使用c语言写一个图片缩放算法

下面是一个简单的图片缩放算法的 C 语言实现，使用了双线性插值法来进行图像的缩放：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef unsigned char BYTE;

// 读取 BMP 图像文件
BYTE* ReadBMP(char* bmpFileHeader, char* bmpInfoHeader, char* bmpPixelData) {
    FILE* fp = fopen("image.bmp", "rb");
    if (fp == NULL) {
        printf("Failed to open the file!\n");
        return NULL;
    }
    BYTE* bmpData = NULL;
    BYTE* bmpPixel = NULL;
    int width = *(int*)(bmpInfoHeader + 4);
    int height = *(int*)(bmpInfoHeader + 8);
    int bitCount = *(int*)(bmpInfoHeader + 14);
    int lineByte = (width * bitCount / 8 + 3) / 4 * 4;
    bmpData = (BYTE*)malloc(lineByte * height);
    memset(bmpData, 0, lineByte * height);
    bmpPixel = (BYTE*)malloc(bitCount / 8);
    memset(bmpPixel, 0, bitCount / 8);
    for (int i = height - 1; i >= 0; i--) {
        for (int j = 0; j < width; j++) {
            fread(bmpPixel, bitCount / 8, 1, fp);
            bmpData[i * lineByte + j * bitCount / 8 + 0] = bmpPixel[0];
            bmpData[i * lineByte + j * bitCount / 8 + 1] = bmpPixel[1];
            bmpData[i * lineByte + j * bitCount / 8 + 2] = bmpPixel[2];
        }
        fseek(fp, lineByte - width * bitCount / 8, SEEK_CUR);
    }
    fclose(fp);
    free(bmpPixel);
    return bmpData;
}

// 写入 BMP 图像文件
void WriteBMP(char* bmpFileHeader, char* bmpInfoHeader, BYTE* bmpData) {
    FILE* fp = fopen("output.bmp", "wb");
    if (fp == NULL) {
        printf("Failed to open the file!\n");
        return;
    }
    int width = *(int*)(bmpInfoHeader + 4);
    int height = *(int*)(bmpInfoHeader + 8);
    int bitCount = *(int*)(bmpInfoHeader + 14);
    int lineByte = (width * bitCount / 8 + 3) / 4 * 4;
    fwrite(bmpFileHeader, 1, 14, fp);
    fwrite(bmpInfoHeader, 1, 40, fp);
    for (int i = height - 1; i >= 0; i--) {
        fwrite(bmpData + i * lineByte, lineByte, 1, fp);
    }
    fclose(fp);
}

// 图像缩放
BYTE* Resize(BYTE* bmpData, int width, int height, float ratio) {
    int newWidth = (int)(width * ratio);
    int newHeight = (int)(height * ratio);
    int bitCount = 24;
    int lineByte = (newWidth * bitCount / 8 + 3) / 4 * 4;
    BYTE* newBmpData = (BYTE*)malloc(lineByte * newHeight);
    memset(newBmpData, 0, lineByte * newHeight);

    for (int i = 0; i < newHeight; i++) {
        for (int j = 0; j < newWidth; j++) {
            int x = (int)(j / ratio);
            int y = (int)(i / ratio);
            float u = j / ratio - x;
            float v = i / ratio - y;
            int pos1 = y * width + x;
            int pos2 = pos1 + 1;
            int pos3 = (y + 1) * width + x;
            int pos4 = pos3 + 1;
            BYTE b1 = bmpData[pos1 * 3 + 0];
            BYTE g1 = bmpData[pos1 * 3 + 1];
            BYTE r1 = bmpData[pos1 * 3 + 2];
            BYTE b2 = bmpData[pos2 * 3 + 0];
            BYTE g2 = bmpData[pos2 * 3 + 1];
            BYTE r2 = bmpData[pos2 * 3 + 2];
            BYTE b3 = bmpData[pos3 * 3 + 0];
            BYTE g3 = bmpData[pos3 * 3 + 1];
            BYTE r3 = bmpData[pos3 * 3 + 2];
            BYTE b4 = bmpData[pos4 * 3 + 0];
            BYTE g4 = bmpData[pos4 * 3 + 1];
            BYTE r4 = bmpData[pos4 * 3 + 2];
            BYTE b = (BYTE)((1 - u) * (1 - v) * b1 + u * (1 - v) * b2 + (1 - u) * v * b3 + u * v * b4);
            BYTE g = (BYTE)((1 - u) * (1 - v) * g1 + u * (1 - v) * g2 + (1 - u) * v * g3 + u * v * g4);
            BYTE r = (BYTE)((1 - u) * (1 - v) * r1 + u * (1 - v) * r2 + (1 - u) * v * r3 + u * v * r4);
            newBmpData[i * lineByte + j * bitCount / 8 + 0] = b;
            newBmpData[i * lineByte + j * bitCount / 8 + 1] = g;
            newBmpData[i * lineByte + j * bitCount / 8 + 2] = r;
        }
    }

    return newBmpData;
}

int main() {
    char bmpFileHeader[14];
    char bmpInfoHeader[40];
    char bmpPixelData[100000];
    BYTE* bmpData = ReadBMP(bmpFileHeader, bmpInfoHeader, bmpPixelData);
    int width = *(int*)(bmpInfoHeader + 4);
    int height = *(int*)(bmpInfoHeader + 8);
    float ratio = 0.5;
    BYTE* newBmpData = Resize(bmpData, width, height, ratio);
    WriteBMP(bmpFileHeader, bmpInfoHeader, newBmpData);
    free(bmpData);
    free(newBmpData);
    return 0;
}

这个示例代码使用了双线性插值算法，可以将原始图像缩小到原来的一半。你可以根据实际需求来修改代码，以适应更多的图像缩放需求。