PSET4 resize recopy method. Help me understand

Question

I don't exactly understand the principle of recopy method used in resize pset4. Can someone explain me without giving the actual solution. Here is my code so far. After running the code I recive a distorted image.I don't understand also the fseek function. From my understanding of fseek it must turn the cursor back to the beginning of the file so it must pass through pixels I wrote and the padding right ?

// iterate over infile's scanlines
for (int i = 0, old_biHeight = abs(old_bi.biHeight); i < old_biHeight; i++)
{
    for (int j = 0; j < n - 1; j++)
    {
        // iterate over pixels in scanline

        for (int k = 0; k < old_bi.biWidth; k++)
        {

            // temporary storage
            RGBTRIPLE triple;


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


            // write RGB triple to infile
            for (int z = 0; z < n; z++)
            {
                fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr);
            }
            //padding
            for (int z = 0; z < old_padding; z++)
            {
                fputc(0x00, outptr);
            }
            //Send infile cursor back
            fseek(inptr, bi.biWidth, SEEK_SET);
        }
    }

    for (int k = 0; k < old_bi.biWidth; k++)
    {
         RGBTRIPLE triple;


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

        for (int z = 0; z < n; z++)
        {
            fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr);
        }

        for (int y = 0; y < new_padding; y++)
        {
        fputc(0x00, outptr);
        }
    }
}

EDIT: please also tell me what's wrong in my code. The code now resizes images but the image is full of randomly colors. Is it a problem with padding ?

EDIT NEW CODE :

    // iterate over infile's scanlines
for (int i = 0, old_biHeight = abs(old_bi.biHeight); i < old_biHeight; i++)
{
    for (int j = 0; j < n - 1; j++)
    {
        // iterate over pixels in scanline

        for (int k = 0; k < old_bi.biWidth; k++)
        {

            // temporary storage
            RGBTRIPLE triple;


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


            // write RGB triple to infile
            for (int z = 0; z < n; z++)
            {
                fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr);
            }
            //padding
            for (int z = 0; z < new_padding; z++)
            {
                fputc(0x00, outptr);
            }
            //Send infile cursor back
            fseek(inptr, new_padding, SEEK_CUR);
        }
    }

    for (int k = 0; k < old_bi.biWidth; k++)
    {
         RGBTRIPLE triple;


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

        for (int z = 0; z < n; z++)
        {
            fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr);
        }

        for (int y = 0; y < new_padding; y++)
        {
        fputc(0x00, outptr);
        }
    }
}

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

Answer 1

Principle of recopy method:
Say you are doubling the size of the image (n = 2). The default code in copy.c will write only 1 pixel - but now you need 2. So, you need to "recopy" the pixel another time. If n were 3, say, you'd need to "recopy" the pixel 2 more times.

That's across - now you also need to "recopy" the entire new row the equivalent amount of times.

fseek:
fseek is for moving around in the original file.

As you move through that file reading the original bytes, a "pointer" is keeping track of where you are in the file. Read a byte, and the pointer moves forward past that byte, ready to read the next byte.

Once you've read the original pixels for a row, you (and that pointer) will have reached the padding. However, the original padding is not likely the same as what's needed for the new file. Therefore, you need to move the pointer (using fseek) to skip over the padding in the original file in order to get to the next row's pixels for "recopy" to the new file.