YOLO源码详解（三）- 前向传播（forward）

本系列作者：木凌 
时间：2016年11月。 
文章连接：http://blog.csdn.net/u014540717

---

一、主函数void forward_network(network net, network_state state)

//network.c
void forward_network(network net, network_state state)
{
    state.workspace = net.workspace;
    int i;
    for(i = 0; i < net.n; ++i){
        state.index = i;
        layer l = net.layers[i];
        //如果delta不为零，那么就把所有的输入值输入乘一个系数，用float *delta指针指向它
        if(l.delta){
            scal_cpu(l.outputs * l.batch, 0, l.delta, 1);
        }
        //从这里开始我们可以一层一层分析了，重复的层就不再分析了，顺序如下：
        //[convolutional]
        //[maxpool]
        //[local]
        //[dropout]
        //[connected]
        //[detection]
        l.forward(l, state);
        state.input = l.output;
    }
}
  
  

   
   
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1、前向传播-convolutional层

//convolutional_layer.c
void forward_convolutional_layer(convolutional_layer l, network_state state)
{
    //获取卷积层输出的长、宽
    int out_h = convolutional_out_height(l);
    int out_w = convolutional_out_width(l);
    int i;

    //初始化，将输出的数据全部赋值０
    fill_cpu(l.outputs*l.batch, 0, l.output, 1);

    /*
       if(l.binary){
       binarize_weights(l.weights, l.n, l.c*l.size*l.size, l.binary_weights);
       binarize_weights2(l.weights, l.n, l.c*l.size*l.size, l.cweights, l.scales);
       swap_binary(&l);
       }
     */

    /*
       if(l.binary){
       int m = l.n;
       int k = l.size*l.size*l.c;
       int n = out_h*out_w;

       char  *a = l.cweights;
       float *b = state.workspace;
       float *c = l.output;

       for(i = 0; i < l.batch; ++i){
       im2col_cpu(state.input, l.c, l.h, l.w, 
       l.size, l.stride, l.pad, b);
       gemm_bin(m,n,k,1,a,k,b,n,c,n);
       c += n*m;
       state.input += l.c*l.h*l.w;
       }
       scale_bias(l.output, l.scales, l.batch, l.n, out_h*out_w);
       add_bias(l.output, l.biases, l.batch, l.n, out_h*out_w);
       activate_array(l.output, m*n*l.batch, l.activation);
       return;
       }
     */

    if(l.xnor){
        binarize_weights(l.weights, l.n, l.c*l.size*l.size, l.binary_weights);
        swap_binary(&l);
        binarize_cpu(state.input, l.c*l.h*l.w*l.batch, l.binary_input);
        state.input = l.binary_input;
    }
    //m是卷积核的个数，k是每个卷积核的参数数量（l.size是卷积核的大小），n是每个输出feature map的像素个数
    int m = l.n;
    int k = l.size*l.size*l.c;
    int n = out_h*out_w;

    if (l.xnor && l.c%32 == 0 && AI2) {
        forward_xnor_layer(l, state);
        printf("xnor\n");
    } else {

        //weights顾名思义，就是卷积核的参数，`$grep -rn "l.weights"`可以看到：
        //l.weights = calloc(c*n*size*size, sizeof(float))
        //说白了a是指向权重的指针，b是指向工作空间指针，c是指向输出的指针
        float *a = l.weights;
        float *b = state.workspace;
        float *c = l.output;

        for(i = 0; i < l.batch; ++i){
            //im2col就是image to column,就是将图像依照卷积核的大小拉伸为列向量，方便矩阵运算
            im2col_cpu(state.input, l.c, l.h, l.w,
                    l.size, l.stride, l.pad, b);
            //这个函数实现矩阵运算，也就是卷积运算
            gemm(0,0,m,n,k,1,a,k,b,n,1,c,n);
            c += n*m;
            //更新输入
            state.input += l.c*l.h*l.w;
        }
    }
    //BN层，加速收敛
    if(l.batch_normalize){
        forward_batchnorm_layer(l, state);
    }
    //添加偏置项
    add_bias(l.output, l.biases, l.batch, l.n, out_h*out_w);
    //非线性变化，leaky RELU层，非常简单，不多做介绍
    activate_array(l.output, m*n*l.batch, l.activation);
    //不太清楚binary和xnor是什么意思，希望有了解的留言，谢谢～
    if(l.binary || l.xnor) swap_binary(&l);
}
  
