge-NMS
t = time.time()
output = [torch.zeros((0, 6 + nm), device=prediction.device)] * bs
# 遍历bs, 每遍历一次就相当于处理一次图片预测出的所有num_boxes
for xi, x in enumerate(prediction): # image index, image inference
# Apply constraints
# x[((x[:, 2:4] < min_wh) | (x[:, 2:4] > max_wh)).any(1), 4] = 0 # width-height
# x.tanspose(0, -1) : torch.Size([4410, 6])
# xc[xi]: torch.Size([4410])
# x: torch.Size([37, 6])
x = x.transpose(0, -1)[xc[xi]] # confidence
# Cat apriori labels if autolabelling
if labels and len(labels[xi]):
lb = labels[xi]
v = torch.zeros((len(lb), nc + nm + 5), device=x.device)
v[:, :4] = lb[:, 1:5] # box
v[range(len(lb)), lb[:, 0].long() + 4] = 1.0 # cls
x = torch.cat((x, v), 0)
# If none remain process next image
if not x.shape[0]:
continue
# Detections matrix nx6 (xyxy, conf, cls)
box, cls, mask = x.split((4, nc, nm), 1)
box = xywh2xyxy(box) # center_x, center_y, width, height) to (x1, y1, x2, y2)
if multi_label:
i, j = (cls > conf_thres).nonzero(as_tuple=False).T
x = torch.cat((box[i], x[i, 4 + j, None], j[:, None].float(), mask[i]), 1)
else: # best class only
# 选择num_cls中cls_score最大的那个作为conf, 其索引即是分类
conf, j = cls.max(1, keepdim=True)
x = torch.cat((box, conf, j.float(), mask), 1)[conf.view(-1) > conf_thres]
# Filter by class
if classes is not None:
x = x[(x[:, 5:6] == torch.tensor(classes, device=x.device)).any(1)]
# Apply finite constraint
# if not torch.isfinite(x).all():
# x = x[torch.isfinite(x).all(1)]
# Check shape
n = x.shape[0] # number of boxes
if not n: # no boxes
continue
# 按照conf进行排序, num_nms的最大值不超过max_nms
x = x[x[:, 4].argsort(descending=True)[:max_nms]] # sort by confidence and remove excess boxes
# Batched NMS
# 进行多标签分类, 如cls=0, 则c = 0 * max_wh, cls=1, 则c = 1 * max_wh
# 即cls=0的所有boxes_wh 都加上 0 * max_wh, cls=1的所有boxes_wh都加上 1 * max_wh, 以此类推
c = x[:, 5:6] * (0 if agnostic else max_wh) # classes
boxes, scores = x[:, :4] + c, x[:, 4] # boxes (offset by class), scores
# 返回的是x对应的位置索引
i = torchvision.ops.nms(boxes, scores, iou_thres) # NMS
i = i[:max_det] # limit detections
if merge and (1 < n < 3E3): # Merge NMS (boxes merged using weighted mean)
# Update boxes as boxes(i,4) = weights(i,n) * boxes(n,4)
iou = box_iou(boxes[i], boxes) > iou_thres # iou matrix
weights = iou * scores[None] # box weights
x[i, :4] = torch.mm(weights, x[:, :4]).float() / weights.sum(1, keepdim=True) # merged boxes
if redundant:
i = i[iou.sum(1) > 1] # require redundancy
# x[i]: 最终保留下来的xyxy, conf, cls
# output[xi]: 将x[i]保存到遍历的bs中相对应的索引位置处
output[xi] = x[i]
if mps:
output[xi] = output[xi].to(device)
if (time.time() - t) > time_limit:
LOGGER.warning(f'WARNING ?? NMS time limit {time_limit:.3f}s exceeded')
break # time limit exceeded
return output