调整 BMP 图像的大小（使其变小）

Question

当缩放系数小于 1 时，我似乎需要一些帮助来调整 BMP 图像的大小。您可以在下面看到我的代码中最关键的部分。代码中的变量 f 是缩放因子。这对我来说似乎合乎逻辑，但它工作不正常。 - 这是我需要调整大小的图像（使其更小）。 - 这是看起来不正确的结果图片。我想我上传失败了，但它看起来像一个没有任何白色中心的绿色小方块。

此外，我尝试调整另一张图片的大小 - 这个美丽的笑脸：结果出乎意料：

这让我觉得 for 循环有问题，尽管对我来说这似乎完全合乎逻辑。

这就是 BMP 的组织方式。

    // BMP-related data types based on Microsoft's own

#include <stdint.h>

// aliases for C/C++ primitive data types
// https://msdn.microsoft.com/en-us/library/cc230309.aspx
typedef uint8_t  BYTE;
typedef uint32_t DWORD;
typedef int32_t  LONG;
typedef uint16_t WORD;

// information about the type, size, and layout of a file
// https://msdn.microsoft.com/en-us/library/dd183374(v=vs.85).aspx
typedef struct
{
    WORD bfType;
    DWORD bfSize;
    WORD bfReserved1;
    WORD bfReserved2;
    DWORD bfOffBits;
} __attribute__((__packed__))
BITMAPFILEHEADER;

// information about the dimensions and color format
// https://msdn.microsoft.com/en-us/library/dd183376(v=vs.85).aspx
typedef struct
{
    DWORD biSize;
    LONG biWidth;
    LONG biHeight;
    WORD biPlanes;
    WORD biBitCount;
    DWORD biCompression;
    DWORD biSizeImage;
    LONG biXPelsPerMeter;
    LONG biYPelsPerMeter;
    DWORD biClrUsed;
    DWORD biClrImportant;
} __attribute__((__packed__))
BITMAPINFOHEADER;

// relative intensities of red, green, and blue
// https://msdn.microsoft.com/en-us/library/dd162939(v=vs.85).aspx
typedef struct
{
    BYTE rgbtBlue;
    BYTE rgbtGreen;
    BYTE rgbtRed;
} __attribute__((__packed__))
RGBTRIPLE;

有人知道如何修复下面的代码吗？

// Copies a BMP file

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

#include "bmp.h"

int main(int argc, char *argv[])
{
    // ensure proper usage
    if (argc != 4)
    {
        fprintf(stderr, "Usage: resize n infile outfile\n");
        return 1;
    }

    // remember filenames
    float f=atof(argv[1]);
    int n=f;
    if(f<=0||f>=1){
        fprintf(stderr, "f, the resize factor, must be between 0 and 1.\n");
        return 1;
    }
    char *infile = argv[2];
    char *outfile = argv[3];

    // open input file
    FILE *inptr = fopen(infile, "r");
    if (inptr == NULL)
    {
        fprintf(stderr, "Could not open %s.\n", infile);
        return 2;
    }

    // open output file
    FILE *outptr = fopen(outfile, "w");
    if (outptr == NULL)
    {
        fclose(inptr);
        fprintf(stderr, "Could not create %s.\n", outfile);
        return 3;
    }

    // read infile's BITMAPFILEHEADER
    BITMAPFILEHEADER bf;
    fread(&bf, sizeof(BITMAPFILEHEADER), 1, inptr);

    // read infile's BITMAPINFOHEADER
    BITMAPINFOHEADER bi;
    fread(&bi, sizeof(BITMAPINFOHEADER), 1, inptr);

    // ensure infile is (likely) a 24-bit uncompressed BMP 4.0
    if (bf.bfType != 0x4d42 || bf.bfOffBits != 54 || bi.biSize != 40 ||
        bi.biBitCount != 24 || bi.biCompression != 0)
    {
        fclose(outptr);
        fclose(inptr);
        fprintf(stderr, "Unsupported file format.\n");
        return 4;
    }
BITMAPFILEHEADER bf_resize = bf;
BITMAPINFOHEADER bi_resize = bi;
bi_resize.biWidth = bi.biWidth * f;
bi_resize.biHeight = bi.biHeight * f;
int padding_resize = (4 - (bi_resize.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;
bi_resize.biSizeImage = (bi_resize.biWidth * sizeof(RGBTRIPLE) + padding_resize) * abs(bi_resize.biHeight);
bf_resize.bfSize = bi_resize.biSizeImage + sizeof(BITMAPFILEHEADER)+sizeof(BITMAPINFOHEADER);
int padding = (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;
int fn = f * 10;
int x = 10 / fn;
int diff = x;
diff--;
int w = 0;
int h = 0;

