计算渐变色的RGB值

渐变色可用于画热度图、分布图等。渐变色示例：

import cv2
import numpy as np

def gradient_ramp(num):
    def arithmetic_progression(vec_1, vec_2, num):
        series = []
        for beg, end in zip(vec_1, vec_2):
            series.append(np.linspace(beg, end, num))
        return np.stack(series, axis=1)

    # red -> yellow
    part_1 = arithmetic_progression([0, 0, 255], [0, 255, 255], 256)
    # yellow -> green
    part_2 = arithmetic_progression([0, 255, 255], [0, 255, 0], 256)
    # green -> cyan
    part_3 = arithmetic_progression([0, 255, 0], [255, 255, 0], 256)
    # cyan -> blue
    part_4 = arithmetic_progression([255, 255, 0], [255, 0, 0], 256)

    ribbon = np.vstack([part_1, part_2[1:], part_3[1:], part_4[1:]])
    ribbon = ribbon.astype(np.uint8)
    total  = ribbon.shape[0]
    indices = np.linspace(0, total - 1, num, dtype=np.int)
    return ribbon[indices] 

if __name__ == '__main__':
    num = 512

    colors = gradient_ramp(num)  # color bar

    canvas = np.zeros((64, num, 3), dtype=np.uint8)
    for idx, color in enumerate(colors):
        canvas[:, idx, :] = color
    cv2.namedWindow('debug')
    cv2.imshow('debug', canvas)
    cv2.waitKey()