Pset4 - Resize problem with vertical scaling

Question

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;
}

//changed fields

BITMAPFILEHEADER bfnew = bf;
BITMAPINFOHEADER binew = bi;

binew.biWidth *= n;
binew.biHeight *= n;

// determine padding for scanlines
int padding = (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;
int newpadding = (4 - (binew.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;

binew.biSizeImage = ((sizeof(RGBTRIPLE) * binew.biWidth) + newpadding) * abs(binew.biHeight);
bfnew.bfSize = binew.biSizeImage + 54;

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

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



// 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);

        // scale width n times
    for (int p = 0; p < n; p++)
    {
        // write RGB triple to outfile
        fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr);
    }

    }

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

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

// close infile
fclose(inptr);

// close outfile
fclose(outptr);

// success
return 0;

}

Have a good day I know this question asked many times but I really did'nt understand logic of padding and vertical scaling .What padding means exactly and what is it doing , I saw my problem solution many times but I want to understand logic behind it there is no sense just copy paste can somebody explain me solution of my problem line by line.Thanks (btw english not my mother tongue sorry for misunderstanding)

Answer 1

---

About you first question: What is padding?

In this type of bmp file, every pixel takes up 3 bytes (that's one of the checks you did on the inputfile: biBitcount = 24 bits = 3 bytes).

However, every line of pixels should be using a multiple of 4 bytes in the file. Therefore, you might have to add 1, 2 or 3 bytes to the end of each pixel line.

Those extra bytes are the padding. You handled in your formula for newpadding (plus the loop in which you use newpadding).

---

About your second question: How does vertical scaling work?

For the horizontal scaling, you made sure that every pixel (represented by a RGBTRIPLE) from the input file was written n times to the output file.

So for the vertical scaling, you should make sure that every (horizontally scaled) pixel line should be written n times to the output file ...

There are several ways to do that, for instance:

• You might try to read every input line n times. Might be difficult to jump back correctly ...
• You might try to collect the output pixel line in some buffer first, before writing it n times to the output file. Might be difficult to prevent memory leaks ...

Try to find some way to do this which looks natural to you.