语义分割学习笔记（三）——SegNet Upsample层解析

1 参数设置

message UpsampleParameter {
  // DEPRECATED. No need to specify upsampling scale factors when
  // exact output shape is given by upsample_h, upsample_w parameters.
  optional uint32 scale = 1 [default = 2];
  // DEPRECATED. No need to specify upsampling scale factors when
  // exact output shape is given by upsample_h, upsample_w parameters.
  optional uint32 scale_h = 2;
  // DEPRECATED. No need to specify upsampling scale factors when
  // exact output shape is given by upsample_h, upsample_w parameters.
  optional uint32 scale_w = 3;
  // DEPRECATED. Specify exact output height using upsample_h. This
  // parameter only works when scale is 2
  optional bool pad_out_h = 4 [default = false];
  // DEPRECATED. Specify exact output width using upsample_w. This
  // parameter only works when scale is 2
  optional bool pad_out_w = 5 [default = false];
  optional uint32 upsample_h = 6;
  optional uint32 upsample_w = 7;
}

可设置参数为：
    scale 
    scale_h scale_w
    pad_out_h pad_out_w
    upsample_h upsample_w

2  top层特征图大小计算

（1）先判断是否指定 upsample_hupsample_w，如果指定，大小为指定大小，否则（2）

（2）判断是否指定 scale_h， 如果未指定， scale_h_ = scale_w_ = scale，否则（3）

（3）scale_h_  = scale_h   scale_w_=scale_w

   只有scale_h_ = scale_w_ =2时，才可以指定pad_out_h，pad_out_w，否则错误，如果是（2）（3）则top特征图大小为：

       upsample_h_ = bottom[0]->height() *  scale_h_ - int(pad_out_h) 

       upsample_w_ = bottom[0]->width() *  scale_w_ - int(pad_out_w) 

注：

（1）如果输入图像的高和宽不是32的整数倍，需要指定upsample_h, upsample_w的大小，不然会出现维度不一致的错误，原因是upsample需要借助编码过程中pool层的位置信息，例如： pool前特征图大小为45， pool后为23，如果直接对23 unsample， 其大小为46， 而pool产生的位置图大小为45，造成upsample时大小不一致；

（2）指定upsample_h  upsample_w的大小时，需要根据编码过程中对应pool特征图的大小，来设定upsample的大小，例如样例proto中输入图像大小为480*360， 以360分析：360—pool1（180）—pool2 （90）—pool3 （45）—pool4（23）—pool5（12）， upsample5需要借助pool4位置信息，需要与pool4大小一致，因此upsamle_h=23 ~

3 源码

#include 
#include 
#include 
#include 

#include "caffe/layers/upsample_layer.hpp"

namespace caffe {

template 
void UpsampleLayer::LayerSetUp(const vector*>& bottom,
      const vector*>& top) {
  UpsampleParameter upsample_param = this->layer_param_.upsample_param();
  CHECK((upsample_param.has_upsample_h() && upsample_param.has_upsample_w())
      || (!upsample_param.has_scale() && upsample_param.has_scale_h()
      && upsample_param.has_scale_w())
      || (!upsample_param.has_scale_h() && !upsample_param.has_scale_w()))
      << "upsample_h & upsample_w are required, else (DEPRECATED) "
      << "scale OR scale_h & scale_w are required.";

  if (upsample_param.has_upsample_h() && upsample_param.has_upsample_w()) {
    upsample_h_ = upsample_param.upsample_h(); //根据upsample_h upsample_w参数设定
    upsample_w_ = upsample_param.upsample_w();
    CHECK_GT(upsample_h_, 1);
    CHECK_GT(upsample_w_, 1);
  } else {
    LOG(INFO) << "Params 'pad_out_{}_' are deprecated. Please declare upsample"
        << " height and width useing the upsample_h, upsample_w parameters.";
    if (!upsample_param.has_scale_h()) { //根据scale设定  没有scale_h直接根据scale设定
      scale_h_ = scale_w_ = upsample_param.scale();
      CHECK_GT(scale_h_, 1);
    } else {
      scale_h_ = upsample_param.scale_h();
      scale_w_ = upsample_param.scale_w();
      CHECK_GT(scale_h_, 1);
      CHECK_GT(scale_w_, 1);
    }
    pad_out_h_ = upsample_param.pad_out_h();
    pad_out_w_ = upsample_param.pad_out_w();
    CHECK(!pad_out_h_ || scale_h_ == 2)  //只有scale_h scale_w=2时，才可以指定
        << "Output height padding compensation requires scale_h == 2, otherwise "
        << "the output size is ill-defined.";
    CHECK(!pad_out_w_ || scale_w_ == 2) 
        << "Output width padding compensation requires scale_w == 2, otherwise "
        << "the output size is ill-defined.";
    upsample_h_ = upsample_w_ = -1;  // flag to calculate in Reshape
  }
}

