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Hi, the HOGDescriptor::detectMultiScale(...) documentation is missing for cpu implementation. But you can take a look at the GPU documentation. From this documentation:

void gpu::HOGDescriptor::detectMultiScale(const GpuMat& img, vector<Rect>& found_locations, double hit_threshold=0, Size win_stride=Size(), Size padding=Size(), double scale0=1.05, int group_threshold=2)

Parameters:

• img – Source image. See gpu::HOGDescriptor::detect() for type limitations.
• found_locations – Detected objects boundaries.
• hit_threshold – Threshold for the distance between features and SVM classifying plane. See gpu::HOGDescriptor::detect() for details.
• win_stride – Window stride. It must be a multiple of block stride.
• padding – Mock parameter to keep the CPU interface compatibility. It must be (0,0).
• scale0 – Coefficient of the detection window increase.
• group_threshold – Coefficient to regulate the similarity threshold. When detected, some objects can be covered by many rectangles. 0 means not to perform grouping. See groupRectangles() .

I recommend you to read the original HoG paper [Navneet Dalal and Bill Triggs. Histogram of oriented gradients for human detection. 2005]. In this paper the authors explain all parameters in detail and show how different parameter settings influence the detection rate.