调整位图文件的大小部分有效 - 放大有效但缩小无效

Question

我编写了一个程序，可以将位图的大小调整为 0-100 之间的一个系数（小于 1 的值会缩小图像）。该程序可以放大文件，但不能缩小文件。

我想我可能错误地使用了 fseek 来跳过像素和行。

感谢您承诺审查此问题。

请注意代码很难看，因为我刚开始学习编码。欢迎任何建议，但可能会造成额外的混乱，所以请向 5 岁的孩子解释你的答案。

您可以运行程序使用类似于“./resize .5 large.bmp test.bmp”的命令

附上两个程序文件 resize.c 和 bmp.h

我还附上了之前和之后的图片，以便更好地了解问题。

之后：

之前：

// Copies a BMP file

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

#include "bmp.h"

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

    // remember filenames
    char* a = NULL;
    float num0 = strtof(argv[1], &a);
    int num = ceil(num0);
    char *infile = argv[2];
    char *outfile = argv[3];

    if (num0 < 0 || num0 > 100)
    {
        fprintf(stderr, "Resize only 0-100. Try again.\n");
        return 5;
    }

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

    // open output file
    FILE *outptr = fopen(outfile, "wb");
    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;
    }

    // calculate old padding
    int padding = (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;
    printf("padding%i ", padding);

    //create new header files to save to
    BITMAPINFOHEADER newbi = bi;
    BITMAPFILEHEADER newbf = bf;

    //determine width and set minimum size
    int biwidth = round(bi.biWidth * num0);
    if (biwidth < 3)
    {
        newbi.biWidth = 3;
    }
    else
    {
        newbi.biWidth = biwidth;

    }
    printf("newbiwidth%i ", newbi.biWidth);

    //determine height and set minimum size
    int biheight = round(bi.biHeight) * num0;
    if (biheight > -3)
    {
        newbi.biHeight = -3;
    }
    else
    {
        newbi.biHeight = biheight;

    }
    printf("newbiheight%i ", newbi.biHeight);

    // determine new padding for scanlines
    int newpadding = (4 - (newbi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;
    printf("newpadding%i ", newpadding);

    //determine new image size
    newbi.biSizeImage = (newbi.biWidth * sizeof(RGBTRIPLE) + newpadding) * abs(newbi.biHeight);
    printf("newbisizeimage%i ", newbi.biSizeImage);

    //determine new file size
    newbf.bfSize = newbi.biSizeImage + sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER);
    printf("newbfsize%i ", newbf.bfSize);

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

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


    // temporary storage

    int factor = zoom(num0, bi.biWidth);
    RGBTRIPLE triple;
    RGBTRIPLE *triple1 = malloc(sizeof(RGBTRIPLE) * factor);
    RGBTRIPLE *sline = malloc(newbi.biWidth * sizeof(RGBTRIPLE));

    //determine whether to loop old or new width
    int biheight1 = 0;
    int biwidth1= 0;
    if ( bi.biWidth > newbi.biWidth)
    biwidth1 = newbi.biWidth;
    else
    biwidth1 = bi.biWidth;

    //determine whether to loop old or new height
    if ( abs(bi.biHeight) > abs(newbi.biHeight))
    biheight1 = abs(newbi.biHeight);
    else
    biheight1 = abs(bi.biHeight);

    // read RGB triple from infile based on shrink or enlarge
    for (int i = 0; i < biheight1; i++)
    {
        printf("H%i  ", i);
        for (int j = 0; j < biwidth1; j++)
        {
            printf("W%i  ", j);

            if (num0 > .5 && num0 < 1)
            fread(triple1, sizeof(RGBTRIPLE), factor, inptr);
            else
            fread(&triple, sizeof(RGBTRIPLE), 1, inptr);

            //store new triple as new scanline

            for (int m = 0; m < num ; m++)
            {
                if (num0 <= .5)
                {
                    sline[j] = triple;
                }
                else if (num0 > .5 && num0 < 1)
                {
                    sline[j] = *triple1;
                }
                else
                {
                    sline[j * num + m] = triple;
                }
                printf("T%i,J%i  ", m, j);
            }

            //skip pixel(s) if image is shrinking
            for (int n = 0; n < num ; n++)
            {
                if (num0 > .5 && num0 < 1)
                {
                    fseek(inptr, sizeof(RGBTRIPLE), SEEK_CUR);
                }
                else if (num0 <= .5)
                {
                        fseek(inptr, (sizeof(RGBTRIPLE) * factor), SEEK_CUR);
                }
            }
        }
        // skip over padding, if any
        fseek(inptr,padding, SEEK_CUR);