  

   
   
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函数剖析

a. im2col_cpu()

这个函数还是很重要的，我们来分析下。这个函数是从caffe中移植过来的

//im2col.c
//From Berkeley Vision's Caffe!
//https://github.com/BVLC/caffe/blob/master/LICENSE
void im2col_cpu(float* data_im,
     int channels,  int height,  int width,
     int ksize,  int stride, int pad, float* data_col) 
{
    int c,h,w;
    int height_col = (height + 2*pad - ksize) / stride + 1;
    int width_col = (width + 2*pad - ksize) / stride + 1;

    int channels_col = channels * ksize * ksize;
    //最外层循环是每个卷积核的参数个数
    for (c = 0; c < channels_col; ++c) {
        int w_offset = c % ksize;
        int h_offset = (c / ksize) % ksize;
        int c_im = c / ksize / ksize;
        //这两层循环是用卷积核把图像遍历一遍，这说起来比较晦涩，一会儿画个图来理解，很简单～
        for (h = 0; h < height_col; ++h) {
            for (w = 0; w < width_col; ++w) {
                int im_row = h_offset + h * stride;
                int im_col = w_offset + w * stride;
                int col_index = (c * height_col + h) * width_col + w;
                data_col[col_index] = im2col_get_pixel(data_im, height, width, channels,
                        im_row, im_col, c_im, pad);
            }
        }
    }
}

float im2col_get_pixel(float *im, int height, int width, int channels,
                        int row, int col, int channel, int pad)
{
    row -= pad;
    col -= pad;

    if (row < 0 || col < 0 ||
        row >= height || col >= width) return 0;
    return im[col + width*(row + height*channel)];
}
  
  

   
   
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我画了下面的图来帮助理解下im2col_cpu()这个函数，为了方便理解，这里假设图像尺寸是5*5， stride=2，kernel_size=3 

float *b指向state.workspace这个工作空间，也就是把原始数据变成行向量放到工作空间里，然后进行卷积计算

b. gemm()

这个函数实现了卷积的运算

//gemm.c
void gemm(int TA, int TB, int M, int N, int K, float ALPHA, 
        float *A, int lda, 
        float *B, int ldb,
        float BETA,
        float *C, int ldc)
{
    gemm_cpu( TA,  TB,  M, N, K, ALPHA,A,lda, B, ldb,BETA,C,ldc);
}

void gemm_cpu(int TA, int TB, int M, int N, int K, float ALPHA,
        float *A, int lda,
        float *B, int ldb,
        float BETA,
        float *C, int ldc)
{
    //printf("cpu: %d %d %d %d %d %f %d %d %f %d\n",TA, TB, M, N, K, ALPHA, lda, ldb, BETA, ldc);
    int i, j;
    for(i = 0; i < M; ++i){
        for(j = 0; j < N; ++j){
            C[i*ldc + j] *= BETA;
        }
    }
    if(!TA && !TB)
        //调用这个函数
        gemm_nn(M, N, K, ALPHA,A,lda, B, ldb,C,ldc);
    else if(TA && !TB)
        gemm_tn(M, N, K, ALPHA,A,lda, B, ldb,C,ldc);
    else if(!TA && TB)
        gemm_nt(M, N, K, ALPHA,A,lda, B, ldb,C,ldc);
    else
        gemm_tt(M, N, K, ALPHA,A,lda, B, ldb,C,ldc);
}

void gemm_nn(int M, int N, int K, float ALPHA,
        float *A, int lda,
        float *B, int ldb,
        float *C, int ldc)
{
    int i,j,k;
    //这个函数同样一会儿画图表示吧，说起来太费劲。。。
    for(i = 0; i < M; ++i){
        for(k = 0; k < K; ++k){
            //关键字请求编译器尽可能的将变量存在CPU内部寄存器中，而不是通过内存寻址访问，以提高效率。
            register float A_PART = ALPHA*A[i*lda+k];
            for(j = 0; j < N; ++j){
                C[i*ldc+j] += A_PART*B[k*ldb+j];
            }
        }
    }
}
  
