caffe卷积层实现

下图是jiayangqing在知乎上的回答,其实过程就是把image转换成矩阵,然后进行矩阵运算

卷积的实现在conv_layer层,conv_layer层继承了base_conv_layer层,base_conv_layer层是卷积操作的基类,包含卷积和反卷积.conv_layer层的前向传播是通过forward_cpu_gemm函数实现,这个函数在vision_layer.hpp里进行了定义,在base_conv_layer.cpp里进行了实现.forward_cpu_gemm函数调用了caffe_cpu_gemm和conv_im2col_cpu,caffe_cpu_gemm的实现在util/math_function.cpp里,conv_im2col_cpu的实现在util/im2col.cpp里

conv_layer层的前向传播代码,先调forward_cpu_gemm函数进行卷积的乘法运算,然后根据是否需要bias项,调用forward_cpu_bias进行加法运算.

template <typename Dtype>
void ConvolutionLayer<Dtype>::Forward_cpu(const vector<Blob<Dtype>*>& bottom,
      const vector<Blob<Dtype>*>& top) {
  const Dtype* weight = this->blobs_[0]->cpu_data();
  for (int i = 0; i < bottom.size(); ++i) {
    const Dtype* bottom_data = bottom[i]->cpu_data();
    Dtype* top_data = top[i]->mutable_cpu_data();
    for (int n = 0; n < this->num_; ++n) {
      this->forward_cpu_gemm(bottom_data + bottom[i]->offset(n), weight,
          top_data + top[i]->offset(n));
      if (this->bias_term_) {
        const Dtype* bias = this->blobs_[1]->cpu_data();
        this->forward_cpu_bias(top_data + top[i]->offset(n), bias);
      }   
    }   
  }
}

num_在vision_layers.hpp中的BaseConvolutionLayer类中定义,表示batchsize(https://blog.csdn.net/sinat_22336563/article/details/69808612,这个博客给做了说明).也就是说.每一层卷积是先把一个batch所有的数据计算完才传给下一层,不是把batch中的一个在整个网络中计算一次,再把batch中的下一个传进整个网络进行计算.这里的bottom[i]->offset(n),相当于指向一个batch中下一个图片或者feature map的内存地址,即对batch中下一个进行计算

offset在blob.hpp中定义.bottom、top都是blob类,所以可以去调用blob这个类的属性或者方法,看具体实现的时候直接去看这个类怎么实现的就OK.

  inline int offset(const int n, const int c = 0, const int h = 0,
      const int w = 0) const {
    CHECK_GE(n, 0); 
    CHECK_LE(n, num());
    CHECK_GE(channels(), 0); 
    CHECK_LE(c, channels());
    CHECK_GE(height(), 0); 
    CHECK_LE(h, height());
    CHECK_GE(width(), 0); 
    CHECK_LE(w, width());
    return ((n * channels() + c) * height() + h) * width() + w;
  }

  inline int offset(const vector<int>& indices) const {
    CHECK_LE(indices.size(), num_axes());
    int offset = 0;
    for (int i = 0; i < num_axes(); ++i) {
      offset *= shape(i);
      if (indices.size() > i) {
        CHECK_GE(indices[i], 0); 
        CHECK_LT(indices[i], shape(i));
        offset += indices[i];
      }   
    }   
    return offset;
  }

https://www.cnblogs.com/neopenx/p/5294682.html

forward_cpu_gemm、forward_cpu_bias函数的实现.is_1x1_用来判断是不是1x1的卷积操作,skip_im2col用来判断是不是需要把图片转换成矩阵

template <typename Dtype>
void BaseConvolutionLayer<Dtype>::forward_cpu_gemm(const Dtype* input,
    const Dtype* weights, Dtype* output, bool skip_im2col) {
  const Dtype* col_buff = input;
  if (!is_1x1_) {
    if (!skip_im2col) {
      conv_im2col_cpu(input, col_buffer_.mutable_cpu_data());
    }
    col_buff = col_buffer_.cpu_data();
  }
  for (int g = 0; g < group_; ++g) {
    caffe_cpu_gemm<Dtype>(CblasNoTrans, CblasNoTrans, conv_out_channels_ /
        group_, conv_out_spatial_dim_, kernel_dim_ / group_,
        (Dtype)1., weights + weight_offset_ * g, col_buff + col_offset_ * g,
        (Dtype)0., output + output_offset_ * g);
  }
}

