darknet源码解读-im2col_cpu

  关于im2col_cpu网上已经有不少优秀的解读博文，我不再复述，可以参考以下几篇文章：

https://blog.csdn.net/mrhiuser/article/details/52672824

https://blog.csdn.net/dwyane12138/article/details/78449898

我这里只是将这段代码单独摘出来做了一个小测试，给定一个输入，看看输出究竟是什么，以佐证自己的理解。代码主要参数如下：

输入：3通道3x3矩阵，使用一维数组表示，如图1。

卷积核尺寸：2x2

填充：0

步长：1

                                                                   图1：输入矩阵的一维数组表示

示例代码：

#include <stdio.h>
#include <stdlib.h>int conv_out_height(h, pad, size, stride) {return (h + 2*pad - size) / stride + 1;
}int conv_out_width(w, pad, size, stride) {return (w + 2*pad - size) / stride + 1;
}int im2col_get_pixel(int *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)];
}//From Berkeley Vision's Caffe!
//https://github.com/BVLC/caffe/blob/master/LICENSE
void im2col_cpu(int* data_im,int channels,  int height,  int width,int ksize,  int stride, int pad, int* 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;for (c = 0; c < channels_col; ++c) { //卷积核参数个数int w_offset = c % ksize;int h_offset = (c / ksize) % ksize;int c_im = c / ksize / ksize;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);}}}
}int main(int argc, char* argv[]) {int *data_im=NULL;int *data_col=NULL;int channels=3,height=3,width=3;int ksize=2,stride=1,pad=0;int out_w,out_h;int workspace_size;int inputs = height * width * channels;data_im = (int*)malloc(inputs * sizeof(int));if (!data_im) {printf("malloc error\n");exit(EXIT_FAILURE);}out_w = conv_out_width(width, pad, ksize, stride);out_h = conv_out_width(height, pad, ksize, stride);workspace_size = out_h * out_w * ksize * ksize * channels;data_col = (int*)malloc(workspace_size * sizeof(int));if (!data_col) {printf("malloc error\n");exit(EXIT_FAILURE);}//init imagefor (int i=0; i<inputs; i++) data_im[i] = i;im2col_cpu(data_im, channels, height, width, ksize, stride, pad, data_col);printf("data_im:\n");for (int i=0; i<inputs; i++) {printf("%-3d", data_im[i]);//if( (i+1) % 4 == 0) printf("\n");}printf("\ndata_col:\n");for (int i=0; i<workspace_size; i++) {printf("%-3d", data_col[i]);//if( (i+1) % 4 == 0) printf("\n");}printf("\n");free(data_im);free(data_col);exit(EXIT_SUCCESS);
}

运行程序，观察结果：