  

   
   
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这个函数的将卷积后的值放入c所指向的内存中（最终生成number of kernel个2*2的feature map） 

函数结束后，开始循环每一个batch，卷积计算结果依次向后放

c. forward_batchnorm_layer()

这个函数实现batch normalization，加速了训练的收敛过程，详细见这篇论文 Batch Normalization: Accelerating Deep Network Training by Reducing Internal Covariate Shift

//batchnorm_layer.c
void forward_batchnorm_layer(layer l, network_state state)
{
    if(l.type == BATCHNORM) copy_cpu(l.outputs*l.batch, state.input, 1, l.output, 1);
    if(l.type == CONNECTED){
        l.out_c = l.outputs;
        l.out_h = l.out_w = 1;
    }
    if(state.train){
        mean_cpu(l.output, l.batch, l.out_c, l.out_h*l.out_w, l.mean);
        variance_cpu(l.output, l.mean, l.batch, l.out_c, l.out_h*l.out_w, l.variance);
        normalize_cpu(l.output, l.mean, l.variance, l.batch, l.out_c, l.out_h*l.out_w);
    } else {
        normalize_cpu(l.output, l.rolling_mean, l.rolling_variance, l.batch, l.out_c, l.out_h*l.out_w);
    }
    scale_bias(l.output, l.scales, l.batch, l.out_c, l.out_h*l.out_w);
}
  
  

   
   
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//blas.c
//计算均值
void mean_cpu(float *x, int batch, int filters, int spatial, float *mean)
{
    float scale = 1./(batch * spatial);
    int i,j,k;
    //注意，这里的均值是不同batch的同一维度的feature的均值
    for(i = 0; i < filters; ++i){
        mean[i] = 0;
        for(j = 0; j < batch; ++j){
            for(k = 0; k < spatial; ++k){
                int index = j*filters*spatial + i*spatial + k;
                mean[i] += x[index];
            }
        }
        mean[i] *= scale;
    }
}
//计算方差
void variance_cpu(float *x, float *mean, int batch, int filters, int spatial, float *variance)
{
    float scale = 1./(batch * spatial - 1);
    int i,j,k;
    for(i = 0; i < filters; ++i){
        variance[i] = 0;
        for(j = 0; j < batch; ++j){
            for(k = 0; k < spatial; ++k){
                int index = j*filters*spatial + i*spatial + k;
                variance[i] += pow((x[index] - mean[i]), 2);
            }
        }
        variance[i] *= scale;
    }
}
//归一化
void normalize_cpu(float *x, float *mean, float *variance, int batch, int filters, int spatial)
{
    int b, f, i;
    for(b = 0; b < batch; ++b){
        for(f = 0; f < filters; ++f){
            for(i = 0; i < spatial; ++i){
                int index = b*filters*spatial + f*spatial + i;
                //公式中的ε=.000001f
                x[index] = (x[index] - mean[f])/(sqrt(variance[f]) + .000001f);
            }
        }
    }
}
  
  

   
   
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//convolutional_layer.c
void scale_bias(float *output, float *scales, int batch, int n, int size)
{
    int i,j,b;
    for(b = 0; b < batch; ++b){
        for(i = 0; i < n; ++i){
            for(j = 0; j < size; ++j){
                //scales就是创建convolutional_layer时分配的l.scales，值全是1
                output[(b*n + i)*size + j] *= scales[i];
            }
        }
    }
}
  
  

   
   
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z这三个函数分别对应论文中的如下公式： 

d. add_bias()

这个函数和scale_bias()一模一样，是什么意思？明白的给解释一下呗～

//convolutional_layer.c
void scale_bias(float *output, float *scales, int batch, int n, int size)
{
    int i,j,b;
    for(b = 0; b < batch; ++b){
        for(i = 0; i < n; ++i){
            for(j = 0; j < size; ++j){
                output[(b*n + i)*size + j] *= scales[i];
            }
        }
    }
}
  