template <typename Dtype>
void BaseConvolutionLayer<Dtype>::forward_cpu_bias(Dtype* output,
    const Dtype* bias) {
  caffe_cpu_gemm<Dtype>(CblasNoTrans, CblasNoTrans, num_output_,
      height_out_ * width_out_, 1, (Dtype)1., bias, bias_multiplier_.cpu_data(),
      (Dtype)1., output);
}

conv_im2col_cpu

inline void conv_im2col_cpu(const Dtype* data, Dtype* col_buff) {
    im2col_cpu(data, 1, conv_in_channels_, conv_in_height_, conv_in_width_,
      kernel_h_, kernel_w_, pad_h_, pad_w_, stride_h_, stride_w_,
      1, 1, col_buff);
  }

template <typename Dtype>
void im2col_cpu(const Dtype* data_im, const int channels,
    const int height, const int width, const int kernel_h, const int kernel_w,
    const int pad_h, const int pad_w,
    const int stride_h, const int stride_w,
    const int dilation_h, const int dilation_w,
    Dtype* data_col) {
  const int output_h = (height + 2 * pad_h -
    (dilation_h * (kernel_h - 1) + 1)) / stride_h + 1;
  const int output_w = (width + 2 * pad_w -
    (dilation_w * (kernel_w - 1) + 1)) / stride_w + 1;
  const int channel_size = height * width;
  for (int channel = channels; channel--; data_im += channel_size) {
    for (int kernel_row = 0; kernel_row < kernel_h; kernel_row++) {
      for (int kernel_col = 0; kernel_col < kernel_w; kernel_col++) {
        int input_row = -pad_h + kernel_row * dilation_h;
        for (int output_rows = output_h; output_rows; output_rows--) {
          if (!is_a_ge_zero_and_a_lt_b(input_row, height)) {
            for (int output_cols = output_w; output_cols; output_cols--) {
              *(data_col++) = 0;
            }
          } else {
            int input_col = -pad_w + kernel_col * dilation_w;
            for (int output_col = output_w; output_col; output_col--) {
              if (is_a_ge_zero_and_a_lt_b(input_col, width)) {
                *(data_col++) = data_im[input_row * width + input_col];
              } else {
                *(data_col++) = 0;
              }
              input_col += stride_w;
            }
          }
          input_row += stride_h;
        }
      }
    }
  }
}

caffe_cpu_gemm函数的实现.cblas_sgemm是cblas库的一个函数(# inclue<cblas.h>),

template<>
void caffe_cpu_gemm<float>(const CBLAS_TRANSPOSE TransA,
    const CBLAS_TRANSPOSE TransB, const int M, const int N, const int K,
    const float alpha, const float* A, const float* B, const float beta,
    float* C) {
  int lda = (TransA == CblasNoTrans) ? K : M;
  int ldb = (TransB == CblasNoTrans) ? N : K;
  cblas_sgemm(CblasRowMajor, TransA, TransB, M, N, K, alpha, A, lda, B,
      ldb, beta, C, N); 
}

template<>
void caffe_cpu_gemm<double>(const CBLAS_TRANSPOSE TransA,
    const CBLAS_TRANSPOSE TransB, const int M, const int N, const int K,
    const double alpha, const double* A, const double* B, const double beta,
    double* C) {
  int lda = (TransA == CblasNoTrans) ? K : M;
  int ldb = (TransB == CblasNoTrans) ? N : K;
  cblas_dgemm(CblasRowMajor, TransA, TransB, M, N, K, alpha, A, lda, B,
      ldb, beta, C, N); 
}

template<>
void caffe_cpu_gemm<Half>(const CBLAS_TRANSPOSE TransA,
    const CBLAS_TRANSPOSE TransB, const int M, const int N, const int K,
    const Half alpha, const Half* A, const Half* B, const Half beta,
    Half* C) {
  DeviceMemPool& pool = Caffe::Get().cpu_mem_pool();
  float* fA = (float*)pool.Allocate(M*K*sizeof(float));
  float* fB = (float*)pool.Allocate(K*N*sizeof(float));
  float* fC = (float*)pool.Allocate(M*N*sizeof(float));
  halves2floats_cpu(A, fA, M*K);
  halves2floats_cpu(B, fB, K*N);
  halves2floats_cpu(C, fC, M*N);
  caffe_cpu_gemm<float>(TransA, TransB, M, N, K, float(alpha), fA, fB, float(beta), fC);
  floats2halves_cpu(fC, C, M*N);
  pool.Free((char*)fA);
  pool.Free((char*)fB);
  pool.Free((char*)fC);
}