1，本文以RGBA 圖片的降取樣為例子
2， 先縱向插值
3，然後橫向插值

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
//縱向差值
/* src 輸入圖片資料指標 
    des 輸出圖片的資料指標
    srcW 輸入圖片的寬
    srcH 輸入圖片的高
    desH 輸出圖片的高 */
int BilinearInterpolationCol(unsigned char * src, unsigned char * des, int
 
 srcW, int srcH, int desH)
{
    float scale = (float)desH / (float)srcH;
    float invscale = (float)srcH / (float)desH;
    int i, j, z;
    float fsy1;
    float fsy2;
    int sy1;
    int sy2;
    int k = 0;
    int m = 0;
    int skip = (int)(ceil(invscale) + 1.0f);
    float * weight = (float *)malloc
 
(skip * sizeof(float));
    int * srcIndex = (int *)malloc(skip * sizeof(int));

    for(i = 0; i < desH; i ++)
    {
        int temp[3];
        memset(weight, 0, sizeof(float) * skip);
        memset(srcIndex, 0, sizeof(int) * skip);
        k = 0;
        m = 0;
        fsy1 = (float) i * invscale;
        fsy2 = fsy1 + invscale;

        sy1 = (int
 
)ceil(fsy1);

        sy2 = (int)floor(fsy2);

        if((float)sy1 - fsy1 > 0.001f)
        {
            srcIndex[k] = (sy1 - 1);
            weight[k] = ((float)sy1 - fsy1) * scale;
            k ++;
        }
        for(m = sy1; m < sy2; m ++)
        {
            srcIndex[k] = m;
            weight[k] = scale;
            k ++;
        }
        if(fsy2 - (float)(sy2) > 0.001f)
        {
            srcIndex[k] = sy2;
            weight[k] = (fsy2 - (float)(sy2)) * scale;
            k ++;
        }
        for(j = 0; j < srcW; j ++)
        {
            float tempFloat[3] = {0.0f, 0.0f, 0.0f};
            for(z = 0; z < skip; z ++)
            {
                srcIndex[z] = min(srcIndex[z], (srcH - 1));
                tempFloat[0] += ((int)src[(srcIndex[z] * srcW + j) * 4 + 0]) * weight[z];
                tempFloat[1] += ((int)src[(srcIndex[z] * srcW + j) * 4 + 1]) * weight[z];
                tempFloat[2] += ((int)src[(srcIndex[z] * srcW + j) * 4 + 2]) * weight[z];

            }
            temp[0] = (int)tempFloat[0];
            temp[1] = (int)tempFloat[1];
            temp[2] = (int)tempFloat[2];

            des[(i * srcW + j) * 4 + 0] = (unsigned char)temp[0];
            des[(i * srcW + j) * 4 + 1] = (unsigned char)temp[1];
            des[(i * srcW + j) * 4 + 2] = (unsigned char)temp[2];
            des[(i * srcW + j) * 4 + 3] = 0;

        }
    }
    free(weight);
    free(srcIndex);
    return 0;

}
/* 橫向插值
    src 輸入圖片資料指標 
    des 輸出圖片的資料指標
    srcW 輸入圖片的寬
    srcH 輸入圖片的高
    desW 輸出圖片的寬 */
int BilinearInterpolationRow(unsigned char * src, unsigned char * des, int srcW, int srcH, int desW)
{
    float scale = (float)desW / (float)srcW;
    float invscale = (float)srcW / (float)desW;
    int i, j, z;
    float fsx1;
    float fsx2;
    int sx1;
    int sx2;
    int k = 0;
    int m = 0;
    int skip = (int)(ceil(invscale) + 1.0f);
    float * weight = (float *)malloc(skip * sizeof(float));
    int * srcIndex = (int *)malloc(skip * sizeof(int));


    for(j = 0; j < desW; j ++)
    {
            int temp[3];

            memset(weight, 0, sizeof(float) * skip);
            memset(srcIndex, 0, sizeof(int) * skip);

            k = 0;
            m = 0;
            fsx1 = (float) j * invscale;
            fsx2 = fsx1 + invscale;

            sx1 = (int)ceil(fsx1);
            sx2 = (int)floor(fsx2);

            if((float)sx1 - fsx1 > 0.001f)
            {
                srcIndex[k] = (sx1 - 1);
                weight[k] = ((float)sx1 - fsx1) * scale;
                k ++;
            }
            for(m = sx1; m < sx2; m ++)
            {
                srcIndex[k] = m;
                weight[k] = scale;
                k ++;
            }
            if(fsx2 - (float)(sx2) > 0.001f)
            {
                srcIndex[k] = sx2;
                weight[k] = (fsx2 - (float)(sx2)) * scale;
                k ++;
            }

        for(i = 0; i < srcH; i ++)
        {
            float tempFloat[3] = {0.0f, 0.0f, 0.0f};

            for(z = 0; z < skip; z ++)
            {
                srcIndex[z] = min(srcIndex[z], (srcW -1));

                tempFloat[0] += ((int)src[(i * srcW + srcIndex[z]) * 4 + 0]) * weight[z];

                tempFloat[1] += ((int)src[(i * srcW + srcIndex[z]) * 4 + 1]) * weight[z];

                tempFloat[2] += ((int)src[(i * srcW + srcIndex[z]) * 4 + 2]) * weight[z];

            }
            temp[0] = (int)tempFloat[0];
            temp[1] = (int)tempFloat[1];
            temp[2] = (int)tempFloat[2];

            des[(i * desW + j) * 4 + 0] = (unsigned char)temp[0];
            des[(i * desW + j) * 4 + 1] = (unsigned char)temp[1];
            des[(i * desW + j) * 4 + 2] = (unsigned char)temp[2];
            des[(i * desW + j) * 4 + 3] = 0;

        }
    }
    free(weight);
    free(srcIndex);
    return 0;

}
/* 對外提供的介面
    RGBA 影象降取樣的差值方法
    src 輸入圖片資料指標 
    des 輸出圖片的資料指標
    srcW 輸入圖片的寬
    srcW 輸出圖片寬
    srcH 輸入圖片的高
    */
int ImageResize(unsigned char * src, unsigned char * des, int srcW, int srcH, int desW, int desH)
{
    // 縱向差值
    unsigned char * temp = (unsigned char *)malloc(srcW * desH * sizeof(char) * 4);
    BilinearInterpolationCol(src, temp, srcW, srcH, desH);
    //橫向差值
    BilinearInterpolationRow(temp, des, srcW, desH, desW);
    free(temp);
    return 0;
}