双三次算法与双线性插值相比，需要计算相邻16个点的输入像素，即w方向坐标分别为w - 1、w、w + 1、 w + 2；h方向坐标分别为h - 1、h、 h + 1、h + 2。如何计算每个点对应的权重呢？opencv的计算方法是这样的：
fx = (float)((dx+0.5)*scale_x - 0.5);
sx = cvFloor(fx);
fx -= sx;
BiBubic(ii - fx); //ii值为-1/0/1/2
#define ALPHA -0.75
/*(a + 2)|x|^3 - (a + 3)|x|^2 + 1 for |x| <= 1
w(x) = { a|x|^3 - 5a|x|^2 + 8a|x| - 4a for 1 < |x| < 20 otherwise
*/
inline float BiBubic(float x)
{float dst_val = 0;float val = ABS(x);if (val <= 1){dst_val = (ALPHA + 2) * val * val * val - (ALPHA + 3) * val * val + 1;}else if (val > 1 && val < 2){dst_val = ALPHA * val * val * val - 5 * ALPHA * val * val + 8 * ALPHA * val - 4 * ALPHA;}return dst_val;
}
然后进行累加求和就可以了。
大致流程如下（细节与opencv不一样，具体需要参考opencv的具体代码）
float scale_h = (float)lSrcHeight / lDstHeight;
float scale_w = (float)lSrcWidth / lDstWidth;
for (dy = 0; dy < lDstHeight; dy++)
{for (dx = 0; dx < lDstWidth; dx++){float src_h = dy * scale_h;float src_w = dx * scale_w;int w_idx = floor(src_w);int h_idx = floor(src_h);float w_weight = src_w - w_idx;float h_weight = src_h - h_idx;float tmp_val = 0;for (ii = -1; ii <= 2; ii++){for (jj = -1; jj <= 2; jj++){if (w_idx + jj < 0 || h_idx + ii < 0 || w_idx + jj >= lSrcWidth || h_idx + ii >= lSrcHeight){continue;}tmp_val += pSrcY[(h_idx + ii) * lSrcStrideY + w_idx + jj] * BiBubic(ii - h_weight) * BiBubic(jj - w_weight);}}tmp_val = MAX(0, tmp_val);tmp_val = MIN(255, tmp_val);pDstY[dy * lDstStrideY + dx] = (MUInt8)tmp_val;}
}