template 
void UpsampleLayer::Reshape(const vector*>& bottom,
      const vector*>& top) {
  CHECK_EQ(4, bottom[0]->num_axes()) << "Input must have 4 axes, "
      << "corresponding to (num, channels, height, width)";
  CHECK_EQ(4, bottom[1]->num_axes()) << "Input mask must have 4 axes, "
      << "corresponding to (num, channels, height, width)";
  CHECK_EQ(bottom[0]->num(), bottom[1]->num());
  CHECK_EQ(bottom[0]->channels(), bottom[1]->channels());
  CHECK_EQ(bottom[0]->height(), bottom[1]->height());
  CHECK_EQ(bottom[0]->width(), bottom[1]->width());

  if (upsample_h_ <= 0 || upsample_w_ <= 0) {
    upsample_h_ = bottom[0]->height() * scale_h_ - int(pad_out_h_); // upsample_h_ = height*scale-pad_out 
    upsample_w_ = bottom[0]->width() * scale_w_ - int(pad_out_w_);
  }
  top[0]->Reshape(bottom[0]->num(), bottom[0]->channels(), upsample_h_,
      upsample_w_);
  channels_ = bottom[0]->channels();
  height_ = bottom[0]->height();
  width_ = bottom[0]->width();
}

template 
void UpsampleLayer::Forward_cpu(const vector*>& bottom,
      const vector*>& top) {
  const Dtype* bottom_data = bottom[0]->cpu_data();
  const Dtype* bottom_mask_data = bottom[1]->cpu_data();
  Dtype* top_data = top[0]->mutable_cpu_data();

  // Initialize
  const int top_count = top[0]->count();
  caffe_set(top_count, Dtype(0), top_data);
  // The main loop
  for (int n = 0; n < bottom[0]->num(); ++n) {
    for (int c = 0; c < channels_; ++c) {
      for (int i = 0; i < height_ * width_; ++i) {
        const int idx = static_cast(bottom_mask_data[i]);
        if (idx >= upsample_h_ * upsample_w_) {
          // this can happen if the pooling layer that created the input mask
          // had an input with different size to top[0]
          LOG(FATAL) << "upsample top index " << idx << " out of range - "
            << "check scale settings match input pooling layer's "
            << "downsample setup";
        }
        top_data[idx] = bottom_data[i];
      }
      // compute offset
      bottom_data += bottom[0]->offset(0, 1);
      bottom_mask_data += bottom[1]->offset(0, 1);
      top_data += top[0]->offset(0, 1);
    }
  }
}

template 
void UpsampleLayer::Backward_cpu(const vector*>& top,
      const vector& propagate_down, const vector*>& bottom) {
  if (propagate_down[0]) {
    const Dtype* top_diff = top[0]->cpu_diff();
    const Dtype* bottom_mask_data = bottom[1]->cpu_data();
    Dtype* bottom_diff = bottom[0]->mutable_cpu_diff();

    const int bottom_count = bottom[0]->count();
    caffe_set(bottom_count, Dtype(0), bottom_diff);
    // The main loop
    for (int n = 0; n < bottom[0]->num(); ++n) {
      for (int c = 0; c < channels_; ++c) {
        for (int i = 0; i < height_ * width_; ++i) {
          const int idx = static_cast(bottom_mask_data[i]);
          if (idx >= height_ * width_ * scale_h_ * scale_w_) {
            // this can happen if the pooling layer that created
            // the input mask had an input with different size to top[0]
            LOG(FATAL) << "upsample top index " << idx << " out of range - "
              << "check scale settings match input pooling layer's downsample setup";
          }
          bottom_diff[i] = top_diff[idx];
        }
        // compute offset
        bottom_diff += bottom[0]->offset(0, 1);
        bottom_mask_data += bottom[1]->offset(0, 1);
        top_diff += top[0]->offset(0, 1);
      }
    }
  }
}


#ifdef CPU_ONLY
STUB_GPU(UpsampleLayer);
#endif

INSTANTIATE_CLASS(UpsampleLayer);
REGISTER_LAYER_CLASS(Upsample);

}  // namespace caffe