quick post about something I did\u00a0last year for a project that never went anywhere.<\/p>\n

the Active Contour Model<\/a>\u00a0(ACM) is an old fashioned way of vectorizing the contour of an object. it works very well when an object is isolated on a flat color but the principle can also\u00a0be used “live” to determine where the “border” between different shapes is.\u00a0In Photoshop, this would correspond to the “magic wand” and the “quick selection tool” respectively.<\/p>\n

ACM uses morphological snakes, the principle of snakes is fairly\u00a0simple to understand, the animations, from this Python implementation’s repo<\/a>\u00a0clearly explain the idea. the snake is basically a polyline that can expand or shrink as needed. below\u00a0the snake is used to enclose a\u00a0starfish shape.<\/p>\n

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the snake is like an elastic, first it’s stretched around the starfish and gradually tightens. there’s\u00a0a major difference between a snake and an elesatic, the snake can go “inside” the creases when an elastic would remain “outside” (thus\u00a0describing the convex hull<\/a>\u00a0of the starfish ).<\/p>\n

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and in this example, the snake behaves more like expanding foam, progressively filling the space around its starting point until it reaches a boundary.<\/p>\n

if the two behaviours seem different, they are\u00a0actually the same ; the snake is driven by gradient\u00a0descent<\/a>, which is a fancy way of saying that the points of the snake are\u00a0always “rolling downhill”.<\/p>\n