BRIEF (Binary Robust Independent Elementary Features)

• BRIEF is not a feature detector—it is a feature descriptor. It describes a keypoint (like one detected using FAST) by encoding intensity comparisons between pairs of pixels in a small image patch.

 

 

Binary Descriptor Computation

• For a detected keypoint, take a square patch (e.g. S × S pixels).
• Pick pairs of points x and y inside this patch.
• For each pair, compare pixel intensities:

 

 

• This gives a 1-bit output per pair.
• Combine many such bits (e.g., 128, 256, or 512 pairs) into a binary string = the BRIEF descriptor.

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Two Steps Required

1. Smoothing (Preprocessing)
   • Apply a smoothing filter (e.g., Gaussian with σ = 2 on a 9×9 window)
   • This removes noise and increases robustness.
2. Pixel Pair Selection
   • Decide where to choose pixel pairs (x, y) in the patch.
   • This spatial arrangement affects performance.

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Method	Detector	Descriptor	Type	Invariance	Speed	Output Size	Notes
SIFT	DoG (Difference of Gaussians)	Gradient histograms (128D)	Float descriptor	Scale, rotation	Slow	128 floats	Very robust, good for wide-baseline matching
SURF	Fast-Hessian (approx. Laplacian of Gaussian)	Haar wavelet response (64D)	Float descriptor	Scale, rotation	Faster than SIFT	64 floats	Uses integral images to speed up filtering
KAZE	Nonlinear scale space (nonlinear diffusion)	Gradient-based, like SURF (64D)	Float descriptor	Scale, rotation	Slower than SURF	64 floats	Preserves edges by nonlinear diffusion
FAST	Pixel intensity comparison (circle of 16)	❌ None (only detector)	—	❌ Not scale or rotation invariant	Very fast	—	Corner detector only; often used with BRIEF/ORB
BRIEF	❌ None (requires external detector like FAST)	Binary string via intensity comparison	Binary descriptor	❌ Not rotation or scale invariant	Very fast	128–512 bits	Needs smoothing; works best with FAST or ORB detector

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• Usage Combinations

 

Detector	Descriptor	Common Pair Name
FAST	BRIEF	FAST+BRIEF
FAST	ORB (variant of BRIEF)	ORB
DoG	SIFT	SIFT
Fast-Hessian	SURF	SURF
KAZE	KAZE descriptor	KAZE