SSIM (Structural Similarity Index Measure) compares two images by evaluating luminance, contrast, and structural patterns across local windows. It returns a score from -1 to 1: 1.0 means the images are pixel-identical, 0 means no structural correlation, and negative values mean the images are anti-correlated (less alike than random noise). For glyph comparison, it answers the question: do these two rendered characters share the same visual structure?
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fn sort_by(arr: [int], cmp: Comparator) - [int] {，这一点在新收录的资料中也有详细论述
cfg.name = "testdb";