I am trying to calculate HOG features for blocks only. I explored hog.cpp listed under opencv/module/gpu/src/. Here is getDescriptor function
void cv::gpu::HOGDescriptor::computeBlockHistograms(const GpuMat& img)
{
computeGradient(img, grad, qangle);
size_t block_hist_size = getBlockHistogramSize();
Size blocks_per_img = numPartsWithin(img.size(), block_size, block_stride);
// block_hists.create(1, block_hist_size * blocks_per_img.area(), CV_32F);
block_hists = getBuffer(1, static_cast(block_hist_size * blocks_per_img.area()), CV_32F, block_hists_buf);
hog::compute_hists(nbins, block_stride.width, block_stride.height, img.rows, img.cols,
grad, qangle, (float)getWinSigma(), block_hists.ptr());
hog::normalize_hists(nbins, block_stride.width, block_stride.height, img.rows, img.cols,
block_hists.ptr(), (float)threshold_L2hys);
}
void cv::gpu::HOGDescriptor::getDescriptors(const GpuMat& img, Size win_stride, GpuMat& descriptors, int descr_format)
{
CV_Assert(win_stride.width % block_stride.width == 0 && win_stride.height % block_stride.height == 0);
computeBlockHistograms(img);
const size_t block_hist_size = getBlockHistogramSize();
Size blocks_per_win = numPartsWithin(win_size, block_size, block_stride);
Size wins_per_img = numPartsWithin(img.size(), win_size, win_stride);
descriptors.create(wins_per_img.area(), static_cast(blocks_per_win.area() * block_hist_size), CV_32F);
switch (descr_format)
{
case DESCR_FORMAT_ROW_BY_ROW:
hog::extract_descrs_by_rows(win_size.height, win_size.width, block_stride.height, block_stride.width,
win_stride.height, win_stride.width, img.rows, img.cols, block_hists.ptr(), descriptors);
break;
case DESCR_FORMAT_COL_BY_COL:
hog::extract_descrs_by_cols(win_size.height, win_size.width, block_stride.height, block_stride.width,
win_stride.height, win_stride.width, img.rows, img.cols, block_hists.ptr(), descriptors);
break;
default:
CV_Error(CV_StsBadArg, "Unknown descriptor format");
}
}
I also explored hig.cu source file. I need to modify hog::extract_descrs_by_rows and hog::extract_descrs_by_cols. Here is the function from hog.cu
//----------------------------------------------------------------------------
// Extract descriptors
template
__global__ void extract_descrs_by_rows_kernel(const int img_block_width, const int win_block_stride_x, const int win_block_stride_y,
const float* block_hists, PtrStepf descriptors)
{
// Get left top corner of the window in src
const float* hist = block_hists + (blockIdx.y * win_block_stride_y * img_block_width +
blockIdx.x * win_block_stride_x) * cblock_hist_size;
// Get left top corner of the window in dst
float* descriptor = descriptors.ptr(blockIdx.y * gridDim.x + blockIdx.x);
// Copy elements from src to dst
for (int i = threadIdx.x; i < cdescr_size; i += nthreads)
{
int offset_y = i / cdescr_width;
int offset_x = i - offset_y * cdescr_width;
descriptor[i] = hist[offset_y * img_block_width * cblock_hist_size + offset_x];
}
}
void extract_descrs_by_rows(int win_height, int win_width, int block_stride_y, int block_stride_x, int win_stride_y, int win_stride_x,
int height, int width, float* block_hists, PtrStepSzf descriptors)
{
const int nthreads = 256;
int win_block_stride_x = win_stride_x / block_stride_x;
int win_block_stride_y = win_stride_y / block_stride_y;
int img_win_width = (width - win_width + win_stride_x) / win_stride_x;
int img_win_height = (height - win_height + win_stride_y) / win_stride_y;
dim3 threads(nthreads, 1);
dim3 grid(img_win_width, img_win_height);
int img_block_width = (width - CELLS_PER_BLOCK_X * CELL_WIDTH + block_stride_x) / block_stride_x;
extract_descrs_by_rows_kernel(
img_block_width, win_block_stride_x, win_block_stride_y, block_hists, descriptors);
cudaSafeCall( cudaGetLastError() );
cudaSafeCall( cudaDeviceSynchronize() );
}
Can some one help to change the code to get HOG descriptors for block only. As I have three different windows so I only interested to calculate HOG descriptors for block once.
You can might use vlfeat to extract HOG descriptor:
http://www.vlfeat.org/overview/hog.html
You can also check wikipedia to find any other resources: https://en.wikipedia.org/wiki/Histogram_of_oriented_gradients
Please take a look at this link, Shah :
http://kgeorge.github.io/2014/06/03/hog-implementation-and-object-detection/
The HOG creation process is explained in detail, with code. Once you understand in which part the block descriptor is computed you can look for the matching part in the OpenCV code.
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