yolov3损失函数代码

Yolov3是一种流行的目标检测模型，它的损失函数设计非常特殊。与传统的目标检测模型不同，yolov3的损失函数不是基于交叉熵或类似的损失函数，而是将目标检测问题定义为一种回归问题，通过对坐标和大小进行回归来预测目标框。

在yolov3的损失函数中，主要包含三部分损失函数：置信度损失、分类损失和坐标损失。置信度损失用于衡量预测的目标框与实际目标框的重叠度，分类损失用于衡量预测的目标框中包含的物体类型是否正确，坐标损失则用于衡量目标框的位置和大小的回归精度。

具体的代码实现如下：

def yolo_loss(args, anchors, num_classes, rescore_confidence=False, print_loss=False):
"""
YOLOv3 loss function.
:param args: YOLOv3 output tensor list.
:param anchors: Anchor box list.
:param num_classes: Number of classes.
:param rescore_confidence: Whether to rescore confidence based on IOU between prediction and target.
:param print_loss: Whether to print loss values for debugging purposes.
:return: Total loss tensor.
"""
# Retrieve model input shape.
input_shape = tf.cast(tf.shape(args[0])[1:3] * 32, tf.float32)
# Tuple of scalars representing the grid shape (width, height).
grid_shape = [tf.cast(tf.shape(args[l])[1:3], tf.float32) for l in range(3)]
# Compute scale factors for box width and height.
scales = [input_shape / grid_shape[l] for l in range(3)]
# Anchor box tensor.
anchors_tensor = tf.reshape(tf.constant(anchors, dtype=tf.float32), [1, 1, 1, 3, 2])
# Element-wise compute inverse of anchor box dimensions.
anchor_dims = anchors_tensor[..., ::-1]
# Extract objectness probability and class predictions from output tensor list.
yolo_outputs = args[:3]
# Extract predicted box coordinates and convert to float.
xy_offset, wh, objectness, class_probs = yolo_head(yolo_outputs, anchors, num_classes, input_shape)
# Compute grid offsets.
grid_offset = [tf.range(tf.cast(grid_shape[l], tf.float32), dtype=tf.float32) for l in range(2)]
grid_offset = tf.meshgrid(grid_offset[1], grid_offset[0])
grid_offset = tf.expand_dims(tf.stack(grid_offset, axis=-1), axis=2)
# Compute true box coordinates and weights.
box_xy, box_wh, box_confidence, box_class_probs, true_box = yolo_boxes_and_scores(y_true, anchors, num_classes, input_shape)
# Compute iou between each predicted box and true box.
iou = yolo_box_iou(xy_offset, wh, true_box[..., 0:4], anchor_dims)
# Parse batch size from input tensor.
batch_size = tf.cast(tf.shape(yolo_outputs[0])[0], tf.float32)
# Compute objectness, class and regression losses.
object_mask = tf.reduce_max(iou, axis=-1, keepdims=True) * y_true[..., 4:5]
object_mask = tf.cast((iou >= object_mask) & (y_true[..., 4:5] > 0), tf.float32)
object_mask_neg = tf.cast((iou < object_mask) & (iou >= 0.5), tf.float32)
object_mask_pos = tf.cast((iou >= object_mask) & (y_true[..., 4:5] > 0), tf.float32)
pred_box_xy = xy_offset * object_mask_pos
pred_box_wh = wh * tf.exp(yolo_outputs[2]) * object_mask_pos
pred_box_confidence = (
(object_mask_pos * objectness) + (object_mask_neg * objectness * rescore_confidence) +
((1 - object_mask_pos - object_mask_neg) * objectness_black_box_rescore)
)
pred_box_class_probs = class_probs * object_mask_pos
true_box_xy = y_true[..., 0:2] / scales[0] - grid_offset
true_box_wh = y_true[..., 2:4] / scales[0]
xy_loss_scale = 2.0 - y_true[..., 2:3] * y_true[..., 3:4] / input_shape / input_shape
wh_loss_scale = 2.0 - y_true[..., 2:3] * y_true[..., 3:4] / input_shape / input_shape
confidence_loss_scale = (1 - y_true[..., 4:5]) + (y_true[..., 4:5] * 4.) * (1 - yolo_outputs[2]) + 1e-8
class_loss_scale = y_true[..., 4:5] * 1.
xy_loss = tf.reduce_sum(tf.square(true_box_xy - pred_box_xy) * xy_loss_scale, axis=-1)
wh_loss = tf.reduce_sum(tf.square(tf.sqrt(true_box_wh) - tf.sqrt(pred_box_wh)) * wh_loss_scale, axis=-1)
confidence_loss = tf.reduce_sum(tf.square(true_box[..., 4:5] - pred_box_confidence) * confidence_loss_scale, axis=-1)
class_loss = tf.reduce_sum(tf.square(true_box[..., 5:] - pred_box_class_probs) * class_loss_scale, axis=-1)
# Normalization factos.
num_positives = tf.reduce_sum(object_mask_pos, axis=[1, 2, 3])
# Compute total YOLOv3 loss.
total_loss = (
xy_loss + wh_loss + confidence_loss + class_loss
)
# Optionally print loss values.
if print_loss:
total_loss = tf.Print(
total_loss, [tf.reduce_mean(xy_loss / num_positives),
tf.reduce_mean(wh_loss / num_positives),
tf.reduce_mean(confidence_loss / num_positives),
tf.reduce_mean(class_loss / num_positives)],
message='loss: '
)
return total_loss