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Computer Vision Metrics: Chapter Six (Part D)

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For Part C of Chapter Six, please click here.

Bibliography references are set off with brackets, i.e. "[XXX]". For the corresponding bibliography entries, please click here.

Here is the basic SIFT descriptor processing flow (note: the matching stage is omitted since this chapter is concerned with feature descriptors and related metrics):

Create a Scale Space Pyramid

An octave scale n/2 image pyramid is used with Gaussian filtered images in a scale space. The amount of Gaussian blur is proportional to the scale, and then the Difference of Gaussians (DoG) method is used to capture the interest point extrema maxima and minima in adjacent images in the pyramid. The image pyramid contains five levels. SIFT also uses a double-scale first pyramid level using pixels at two times the original magnification to help preserve fine details. This technique increases the number of stable keypoints by about four times, which is quite significant. Otherwise, computing the Gaussian blur across the original image would have the effect of throwing away the high-frequency details. See Figure 6-15 and 6-16.

Figure 6-15. SIFT DoG as the simple arithmetic difference between the Gaussian filtered images in the pyramid scale

Figure 6-16. SIFT interest point or keypoint detection using scale invariant extrema detection, where the dark pixel in the middle octave is compared within a 3x3x3 area against its 26 neighbors in adjacent DOG octaves, which includes the eight neighbors at the local scale plus the nine neighbors at adjacent octave scales (up or down)

Identify Scale-Invariant Interest Points

As shown in Figure 6-16, the candidate interest points are chosen from local maxima or minima as compared between the 26 adjacent pixels in the DOG images from the three adjacent octaves in the pyramid. In other words, the interest points are scale invariant.

The selected interest points are further qualified to achieve invariance by analyzing local contrast, local noise, and local edge presence within the local 26 pixel neighborhood. Various methods may be used beyond those in the original method, and several techniques are used together to select the best interest points...