YOLO源码详解（二）- 函数剖析

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

---

1、网络参数解析函数：parse_network_cfg

network parse_network_cfg(char *filename)
{
    //read_cfg这个函数将所有的参数读到一个图中，如下所示，希望能帮助理解下代码
    list *sections = read_cfg(filename);

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//首先我们看一下list，section, node, kvp这四个结构体
typedef struct{
    int size;
    node *front;
    node *back;
} list;

typedef struct{
    char *type;
    list *options;
} section;

typedef struct node{
    void *val;
    struct node *next;
    struct node *prev;
} node;

typedef struct{
    char *key;
    char *val;
    int used;
} kvp;

//再看下yolo.cfg文件
[net]
batch=64
subdivisions=64
height=448
width=448
channels=3
momentum=0.9
decay=0.0005

learning_rate=0.001
policy=steps
steps=200,400,600,20000,30000
scales=2.5,2,2,.1,.1
max_batches = 40000

[crop]
crop_width=448
crop_height=448
flip=0
.
.
.
/*用矩形表示list，椭圆表示section，圆形表示node，六边形表示kvp，为了表达方便，我就把section和kvp放到了node里面，其实这样表达有失妥当，大家理解就行。根据作者代码我们就可以得出以下的参数网络，如有表达错误或不妥，欢迎指正*/

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我们来大概解释下该参数网，首先创建一个list，取名sections，记录一共有多少个section（一个section存储了CNN一层所需参数）；然后创建一个node，该node的void类型的指针指向一个新创建的section；该section的char类型指针指向.cfg文件中的某一行（line），然后将该section的list指针指向一个新创建的node，该node的void指针指向一个kvp结构体，kvp结构体中的key就是.cfg文件中的关键字（如：batch，subdivisions等），val就是对应的值；如此循环就形成了上述的参数网络图。

    node *n = sections->front;
    if(!n) error("Config file has no sections");
    network net = make_network(sections->size - 1);
    net.gpu_index = gpu_index;
    size_params params;

    section *s = (section *)n->val;
    list *options = s->options;
    if(!is_network(s)) error("First section must be [net] or [network]");
    parse_net_options(options, &net);

    params.h = net.h;
    params.w = net.w;
    params.c = net.c;
    params.inputs = net.inputs;
    params.batch = net.batch;
    params.time_steps = net.time_steps;
    params.net = net;

    size_t workspace_size = 0;
    n = n->next;
    int count = 0;
    free_section(s);
    while(n){
        params.index = count;
        fprintf(stderr, "%d: ", count);
        s = (section *)n->val;
        options = s->options;
        layer l = {0};
        LAYER_TYPE lt = string_to_layer_type(s->type);
        if(lt == CONVOLUTIONAL){
            l = parse_convolutional(options, params);
        }else if(lt == LOCAL){
            l = parse_local(options, params);
        }else if(lt == ACTIVE){
            l = parse_activation(options, params);
        }else if(lt == RNN){
            l = parse_rnn(options, params);
        }else if(lt == GRU){
            l = parse_gru(options, params);
        }else if(lt == CRNN){
            l = parse_crnn(options, params);
        }else if(lt == CONNECTED){
            l = parse_connected(options, params);
        }else if(lt == CROP){
            l = parse_crop(options, params);
        }else if(lt == COST){
            l = parse_cost(options, params);
        }else if(lt == REGION){
            l = parse_region(options, params);
        }else if(lt == DETECTION){
            l = parse_detection(options, params);
        }else if(lt == SOFTMAX){
            l = parse_softmax(options, params);
            net.hierarchy = l.softmax_tree;
        }else if(lt == NORMALIZATION){
            l = parse_normalization(options, params);
        }else if(lt == BATCHNORM){
            l = parse_batchnorm(options, params);
        }else if(lt == MAXPOOL){
            l = parse_maxpool(options, params);
        }else if(lt == REORG){
            l = parse_reorg(options, params);
        }else if(lt == AVGPOOL){
            l = parse_avgpool(options, params);
        }else if(lt == ROUTE){
            l = parse_route(options, params, net);
        }else if(lt == SHORTCUT){
            l = parse_shortcut(options, params, net);
        }else if(lt == DROPOUT){
            l = parse_dropout(options, params);
            l.output = net.layers[count-1].output;
            l.delta = net.layers[count-1].delta;
#ifdef GPU
            l.output_gpu = net.layers[count-1].output_gpu;
            l.delta_gpu = net.layers[count-1].delta_gpu;
#endif
        }else{
            fprintf(stderr, "Type not recognized: %s\n", s->type);
        }
        l.dontload = option_find_int_quiet(options, "dontload", 0);
        l.dontloadscales = option_find_int_quiet(options, "dontloadscales", 0);
        option_unused(options);
        net.layers[count] = l;
        if (l.workspace_size > workspace_size) workspace_size = l.workspace_size;
        free_section(s);
        n = n->next;
        ++count;
        if(n){
            params.h = l.out_h;
            params.w = l.out_w;
            params.c = l.out_c;
            params.inputs = l.outputs;
        }
    }
    free_list(sections);
    net.outputs = get_network_output_size(net);
    net.output = get_network_output(net);
    if(workspace_size){
        //printf("%ld\n", workspace_size);
#ifdef GPU
        if(gpu_index >= 0){
            net.workspace = cuda_make_array(0, (workspace_size-1)/sizeof(float)+1);
        }else {
            net.workspace = calloc(1, workspace_size);
        }
#else
        net.workspace = calloc(1, workspace_size);
#endif
    }
    return net;
}