// write outfile's BITMAPFILEHEADER
fwrite(&bf_resize, sizeof(BITMAPFILEHEADER), 1, outptr);

// write outfile's BITMAPINFOHEADER
fwrite(&bi_resize, sizeof(BITMAPINFOHEADER), 1, outptr);

     // iterate over infile's scanlines
       for (int i = 0, biHeight = abs(bi.biHeight); i < biHeight; i++)
    {
        if(h==0){
        // iterate over pixels in scanline
        for (int j = 0; j < bi.biWidth; j++)
        {
            // temporary storage
            RGBTRIPLE triple;

            // read RGB triple from infile
            fread(&triple, sizeof(RGBTRIPLE), 1, inptr);
            if(w==0){
            // write RGB triple to outfile
            fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr);
            w++;
            }
            else{
                w++;
            }
            if(w==diff){
               w=0; 
            }
        }
        }
         h++;
        if(h==diff){
            h=0;
        }
        // skip over padding, if any
        fseek(inptr, padding, SEEK_CUR);

        // then add it back (to demonstrate how)
        for (int k = 0; k < padding; k++)
        {
            fputc(0x00, outptr);
        }
    }
    // close infile
    fclose(inptr);

    // close outfile
    fclose(outptr);

    // success
    return 0;
}

Answer 1

我假设您想通过使用变量 w、h 和 diff 跳过行和列来缩小图像。例如，如果我们设置比例因子f赋值为0.5，diff赋值为1，每隔一个 rows/columns 将被跳过以将图像缩放 0.5 倍。然后在循环中有两个关键问题 i 和 j:

您正在 if(w==diff){ w=0; } w++; 之后重置 w。然后 w 一直为 0，没有跳过任何列。
您正在将 if(h==0){ 条件放在外部块中。然后是像素 h==0 时不读取。为了缩小图像，你需要保持无论条件如何读取每个像素，并写入像素如果条件满足。

那么循环将被改进为：

    // iterate over infile's scanlines
    for (int i = 0, biHeight = abs(bi.biHeight); i < biHeight; i++) {
        // iterate over pixels in scanline
        for (int j = 0; j < bi.biWidth; j++) {
            // temporary storage
            RGBTRIPLE triple;

            // read RGB triple from infile
            fread(&triple, sizeof(RGBTRIPLE), 1, inptr);
            if (w == 0 && h == 0) {
                // write RGB triple to outfile
                fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr);
            }
            w++;
            if (w > diff){
               w = 0;
            }
        }
        // skip over padding, if any
        fseek(inptr, padding, SEEK_CUR);

        // padding to the output file, if any
        if (h == 0) {
            for (int k = 0; k < padding_resize; k++) {
                fputc(0x00, outptr);
            }
        }

        h++;
        if (h > diff){
            h = 0;
        }
    }

它将在有限的条件下工作：f=0.5 以及图像宽度和高度是偶数。但在一般情况下，它仍然不能很好地工作。例如，如果我们将 f 设置为 0.4， bi_resize.biWidth 和 bi_resize.biHeight 将计算为将原始大小乘以 0.4，而 diff 将被计算为 1。然后它们会导致 header 信息与实际像素。

这是解决问题的另一个提示：

处理图像的常用方法是存储整个像素一开始进入内存。然后你可以random-access以下任意像素过程和代码将更加直接。基本思想是迭代 destination 坐标并返回到源图像的坐标以进行拾取像素值。
您发布的输入图像太小，无法确定问题所在，因为细线和小点很容易折叠，即使过滤如果算法合适。最好用大图来评价。