  

   
   
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2、前向传播-maxpool layer

//池化层相对要简单很多，如果理解了卷基层，这一层就很好理解

void forward_maxpool_layer(const maxpool_layer l, network_state state)
{
    int b,i,j,k,m,n;
    int w_offset = -l.pad;
    int h_offset = -l.pad;

    int h = l.out_h;
    int w = l.out_w;
    int c = l.c;

    for(b = 0; b < l.batch; ++b){
        for(k = 0; k < c; ++k){
            //注意这里的h和w是maxpooling层输出的高度和宽度
            for(i = 0; i < h; ++i){
                for(j = 0; j < w; ++j){
                    int out_index = j + w*(i + h*(k + c*b));
                    float max = -FLT_MAX;
                    int max_i = -1;
                    //寻找最大值
                    for(n = 0; n < l.size; ++n){
                        for(m = 0; m < l.size; ++m){
                            int cur_h = h_offset + i*l.stride + n;
                            int cur_w = w_offset + j*l.stride + m;
                            int index = cur_w + l.w*(cur_h + l.h*(k + b*l.c));
                            int valid = (cur_h >= 0 && cur_h < l.h &&
                                         cur_w >= 0 && cur_w < l.w);
                            float val = (valid != 0) ? state.input[index] : -FLT_MAX;
                            max_i = (val > max) ? index : max_i;
                            max   = (val > max) ? val   : max;
                        }
                    }
                    l.output[out_index] = max;
                    l.indexes[out_index] = max_i;
                }
            }
        }
    }
}
  
  

   
   
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3、前向传播-local layer

顾名思义，local层就是只看前一层的一部分，作者这里选择了左上角的区域

void forward_local_layer(const local_layer l, network_state state)
{
    int out_h = local_out_height(l);
    int out_w = local_out_width(l);
    int i, j;
    int locations = out_h * out_w;

    for(i = 0; i < l.batch; ++i){
        copy_cpu(l.outputs, l.biases, 1, l.output + i*l.outputs, 1);
    }

    for(i = 0; i < l.batch; ++i){
        float *input = state.input + i*l.w*l.h*l.c;
        im2col_cpu(input, l.c, l.h, l.w, 
                l.size, l.stride, l.pad, l.col_image);
        float *output = l.output + i*l.outputs;
        for(j = 0; j < locations; ++j){
            float *a = l.weights + j*l.size*l.size*l.c*l.n;
            float *b = l.col_image + j;
            float *c = output + j;

            int m = l.n;
            //n=1说明了作者只取了左上角的local区域，很容易想明白～
            int n = 1;
            int k = l.size*l.size*l.c;

            gemm(0,0,m,n,k,1,a,k,b,locations,1,c,locations);
        }
    }
    activate_array(l.output, l.outputs*l.batch, l.activation);
}
  
  

   
   
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4、前向传播-dropout layer

dropout层主要是为了防止过拟合，详细可以自己搜索，这里不做过多介绍

void forward_dropout_layer(dropout_layer l, network_state state)
{
    int i;
    //dropout层只在训练阶段有效
    if (!state.train) return;
    for(i = 0; i < l.batch * l.inputs; ++i){
        //产生0~1之间的随机数
        float r = rand_uniform(0, 1);
        l.rand[i] = r;
        //如果小于给定概率值，就把相应的输入项赋0
        if(r < l.probability) state.input[i] = 0;
        else state.input[i] *= l.scale;
    }
}
  
  

   
   
