分类VS分割——从原理和代码角度来分析对比(点云深度学习)
分类任务与分割任务有什么联系吗？
答案是肯定的。
分割其实就是对每一个像素进行分类。在代码上，分割与分类的区别就更小了，都是用全连接层输出一定的数目，这个数目就是你要分类/分割的个数。
以PointNet为例，先看看网络架构：
可以看到网络在得到global feature之前，分类和分割是公用一套网络的。它们的代码自然也一样。这部分代码位于pointnet_cls.py和pointnet_seg.py中，完全相同。
def get_model(point_cloud, is_training, bn_decay=None):
""" Classification PointNet, input is BxNx3, output Bx40 """
batch_size = point_cloud.get_shape()[0].value
num_point = point_cloud.get_shape()[1].value
end_points = {} with tf.variable_scope('transform_net1') as sc:
transform = input_transform_net(point_cloud, is_training, bn_decay, K=3)
point_cloud_transformed = tf.matmul(point_cloud, transform)
input_image = tf.expand_dims(point_cloud_transformed, -1)
net = tf_util.conv2d(input_image, 64, [1,3],
padding='VALID', stride=[1,1],
bn=True, is_training=is_training,
scope='conv1', bn_decay=bn_decay)
net = tf_util.conv2d(net, 64, [1,1],
padding='VALID', stride=[1,1],
bn=True, is_training=is_training,
scope='conv2', bn_decay=bn_decay) with tf.variable_scope('transform_net2') as sc:
transform = feature_transform_net(net, is_training, bn_decay, K=64)
end_points['transform'] = transform
net_transformed = tf.matmul(tf.squeeze(net, axis=[2]), transform)
net_transformed = tf.expand_dims(net_transformed, [2])
net = tf_util.conv2d(net_transformed, 64, [1,1],
padding='VALID', stride=[1,1],
bn=True, is_training=is_training,
scope='conv3', bn_decay=bn_decay)
net = tf_util.conv2d(net, 128, [1,1],
padding='VALID', stride=[1,1],
bn=True, is_training=is_training,
scope='conv4', bn_decay=bn_decay)
net = tf_util.conv2d(net, 1024, [1,1],
padding='VALID', stride=[1,1],
bn=True, is_training=is_training,
scope='conv5', bn_decay=bn_decay)再往后看，就出现一些区别了。
分类任务：
# Symmetric function: max pooling
net = tf_util.max_pool2d(net, [num_point,1],
padding='VALID', scope='maxpool')
net = tf.reshape(net, [batch_size, -1])
net = tf_util.fully_connected(net, 512, bn=True, is_training=is_training,
scope='fc1', bn_decay=bn_decay)
net = tf_util.dropout(net, keep_prob=0.7, is_training=is_training,
scope='dp1')
net = tf_util.fully_connected(net, 256, bn=True, is_training=is_training,
scope='fc2', bn_decay=bn_decay)
net = tf_util.dropout(net, keep_prob=0.7, is_training=is_training,
scope='dp2')
net = tf_util.fully_connected(net, 40, activation_fn=None, scope='fc3') return net, end_points代码布局如同网络中描绘的一样。池化操作后，做全连接层，最后输出40，对应40类物体分类。
再来看分割：
global_feat = tf_util.max_pool2d(net, [num_point,1],
padding='VALID', scope='maxpool')
print(global_feat)
global_feat_expand = tf.tile(global_feat, [1, num_point, 1, 1])
concat_feat = tf.concat(3, [point_feat, global_feat_expand])
print(concat_feat)
net = tf_util.conv2d(concat_feat, 512, [1,1],
padding='VALID', stride=[1,1],
bn=True, is_training=is_training,
scope='conv6', bn_decay=bn_decay)
net = tf_util.conv2d(net, 256, [1,1],
padding='VALID', stride=[1,1],
bn=True, is_training=is_training,
scope='conv7', bn_decay=bn_decay)
net = tf_util.conv2d(net, 128, [1,1],
padding='VALID', stride=[1,1],
bn=True, is_training=is_training,
scope='conv8', bn_decay=bn_decay)
net = tf_util.conv2d(net, 128, [1,1],
padding='VALID', stride=[1,1],
bn=True, is_training=is_training,
scope='conv9', bn_decay=bn_decay)
net = tf_util.conv2d(net, 50, [1,1],
padding='VALID', stride=[1,1], activation_fn=None,
scope='conv10')
net = tf.squeeze(net, [2]) # BxNxC
return net, end_points除了增加全局特征与点特征的拼接外，也是做了全连接操作，注意此处的全连接使用1*1的卷积实现的，但是本质上和使用fully_connect效果一样。最后的输出是50，对应的是分割任务的50个parts。
最后的损失函数也是一样的。这里就不贴出来了。
所以，总的来说，分割就是一种特殊的分类。当然，为了提高分割效果，可以对损失函数做相应的改进，如平滑等