多尺度檢測環路如何在OpenCV的HOG檢測中工作？

Question

在關於HOG的Dalal和Triggs文件中，似乎多尺度檢測通過掃描圖像金字塔來工作。但是我找不到執行金字塔掃描/循環的modules/objdetect/src/hog.cpp中的哪一部分。我的理解錯了，還是我讀了錯誤的源文件？

Answer 1

如果你看看源代碼，這個功能

void HOGCache::init(const HOGDescriptor* _descriptor, 
     const Mat& _img, Size _paddingTL, Size _paddingBR, 
     bool _useCache, Size _cacheStride) 

你會看到下面的評論

// Initialize 2 lookup tables, pixData & blockData. 
// Here is why: 
// 
// The detection algorithm runs in 4 nested loops (at each pyramid layer): 
// loop over the windows within the input image 
// loop over the blocks within each window 
//  loop over the cells within each block 
//  loop over the pixels in each cell 
// 
// As each of the loops runs over a 2-dimensional array, 
// we could get 8(!) nested loops in total, which is very-very slow. 
// 
// To speed the things up, we do the following: 
// 1. loop over windows is unrolled in the HOGDescriptor::{compute|detect} methods; 
//   inside we compute the current search window using getWindow() method. 
//   Yes, it involves some overhead (function call + couple of divisions), 
//   but it's tiny in fact. 
// 2. loop over the blocks is also unrolled. Inside we use pre-computed blockData[j] 
//   to set up gradient and histogram pointers. 
// 3. loops over cells and pixels in each cell are merged 
//  (since there is no overlap between cells, each pixel in the block is processed once) 
//  and also unrolled. Inside we use PixData[k] to access the gradient values and 
//  update the histogram 
// 

正如註釋說明，這些環展開了優化的目的，這也許是爲什麼通過源代碼快速掃描很難找到它們。