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5、前向传播-connected layer

void forward_connected_layer(connected_layer l, network_state state)
{
    int i;
    fill_cpu(l.outputs*l.batch, 0, l.output, 1);
    int m = l.batch;
    int k = l.inputs;
    int n = l.outputs;
    float *a = state.input;
    float *b = l.weights;
    float *c = l.output;
    //注意这里的TB=1了，所以调用了gemm_tn()这个函数，下面会有介绍
    gemm(0,1,m,n,k,1,a,k,b,k,1,c,n);
    if(l.batch_normalize){
        if(state.train){
            mean_cpu(l.output, l.batch, l.outputs, 1, l.mean);
            variance_cpu(l.output, l.mean, l.batch, l.outputs, 1, l.variance);
            //将l.rolling_mean所有值赋0.95（移动平均什么意思呢？自己百度吧，数据分析用的很多～）
            scal_cpu(l.outputs, .95, l.rolling_mean, 1);
            //将l.rolling_mean的值加上0.5*l.mean
            axpy_cpu(l.outputs, .05, l.mean, 1, l.rolling_mean, 1);
            //将l.rolling_variance所有值赋0.95
            scal_cpu(l.outputs, .95, l.rolling_variance, 1);
            //将l.rolling_variance的值加上0.5*l.variance
            axpy_cpu(l.outputs, .05, l.variance, 1, l.rolling_variance, 1);

            //将l.output的值赋值到l.x，此时l.x是没有经过BN的
            copy_cpu(l.outputs*l.batch, l.output, 1, l.x, 1);
            //BN
            normalize_cpu(l.output, l.mean, l.variance, l.batch, l.outputs, 1);
            //将l.output的值赋值到l.x_norm，此时l.x_norm是经过BN之后的数据
            copy_cpu(l.outputs*l.batch, l.output, 1, l.x_norm, 1);
        } else {
            normalize_cpu(l.output, l.rolling_mean, l.rolling_variance, l.batch, l.outputs, 1);
        }
        scale_bias(l.output, l.scales, l.batch, l.outputs, 1);
    }
    for(i = 0; i < l.batch; ++i){
        axpy_cpu(l.outputs, 1, l.biases, 1, l.output + i*l.outputs, 1);
    }
    //线性变换返回值不变
    activate_array(l.output, l.outputs*l.batch, l.activation);
}
  
  

   
   
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//gemm.c
void gemm_nt(int M, int N, int K, float ALPHA, 
        float *A, int lda, 
        float *B, int ldb,
        float *C, int ldc)
{
    int i,j,k;
    //M=batch，每个样本有N(yolo.train.cfg中是1715=S×S×(B∗5+C))个输出
    for(i = 0; i < M; ++i){
        for(j = 0; j < N; ++j){
            register float sum = 0;
            //K是inputs，即输入个数
            for(k = 0; k < K; ++k){
                //输入项和权重项对应相乘相加
                sum += ALPHA*A[i*lda+k]*B[j*ldb + k];
            }
            C[i*ldc+j] += sum;
        }
    }
}
  
  

   
   
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6、前向传播-detectiondetection layer

这是最后一层了，这也是作者论文的精髓所在，希望大家能对比论文认真看一下。 
前向传播终于要结束了，这篇博文有点小长～～

void forward_detection_layer(const detection_layer l, network_state state)
{
    int locations = l.side*l.side;
    int i,j;
    memcpy(l.output, state.input, l.outputs*l.batch*sizeof(float));
    //if(l.reorg) reorg(l.output, l.w*l.h, size*l.n, l.batch, 1);
    int b;
    //这里的softmax=0，所以最后竟然都没有softmax层……
    if (l.softmax){
        for(b = 0; b < l.batch; ++b){
            int index = b*l.inputs;
            for (i = 0; i < locations; ++i) {
                int offset = i*l.classes;
                softmax(l.output + index + offset, l.classes, 1,
                        l.output + index + offset);
            }
        }
    }
    //训练的时候才需要cost function
    if(state.train){
        float avg_iou = 0;
        float avg_cat = 0;
        float avg_allcat = 0;
        float avg_obj = 0;
        float avg_anyobj = 0;
        int count = 0;
        *(l.cost) = 0;
        int size = l.inputs * l.batch;
        /*void *memset(void *s, int ch, size_t n);
        memset是计算机中C/C++语言函数。将s所指向的某一块内存中的前n个 字节的内容全部设置
        为ch指定的ASCII值*/
        memset(l.delta, 0, size * sizeof(float));//l.delta存放的loss function的没一项
        for (b = 0; b < l.batch; ++b){
            int index = b*l.inputs;
            //locations = 7*7
            for (i = 0; i < locations; ++i) {
                //coords包括x, y, w, h，1代表的是置信度
                //truth_index是真实值的坐标索引
                int truth_index = (b*locations + i)*(1+l.coords+l.classes);
                int is_obj = state.truth[truth_index];
                //计算置信度的损失
                for (j = 0; j < l.n; ++j) {
                    //p_index是预测值的坐标索引，每个网格预测l.n个框，这里l.n=3（cfg文件中的num值）,论文中是2
                    int p_index = index + locations*l.classes + i*l.n + j;
                    l.delta[p_index] = l.noobject_scale*(0 - l.output[p_index]);
                    //对应论文公式，这里先假设B个框中都没有物体
                    *(l.cost) += l.noobject_scale*pow(l.output[p_index], 2);
                    avg_anyobj += l.output[p_index];
                }

