Padding, pset4, resize. weird xxd output

Question

I have a padding problem in the resize (pset4). I can resize any image that doesnt need padding in their new version, but when it comes to padding it just doesn't work. I checked the headers with peek and they're not the problem. The program also calculates the values of oldpadding and newpadding correctly. (xxd output for small.bmp resized by 2.)It also has a weird xxd output. As you see above, it shows 8 pixels as the wiidth of the image despite it says 6 when i peek in the bitmapinfoheader. I would appreciate every little tip. Because this problem is driving me crazy.

//change the size

LONG cbiwidth = bi.biWidth*n;

// determine padding for scanlines
int newpadding =  (4 - (cbiwidth * sizeof(RGBTRIPLE)) % 4) % 4;
int oldpadding =  (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;    
LONG cbiheight = bi.biHeight*n;

DWORD cbisizeimage = abs(cbiheight)* (abs(cbiwidth)*sizeof(RGBTRIPLE) + newpadding) ;

DWORD cbfsize = cbisizeimage +sizeof(bf)+ sizeof(bi);    

// write outfile's BITMAPFILEHEADER
fwrite(&bf.bfType, sizeof(WORD), 1, outptr);
fwrite(&cbfsize, sizeof(DWORD),1,outptr);
fwrite(&bf.bfReserved1, sizeof(WORD)*2 + sizeof(DWORD),1,outptr);
// write outfile's BITMAPINFOHEADER
fwrite(&bi, sizeof(DWORD),1,outptr);
fwrite(&cbiwidth, sizeof(LONG),1,outptr);
fwrite(&cbiheight, sizeof(LONG),1,outptr);
fwrite(&bi.biPlanes, sizeof(WORD)*2 + sizeof(DWORD)*1,1,outptr);
fwrite(&cbisizeimage, sizeof(DWORD),1, outptr);
fwrite(&bi.biXPelsPerMeter, sizeof(LONG)*2 +sizeof(DWORD)*2, 1, outptr);

fseek(inptr,sizeof(BITMAPINFOHEADER) +sizeof(BITMAPFILEHEADER),SEEK_SET);


// iterate over infile's scanlines
for (int i = 0, biHeight = abs(bi.biHeight); i < biHeight; i++)
{
     for(int h=0; h < n; h++)
     {
            fseek(inptr,(-(sizeof(RGBTRIPLE)*(bi.biWidth)+oldpadding))*i, SEEK_CUR);
        // 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);

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

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

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

Answer 1

For the strange xxd output, it's because you have given a wrong value for the columns -c flag. Since the small.bmp image has a width of 3 pixels, it has as -c

• 3 pixels * 3 bytes each pixel == 9 bytes
• + 3 bytes of padding to be a multiple of 4
• we have 12 bytes

That's why the command for the xxd on small is

xxd -c 12 -g 3 -s 54 small.bmp

Now If you resize it by 2, you have:

• 6 pixels * 3 bytes each pixel == 18 bytes
• + 2 bytes of padding to be a multiple of 4
• we have 20 bytes

So the xxd command for small2.bmp should be

xxd -c 20 -g 3 -s 54 small2.bmp

Now if you could kindly re-execute the correct command and edit your question with the new output and what is the problem exactly you're facing with the padding.

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