双线性插值的实现

双线性插值根据周围4个点及与4个点的距离，来计算待插值像素的值
http://baike.baidu.com/link?url=4it85nPzCC9J0oZZGU7HEGuXbcDcOmzhtKuwTP9Nar9HLRyZdwKaocnn1yZOFJ7pdZjnXmTUpiKStH5nPs1qTq2V57th0nHQqc1m0_fzQXIklY4hFLnYg3T5kbqTibxqIUa_JFNV0sy8Y3_YX_XopK

下面是根据插值算法做的一个实现

void bilinear_c_int(uchar *pSrc, int srcWidth, int srcHeight, uchar *pDst, int dstWidth, 
                    int dstHeight) {

	const int N = 8;
	const int FACTOR = pow(2, N);

	float fx = (float)srcWidth / dstWidth;
	float fy = (float)srcHeight / dstHeight;

	int *x = new int[dstWidth];
	int *a = new int[dstWidth];

	int *y = new int[dstHeight];
	int *b = new int[dstHeight];

	for (int j = 0; j < dstWidth; j++) {
		x[j] = (int)(fx * j);
		a[j] = (fx*j - x[j]) * FACTOR ;
		if (x[j] > srcWidth - 2) {
			x[j] = srcWidth - 2;
			a[j] = FACTOR;
		}
	}

	for (int i = 0; i < dstHeight; i++) {
		y[i] = (int)(fy * i);
		b[i] = (fy *i - y[i]) * FACTOR;

		if (y[i] > srcHeight - 2) {
			y[i] = srcHeight - 2;
			b[i] = FACTOR;
		}
	}

	int index, p0, p1;
	for (int i = 0; i < dstHeight; i++) {
		for (int j = 0; j < dstWidth; j++) {
			index = y[i] * srcWidth + x[j];

			p0 = pSrc[index] * (FACTOR - a[j]) + pSrc[index + 1] * a[j];
			p1 = pSrc[index + srcWidth + 1] * (FACTOR - a[j]) + pSrc[index + srcWidth + 1] * a[j];
			pDst[i * dstWidth + j] = (p0 * (FACTOR - b[i]) + p1 * b[i]) >> (2 * N);
		}
	}

	delete[] x;
	delete[] a;
	delete[] y;
	delete[] b;
}