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Implementation of summed area table / integral image in python.
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| class SummedAreaTable(object): | |
| def __init__(self, size, data): | |
| """ | |
| Just because I dislike a 2d array / list. | |
| data should be a List of Integer. | |
| """ | |
| width, height = size | |
| assert width * height == len(data), "invalid data length and or data size" | |
| self.size = size | |
| self.data = data | |
| self.memo = [None for _ in xrange(width * height)] | |
| self.generate() | |
| def get(self, x, y): | |
| """ | |
| get the value of self, at point x, y. | |
| it's possible for the value at that point hadn't been generated. | |
| """ | |
| width, height = self.size | |
| index = y * width + x | |
| if(x < 0 or y < 0): | |
| # value at negative-indexed point is always 0 | |
| return 0 | |
| elif self.memo[index] is not None: | |
| # if the value at point x, y has already been generated | |
| return self.memo[index] | |
| else: | |
| # calculate the value at point x, y if it hasn't been generated | |
| cummulative = self.get(x - 1, y) + self.get(x, y - 1) - self.get(x - 1, y - 1) + self.data[index] | |
| self.memo[index] = cummulative | |
| return cummulative | |
| def total(self, x0, y0, x1, y1): | |
| """ | |
| get the cummulative value of this instance from point (x0, y0) to (x1, y1) | |
| """ | |
| a = self.get(x0 - 1, y0 - 1) | |
| b = self.get(x0 - 1, y1) | |
| c = self.get(x1, y0 - 1) | |
| d = self.get(x1, y1) | |
| return d - b - c + a | |
| def generate(self): | |
| width, height = self.size | |
| self.memo = [self.get(x, y) for y in xrange(height) for x in xrange(width)] | |
| def example(): | |
| w = h = 10 | |
| haar = SummedAreaTable(size=(w, h), data=range(w * h)) | |
| print haar.total(2, 2, 4, 4) | |
| print sum((y * w + x for y in xrange(h) for x in xrange(w) if 2 <= y <= 4 and 2 <= x <= 4)) | |
| if __name__ == '__main__': | |
| example() |
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