无法正确调整 .bmp 图像的大小(CS50 pset 4，调整大小，不太舒服)

Question

我目前正在学习 CS50 类(class)，但我完全陷入了调整大小的问题(不太舒服)。任务是制作一个程序，该程序接受输入 .bmp 文件，将其缩放 n 次并将其写入输出 .bmp 文件。但我的代码只是有点扭曲了输入图像，根本没有改变大小。这是为测试提供的 bmp(3x3 像素，此处放大):https://imgur.com/1v55tjz当我尝试调整它的大小时，它变成了这个(仍然是 3x3):https://imgur.com/JAKQMfY

这是调整大小的代码:

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

#include "bmp.h"

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

    // remember filenames
    int n = atoi(argv[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;
    }

    // create file- and infoheader for output file
    BITMAPFILEHEADER outBF = bf;
    BITMAPINFOHEADER outBI = bi;

    //Scale width and height by n
    outBI.biWidth *= n;
    outBI.biHeight *= n;

    //Calculate padding for input and output file
    int outPadding = (4 - (outBI.biWidth * sizeof(RGBTRIPLE) % 4) % 4);
    int inPadding = (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;

    //Calculate biSizeImage and bfSize for output file
    outBI.biSizeImage = ((sizeof(RGBTRIPLE) * outBI.biWidth) + outPadding * abs(outBI.biHeight));
    outBF.bfSize = outBI.biSizeImage + sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER);

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

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

    //Instantiate two counters to keep track of whether we have reached the n - 1 times of the recopy method and to be able to fseek to the next line
    int countIterations = 1;
    int countPositionInFile = 0;


    // iterate over infile's scanlines
    for (int i = 0, biHeight = abs(bi.biHeight); i < biHeight; i++)
    {
        countIterations = 1;
        for(int j = 0; j < n; j++)
        {
            // iterate over pixels in scanline
            for (int k = 0; k < bi.biWidth; k++)
            {
                // temporary storage
                RGBTRIPLE triple;

                // read RGB triple from infile
                fread(&triple, sizeof(RGBTRIPLE), 1, inptr);

                // resize horizontally by writing every pixel in row n times
                for(int l = 0; l < n; l++)
                {
                    // write RGB triple to outfile
                    fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr);
                }
            }

            // skip over padding, if any
            fseek(inptr, inPadding, SEEK_CUR);

            //Add padding for this line
            for (int l = 0; l < outPadding; l++)
            {
            fputc(0x00, outptr);
            }

            if(countIterations == n)
            {
                countPositionInFile++;
            }

            int newPosition = (sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER) + (countPositionInFile * (bi.biWidth + inPadding)));

            // return the "cursor" to the start of the file
            fseek(inptr, newPosition, SEEK_SET);
            countIterations++;
        }

    }

    // close infile
    fclose(inptr);

    // close outfile
    fclose(outptr);

    // success
    return 0;
}    

这是 bmp.h 文件:

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

#include <stdint.h>

/**
 * Common Data Types 
 *
 * The data types in this section are essentially aliases for C/C++ 
 * primitive data types.
 *
 * Adapted from https://msdn.microsoft.com/en-us/library/cc230309.aspx.
 * See http://en.wikipedia.org/wiki/Stdint.h for more on stdint.h.
 */
typedef uint8_t  BYTE;
typedef uint32_t DWORD;
typedef int32_t  LONG;
typedef uint16_t WORD;

/**
 * BITMAPFILEHEADER
 *
 * The BITMAPFILEHEADER structure contains information about the type, size,
 * and layout of a file that contains a DIB [device-independent bitmap].
 *
 * Adapted from 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; 

/**
 * BITMAPINFOHEADER
 *
 * The BITMAPINFOHEADER structure contains information about the 
 * dimensions and color format of a DIB [device-independent bitmap].
 *
 * Adapted from 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; 

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

我希望有人有这个任务的经验并且可以提供帮助，我现在感觉很困难，但也许我已经盯着它看太久了。提前致谢!

Answer 1

文件应该用二进制标志打开:

FILE *inptr = fopen(infile, "rb");
FILE *outptr = fopen(outfile, "wb");

您正在忽略 outBI 和 outBF，您正在将旧 header 写入新文件:

fwrite(&bf, sizeof(BITMAPFILEHEADER), 1, outptr);//old header
fwrite(&bi, sizeof(BITMAPINFOHEADER), 1, outptr);//old header

这里使用两种不同的方法来计算填充:

int outPadding = (4 - (outBI.biWidth * sizeof(RGBTRIPLE)  % 4) % 4);
int inPadding =  (4 - (bi.biWidth *    sizeof(RGBTRIPLE)) % 4) % 4;
                                                        ^         ^

他们不可能都是对的，除非是偶然的。在这种情况下，我们使用 24 位图，您可以将填充计算为

int outPadding = outBI.biWidth % 4;
int inPadding = bi.biWidth % 4;

这将匹配您使用 fseek(inptr, inPadding, SEEK_CUR); 和 for(...) fputc(0x00, outptr); 跳过填充的方式

我无法理解这段代码:

int newPosition = (sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER) 
    + (countPositionInFile * (bi.biWidth + inPadding)));<==remove
fseek(inptr, newPosition, SEEK_SET);<==remove

那应该是 (bi.biWidth * 3) 或者你可以删除它并使用 ftell 在开始阅读该行之前保存位置，然后使用 fseek 回到那个位置。

位图高度应该从下往上读取，但在这种情况下没有区别。