                int best_index = -1;
                float best_iou = 0;
                float best_rmse = 20;

                if (!is_obj){
                    continue;
                }

                int class_index = index + i*l.classes;                
                for(j = 0; j < l.classes; ++j) {
                    //计算类别的损失
                    l.delta[class_index+j] = l.class_scale * (state.truth[truth_index+1+j] - l.output[class_index+j]);
                    *(l.cost) += l.class_scale * pow(state.truth[truth_index+1+j] - l.output[class_index+j], 2);
                    if(state.truth[truth_index + 1 + j]) avg_cat += l.output[class_index+j];
                    avg_allcat += l.output[class_index+j];
                }

                //计算位置信息的损失
                box truth = float_to_box(state.truth + truth_index + 1 + l.classes);
                truth.x /= l.side;
                truth.y /= l.side;

                /*寻找最后预测框（We only predict one set of class probabilities per grid cell, 
                regardless of the number of boxes B）*/
                for(j = 0; j < l.n; ++j){
                    int box_index = index + locations*(l.classes + l.n) + (i*l.n + j) * l.coords;
                    box out = float_to_box(l.output + box_index);
                    out.x /= l.side;
                    out.y /= l.side;

                    if (l.sqrt){
                        out.w = out.w*out.w;
                        out.h = out.h*out.h;
                    }

                    //计算iou的值
                    float iou  = box_iou(out, truth);
                    //iou = 0;
                    //计算均方根误差（root-mean-square error）
                    float rmse = box_rmse(out, truth);
                    //选出iou最大或者均方根误差最小的那个框作为最后预测框～
                    if(best_iou > 0 || iou > 0){
                        if(iou > best_iou){
                            best_iou = iou;
                            best_index = j;
                        }
                    }else{
                        if(rmse < best_rmse){
                            best_rmse = rmse;
                            best_index = j;
                        }
                    }
                }
                //强制确定一个最后预测框
                if(l.forced){
                    if(truth.w*truth.h < .1){
                        best_index = 1;
                    }else{
                        best_index = 0;
                    }
                }
                //随机确定一个最后预测框～
                if(l.random && *(state.net.seen) < 64000){
                    best_index = rand()%l.n;
                }

                //预测的框的索引
                int box_index = index + locations*(l.classes + l.n) + (i*l.n + best_index) * l.coords;
                //真实框的索引
                int tbox_index = truth_index + 1 + l.classes;

                box out = float_to_box(l.output + box_index);
                out.x /= l.side;
                out.y /= l.side;
                if (l.sqrt) {
                    out.w = out.w*out.w;
                    out.h = out.h*out.h;
                }
                float iou  = box_iou(out, truth);

