pset4 resize - fseek help (I think)

Question

I'm on the verge of giving up. I've been stuck on this problem for weeks. I managed the headers, and I think I set up the loops right to write the pixels correctly. I just can't wrap my head around fseek however. I read the man page, and read stack exchange, but I just don't get it.

Can anyone help me, why my code is failing to resize, and what direction should I progress? I'm super frustrated with it.

// Saving original width and height
int oriwidth = bi.biWidth;
int oriheight = bi.biHeight;

// Modifying to new width and height
bi.biWidth = bi.biWidth * n;
bi.biHeight = bi.biHeight * n;

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

// Modifying sizes
bi.biSizeImage = ((bi.biWidth * abs(bi.biHeight)) * sizeof(RGBTRIPLE)) + (padding * abs(bi.biHeight));
bf.bfSize = bi.biSizeImage + 54;

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

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


// iterate over infile's scanlines
for (int i = 0; i < abs(oriheight); i++)
{   

    // temporary storage
    RGBTRIPLE triple; 

    for (int s = 0; s < n; s++)
    {

    fseek(inptr, 54 + ((i + 1) * oriwidth * (sizeof(triple) + oripadding)), SEEK_SET);

        // iterate over pixels in scanline
        for (int j = 0; j < oriwidth; j++)
        {

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

            // write RGB triple to outfile
            for (int k = 0; k < n; k++)
            {
                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 < padding; k++)
    {
        fputc(0x00, outptr);
    }
}

Answer 1

Your approach is correct, but you have three errors to deal with, one of which is irrelevant.

First, as you guessed, your calculation for fseek is in error. In simple terms, you've placed parentheses incorrectly. Hint: padding only exists once on each line of pixels. Look carefully at how you laid out the line and you should be able to resolve it.

fseek(inptr, 54 + ((i + 1) * oriwidth * (sizeof(triple) + oripadding)), SEEK_SET);

I like your approach. Most people, myself included, use relative positioning for the fseek based on the previous position, but you encoded an absolute position based on the start of the file. I think in the long run, it's more elegant.

The second problem is the addition of padding to each output line. You misplaced the code to doing that outside the for loop that generated each line. In essence, you are outputting n lines of pixels and then adding the padding once after multiple lines. Instead, the 'add padding' code should be inside the appropriate for loop so that padding is appended to every line.

Finally, the irrelevant error. The line of code that skips over the padding at the end of each line in the input file used the output padding var instead of the input padding var. It is irrelevant though, because of how you calculate the fseek above! By fseeking relative to the beginning of the file, it rendered this fseek totally unnecessary. The error has no impact and that line of code could be completely removed. Nice.

That should get you going. Try to fix the fseek on your own now, and if you can't, leave a message and I'll give you another push. ;-)

If this answers your question, please click the check mark to accept. Let's keep up on forum maintenance. ;-)