双三次插值是图像空域变换的经典算法，可用于图像旋转和放缩等灰度插值。
Bicubic interpolation is a classical algorithm used in spacial transformation of image such as image rotation and zooming.
当原始图像点乘以变换矩阵，得到新的图像位置点时，其位置可能不是整数值，该位置的灰度值需要通过灰度插值获得。
When the original image point multiply the transformation matrix, a new image position, the position may not integer, this position gray value by gray interpolation gain needed.
在常用的方法中，基于区域灰度平均的算法使图像模糊，而基于样条插值的算法计算耗时。
In the common methods, the algorithm based on the average of region gray makes image blur, and the algorithm based on spline interpolation wastes time.
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