这是我根据你的代码重写的：

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

#include "bmp.h"

int main(int argc, char *argv[])
{
    // ensure proper usage
    if (argc != 4) {
        fprintf(stderr, "Usage: resize n infile outfile\n");
        return 1;
    }

    // read the scaling factor
    float f = atof(argv[1]);
    if (f <= 0 || f > 1) {
        fprintf(stderr, "f, the resize factor, must be between 0 and 1.\n");
        return 1;
    }
    char *infile = argv[2];
    char *outfile = argv[3];

    // open input file
    FILE *inptr = fopen(infile, "r");
    if (inptr == NULL) {
        fprintf(stderr, "Could not open %s.\n", infile);
        return 2;
    }

    // open output file
    FILE *outptr = fopen(outfile, "w");
    if (outptr == NULL) {
        fclose(inptr);
        fprintf(stderr, "Could not create %s.\n", outfile);
        return 3;
    }

    // read infile's BITMAPFILEHEADER
    BITMAPFILEHEADER bf;
    fread(&bf, sizeof(BITMAPFILEHEADER), 1, inptr);

    // read infile's BITMAPINFOHEADER
    BITMAPINFOHEADER bi;
    fread(&bi, sizeof(BITMAPINFOHEADER), 1, inptr);

    // ensure infile is (likely) a 24-bit uncompressed BMP 4.0
    if (bf.bfType != 0x4d42 || bf.bfOffBits != 54 || bi.biSize != 40 ||
        bi.biBitCount != 24 || bi.biCompression != 0) {
        fclose(outptr);
        fclose(inptr);
        fprintf(stderr, "Unsupported file format.\n");
        return 4;
    }

    BITMAPFILEHEADER bf_resize = bf;
    BITMAPINFOHEADER bi_resize = bi;
    bi_resize.biWidth = bi.biWidth * f;
    bi_resize.biHeight = bi.biHeight * f;
    int padding = bi.biWidth % 4;       // you can simplify the calculation
    int padding_resize = bi_resize.biWidth % 4;
    bi_resize.biSizeImage = (bi_resize.biWidth * sizeof(RGBTRIPLE) + padding_resize) * bi_resize.biHeight;
    bf_resize.bfSize = bi_resize.biSizeImage + sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER);

    // allocate mamory for the rgb triplets of the original (input) image
    RGBTRIPLE *pix = malloc(sizeof(RGBTRIPLE) * bi.biWidth * bi.biHeight);
    if (pix == NULL) {
        fprintf(stderr, "malloc failed.\n");
        return 5;
    }
    // temporary storage
    RGBTRIPLE triple;

    // read the entire pixels of the original image and store into the memory
    for (int i = 0; i < bi.biHeight; i++) {
        for (int j = 0; j < bi.biWidth; j++) {
            fread(&triple, sizeof(RGBTRIPLE), 1, inptr);
            pix[i * bi.biWidth + j] = triple;
        }
        // skip over padding, if any
        fseek(inptr, padding, SEEK_CUR);
    }

    // write outfile's header
    fwrite(&bf_resize, sizeof(BITMAPFILEHEADER), 1, outptr);
    fwrite(&bi_resize, sizeof(BITMAPINFOHEADER), 1, outptr);

    // write the pixels of destination (resized) image
    for (int i = 0; i < bi_resize.biHeight; i++) {
        for (int j = 0; j < bi_resize.biWidth; j++) {
            // calculate the corresponding coorinates in the original image
            int m = (i / f + 0.5);              // +0.5 for rounding
            if (m > bi.biHeight - 1) {          // limit the value
                m = bi.biHeight - 1;
            }
            int n = (j / f + 0.5);
            if (n > bi.biWidth - 1) {
                n = bi.biWidth - 1;
            }
            // pick the pixel value at the coordinate
            triple = pix[m * bi.biWidth + n];
            // write RGB triplet to outfile
            fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr);
        }
        // padding for the output image, if any
        for (int j = 0; j < padding_resize; j++) {
            fputc(0x00, outptr);
        }
    }
    free(pix);
    fclose(inptr);
    fclose(outptr);

    return 0;
}

输入图像： f=0.4 的输出图像：

调整 BMP 图像的大小（使其变小）

Resizing a BMP image (making it smaller)

c

resize

pixel

bmp

cs50