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２、加载权重函数：load_weights(&net, weightfile)

void load_weights(network *net, char *filename)
{
    //调用load_weights_upto(net, filename, net->n)函数
    load_weights_upto(net, filename, net->n);
}

void load_weights_upto(network *net, char *filename, int cutoff)
{
#ifdef GPU
    if(net->gpu_index >= 0){
        cuda_set_device(net->gpu_index);
    }
#endif
    fprintf(stderr, "Loading weights from %s...", filename);
    //fflush()函数冲洗流中的信息，该函数通常用于处理磁盘文件。
    fflush(stdout);
    FILE *fp = fopen(filename, "rb");
    if(!fp) file_error(filename);

    int major;
    int minor;
    int revision;
    /*size_t fread ( void *buffer, size_t size, size_t count, FILE *stream)
    fread是一个函数。从一个文件流中读数据，最多读取count个项，每个项size个字节，如果调用成功返回实
    际读取到的项个数（小于或等于count），如果不成功或读到文件末尾返回0。*/
    fread(&major, sizeof(int), 1, fp);
    fread(&minor, sizeof(int), 1, fp);
    fread(&revision, sizeof(int), 1, fp);
    fread(net->seen, sizeof(int), 1, fp);
    int transpose = (major > 1000) || (minor > 1000);

    int i;
    //cutoff顾名思义，之后的参数不加载，fine tuning的时候用
    for(i = 0; i < net->n && i < cutoff; ++i){
        //读取各层权重
        layer l = net->layers[i];
        if (l.dontload) continue;
        if(l.type == CONVOLUTIONAL){
            load_convolutional_weights(l, fp);
        }
        //我们看下卷积层的权重加载

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void load_convolutional_weights(layer l, FILE *fp)
{
    if(l.binary){
        //load_convolutional_weights_binary(l, fp);
        //return;
    }
    //卷积层的参数个数，卷积核个数×通道数×卷积核长度×卷积核宽度
    int num = l.n*l.c*l.size*l.size;
    //这个估计没用上吧，直接给０了
    if(0){
        fread(l.biases + ((l.n != 1374)?0:5), sizeof(float), l.n, fp);
        if (l.batch_normalize && (!l.dontloadscales)){
            fread(l.scales + ((l.n != 1374)?0:5), sizeof(float), l.n, fp);
            fread(l.rolling_mean + ((l.n != 1374)?0:5), sizeof(float), l.n, fp);
            fread(l.rolling_variance + ((l.n != 1374)?0:5), sizeof(float), l.n, fp);
        }
        fread(l.weights + ((l.n != 1374)?0:5*l.c*l.size*l.size), sizeof(float), num, fp);
    }else{
        fread(l.biases, sizeof(float), l.n, fp);
        //如果使用了Batch Normalizationn(https://www.zhihu.com/question/38102762,
                                    　　https://arxiv.org/abs/1502.03167)，
        那就加载三个参数
        if (l.batch_normalize && (!l.dontloadscales)){
            fread(l.scales, sizeof(float), l.n, fp);
            fread(l.rolling_mean, sizeof(float), l.n, fp);
            fread(l.rolling_variance, sizeof(float), l.n, fp);
        }
        fread(l.weights, sizeof(float), num, fp);
    }
    if(l.adam){
        fread(l.m, sizeof(float), num, fp);
        fread(l.v, sizeof(float), num, fp);
    }
    //if(l.c == 3) scal_cpu(num, 1./256, l.weights, 1);
    if (l.flipped) {
        transpose_matrix(l.weights, l.c*l.size*l.size, l.n);
    }
    //if (l.binary) binarize_weights(l.weights, l.n, l.c*l.size*l.size, l.weights);
#ifdef GPU
    if(gpu_index >= 0){
        push_convolutional_layer(l);
    }
#endif
}