                //printf("%d,", best_index);
                int p_index = index + locations*l.classes + i*l.n + best_index;
                //前面假设了B个框中都没有物体来计算损失，这里再把有物体的那个减掉
                *(l.cost) -= l.noobject_scale * pow(l.output[p_index], 2);
                *(l.cost) += l.object_scale * pow(1-l.output[p_index], 2);
                avg_obj += l.output[p_index];
                l.delta[p_index] = l.object_scale * (1.-l.output[p_index]);

                if(l.rescore){
                    l.delta[p_index] = l.object_scale * (iou - l.output[p_index]);
                }

                l.delta[box_index+0] = l.coord_scale*(state.truth[tbox_index + 0] - l.output[box_index + 0]);
                l.delta[box_index+1] = l.coord_scale*(state.truth[tbox_index + 1] - l.output[box_index + 1]);
                l.delta[box_index+2] = l.coord_scale*(state.truth[tbox_index + 2] - l.output[box_index + 2]);
                l.delta[box_index+3] = l.coord_scale*(state.truth[tbox_index + 3] - l.output[box_index + 3]);
                if(l.sqrt){
                    l.delta[box_index+2] = l.coord_scale*(sqrt(state.truth[tbox_index + 2]) - l.output[box_index + 2]);
                    l.delta[box_index+3] = l.coord_scale*(sqrt(state.truth[tbox_index + 3]) - l.output[box_index + 3]);
                }

                //把iou作为损失，这包含了x,y,w,h四个参数，其实后来没用iou来计算损失，而是论文中给的公式
                *(l.cost) += pow(1-iou, 2);
                avg_iou += iou;
                ++count;
            }
        }
        //论文中没用到
        if(0){
            float *costs = calloc(l.batch*locations*l.n, sizeof(float));
            for (b = 0; b < l.batch; ++b) {
                int index = b*l.inputs;
                for (i = 0; i < locations; ++i) {
                    for (j = 0; j < l.n; ++j) {
                        int p_index = index + locations*l.classes + i*l.n + j;
                        costs[b*locations*l.n + i*l.n + j] = l.delta[p_index]*l.delta[p_index];
                    }
                }
            }
            int indexes[100];
            top_k(costs, l.batch*locations*l.n, 100, indexes);
            float cutoff = costs[indexes[99]];
            for (b = 0; b < l.batch; ++b) {
                int index = b*l.inputs;
                for (i = 0; i < locations; ++i) {
                    for (j = 0; j < l.n; ++j) {
                        int p_index = index + locations*l.classes + i*l.n + j;
                        if (l.delta[p_index]*l.delta[p_index] < cutoff) l.delta[p_index] = 0;
                    }
                }
            }
            free(costs);
        }

        //前面的*(l.cost)其实可以注释掉了，因为前面都没用，到这里才计算loss
        *(l.cost) = pow(mag_array(l.delta, l.outputs * l.batch), 2);

        //打印log
        printf("Detection Avg IOU: %f, Pos Cat: %f, All Cat: %f, Pos Obj: %f, Any Obj: %f, count: %d\n", avg_iou/count, avg_cat/count, avg_allcat/(count*l.classes), avg_obj/count, avg_anyobj/(l.batch*locations*l.n), count);
        //if(l.reorg) reorg(l.delta, l.w*l.h, size*l.n, l.batch, 0);
    }
}
  
  

   
   
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二、总结

读到这里，我们已经完全掌握了YOLO代码的框架，我们大概总结下darknet的优缺点。 
优点：

1. 代码依赖项少，只有cuda，甚至连opencv都可以不需要，如果你在cpu平台，cuda都可以扔了（当然darknet的cup代码并没有做什么优化，跑起来就很慢）。这对于做工程的人来说是非常好的消息，因为我们可以很easy的将代码移植到其他平台

缺点：

1. 在darknet中，所有层的lr都一样，这对微调造成了很大的困难，因为微调需要把前面几层的lr都设置的很小很小，然后主要训练最后一层的权重

2. 总的来说就是darknet的接口确实很差，如果想把网络改成inception或者resnet的构架，需要改大量的代码，这对于验证模型可行性来说，非常浪费时间。你也应该能理解到为什么我们想要将代码移植到mxnet或者caffe上，然后在mxnet上做模型压缩了

下一篇是反向传播部分的代码，这部分是很重要的，但对与移植或者进一步改进网络来说，其实没必要理解反向传播部分的代码，但如果你想对CNN更深入的了解，可以继续看一下篇关于反向传播部分的内容。

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(END)