        //write new scanline to file
        for (int k = 0; k < num; k++)
        {
            printf("F%i  ", k);
            fwrite(sline, (newbi.biWidth * 3), 1, outptr);

            // add padding if any
            for (int h = 0; h < newpadding; h++)
            {
                fputc(0x00, outptr);
            }
        }

            //skip scanline(s) if shrinking
            for (int o = 0; o < num ; o++)
            {
                if (num0 > .5 && num0 < 1)
                {
                    fseek(inptr, (bi.biWidth + padding), SEEK_CUR);
                }
                else if (num0 <= .5)
                {
                    fseek(inptr, ((bi.biWidth + padding) * factor), SEEK_CUR);
                }
            }

    }

    //free memory
    free(sline);
    free(triple1);


    // close infile
    fclose(inptr);

    // close outfile
    fclose(outptr);


    // success
    return 0;
}


//determine shrink factor
int zoom(float number, int biwidth)
{

    int zoom1;
    int a;
    int b;
    a = (biwidth * number);
    b = (biwidth - a);
    if ( a > b && b != 0)
    {
        zoom1 = a/b;
    }
    else if (a < b && a != 0)
    {
        zoom1 = b/a;
    }
    else if (b <= 0)
    {
        zoom1 = 1;
    }
    else
    {
        zoom1 = 1;
    }
    return zoom1;
}


    // 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;

Answer 1

@CraigEstey 发布了以下解决方案：

用于跳过扫描线的 fseek 不完整。 bi.biWidth 需要乘以 sizeof(RGBTRIPLE)

使用 fseek(inptr, ((bi.biWidth * sizeof(RGBTRIPLE) + padding), SEEK_CUR); 而不是 fseek(inptr, (bi.biWidth + padding), SEEK_CUR);

该程序现在可以通过此修复来放大和缩小图像。

Answer 2

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

int main()
{

    struct rgb { unsigned char r, g, b; };

    float scale = 1.5;//0 to 100!

    // open input and output file
    FILE* inptr  = fopen("in.bmp", "rb");
    FILE* outptr = fopen("out.bmp","wb");

    BITMAPFILEHEADER bf;    fread(&bf, sizeof(BITMAPFILEHEADER), 1, inptr);
    BITMAPINFOHEADER bi;    fread(&bi, sizeof(BITMAPINFOHEADER), 1, inptr);


    // calculate old padding
    int pad = (4 - (bi.biWidth * 3) % 4) % 4;

    //create new header files
    BITMAPINFOHEADER bi2 = bi;
    BITMAPFILEHEADER bf2 = bf;

    //image old
    int width = bi.biWidth;
    int height = bi.biHeight;

    //image new
    int width2 = ceil(bi.biWidth * scale);
    int height2 = ceil(bi.biHeight *scale);  

    //new header
    bi2.biWidth = width2;
    bi2.biHeight = height2;
    int pad2 = (4 - (width2 * 3) % 4) % 4;
    bi2.biSizeImage = (width2 * 3 + pad2) * height2;
    bf2.bfSize = bi2.biSizeImage + 54;        


    //header save
    fwrite(&bf2, sizeof(BITMAPFILEHEADER), 1, outptr);
    fwrite(&bi2, sizeof(BITMAPINFOHEADER), 1, outptr);


    rgb *p  = (rgb*)malloc(width*height* sizeof(rgb));   //image old
    rgb *p2 = (rgb*)malloc(width2*height2* sizeof(rgb));  //image2 new


    //write p
    int say = 0;
    for (int i = 0; i < height; i++)
    {
        for (int j = 0; j < width; j++)
        {

            fread(p + say, 3, 1, inptr);
            say++;
        }
        fseek(inptr, pad, SEEK_CUR);
    }

    //write p---->p2
    int say2 = 0;
    for (int i = 0; i < height2; i++)
    {
        for (int j = 0; j < width2; j++)
        {
            int x = j / scale ;
            int y = i / scale ;
            int say = y * width + x;

            memcpy(p2+say2, p + say, sizeof(rgb));
            say2++;
        }
    }

    //write file
    say = 0;
    for (int i = 0; i < height2; i++)
    {
        for (int j = 0; j < width2; j++)
        {
            fwrite(p2+say, sizeof(rgb), 1, outptr);
            say++;
        }
        for (int h = 0; h < pad2; h++)
                fputc(0x00, outptr);
    }



    fclose(inptr);
    fclose(outptr);


    return 0;
}

调整位图文件的大小部分有效 - 放大有效但缩小无效

Resizing Bitmap Files Partially Working - Enlarge Works but Reducing Doesn't

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image

image-resizing

cs50