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        if(l.type == CONNECTED){
            load_connected_weights(l, fp, transpose);
        }
        if(l.type == BATCHNORM){
            load_batchnorm_weights(l, fp);
        }
        if(l.type == CRNN){
            load_convolutional_weights(*(l.input_layer), fp);
            load_convolutional_weights(*(l.self_layer), fp);
            load_convolutional_weights(*(l.output_layer), fp);
        }
        if(l.type == RNN){
            load_connected_weights(*(l.input_layer), fp, transpose);
            load_connected_weights(*(l.self_layer), fp, transpose);
            load_connected_weights(*(l.output_layer), fp, transpose);
        }
        if(l.type == GRU){
            load_connected_weights(*(l.input_z_layer), fp, transpose);
            load_connected_weights(*(l.input_r_layer), fp, transpose);
            load_connected_weights(*(l.input_h_layer), fp, transpose);
            load_connected_weights(*(l.state_z_layer), fp, transpose);
            load_connected_weights(*(l.state_r_layer), fp, transpose);
            load_connected_weights(*(l.state_h_layer), fp, transpose);
        }
        if(l.type == LOCAL){
            int locations = l.out_w*l.out_h;
            int size = l.size*l.size*l.c*l.n*locations;
            fread(l.biases, sizeof(float), l.outputs, fp);
            fread(l.weights, sizeof(float), size, fp);
#ifdef GPU
            if(gpu_index >= 0){
                push_local_layer(l);
            }
#endif
        }
    }
    fprintf(stderr, "Done!\n");
    fclose(fp);
}

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３、train_network(network net, data d)

这个函数是核心～

//network.c
float train_network(network net, data d)
{
    //先看以下data的结构

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//data.h
typedef struct{
    int w, h;
    matrix X;
    matrix y;
    int shallow;
    int *num_boxes;
    box **boxes;
} data;

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//matrix.h
//这里rows是一次加载到内存中的样本的个数（batch*net.subdivisions），cols就是样本的维度，**vals指向的是样本的值
typedef struct matrix{
    int rows, cols;
    float **vals;
} matrix;

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    assert(d.X.rows % net.batch == 0);
    int batch = net.batch;
    //注意，n现在表示加载一次数据可以训练几次，其实就是subdivisions
    int n = d.X.rows / batch;
    float *X = calloc(batch*d.X.cols, sizeof(float));
    float *y = calloc(batch*d.y.cols, sizeof(float));

    int i;
    float sum = 0;
    for(i = 0; i < n; ++i){
        //完成数据拷贝，从ｄ拷贝到Ｘ和ｙ中
        get_next_batch(d, batch, i*batch, X, y);

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//data.c
void get_next_batch(data d, int n, int offset, float *X, float *y)
{
    int j;
    for(j = 0; j < n; ++j){
        //offset就是第几个batch(i*batch)了，j表示的是每个batch中的第几个样本（图像）
        int index = offset + j;
        //void *memcpy(void *dest, const void *src, size_t n);
        //memcpy函数的功能是从源src所指的内存地址的起始位置开始拷贝n个字节到目标dest所指的内存地址的起始位置中
        memcpy(X+j*d.X.cols, d.X.vals[index], d.X.cols*sizeof(float));
        memcpy(y+j*d.y.cols, d.y.vals[index], d.y.cols*sizeof(float));
    }
}

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        float err = train_network_datum(net, X, y);

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//network.c
float train_network_datum(network net, float *x, float *y)
{
#ifdef GPU
    if(gpu_index >= 0) return train_network_datum_gpu(net, x, y);
#endif
    network_state state;
    *net.seen += net.batch;
    state.index = 0;
    state.net = net;
    state.input = x;
    state.delta = 0;
    state.truth = y;
    state.train = 1;
    //这两个函数才是整个网络的重中之重，我们下一节讲
    forward_network(net, state);
    backward_network(net, state);
    float error = get_network_cost(net);

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//network.c
float get_network_cost(network net)
{
    int i;
    float sum = 0;
    int count = 0;
    for(i = 0; i < net.n; ++i){
        //获取各层的损失，用`$grep -rn "l.cost"`可以看出，只有detection层有cost，因为这个复制函数在前向传播里，我们下节再详细分析
        if(net.layers[i].cost){
            sum += net.layers[i].cost[0];
            ++count;
        }
    }
    //返回平均损失
    return sum/count;
}

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    if(((*net.seen)/net.batch)%net.subdivisions == 0) update_network(net);
    return error;
}

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        sum += err;
    }
    free(X);
    free(y);
    return (float)sum/(n*batch);
}

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到这里我们只剩下最重要的两个函数： 
forward_network(net, state); 
backward_network(net, state);

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(END)