pset4 doesn't pass resize

Question

The output image appears to be correct, but only passes tests when resizing by 1. I tried to use peek and xxd to compare with staff implementation, my output appears to be equal.

// read infile's BITMAPFILEHEADER
BITMAPFILEHEADER bf;
fread(&bf, sizeof(BITMAPFILEHEADER), 1, inptr);

    // read infile's BITMAPINFOHEADER
    BITMAPINFOHEADER bi;
    fread(&bi, sizeof(BITMAPINFOHEADER), 1, inptr);

    //store old header values
    int width = bi.biWidth;
    int height = bi.biHeight;

    //update headers for the resized image
    bi.biWidth *= n;
    bi.biHeight *= n;
    bi.biSizeImage *= n * n;

    bf.bfSize = bi.biSizeImage + 54;

    // 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;
    }

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

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

    //size of output line in bytes
    int size = bi.biWidth * sizeof(RGBTRIPLE);

    // iterate over infile's scanlines
    height = abs(height);
    for (int i = 0; i < height; i++)
    {
        // iterate over pixels in scanline
        for (int j = 0; j < width; j++)
        {
            // temporary storage
            RGBTRIPLE triple;

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

            // stores position of output to go back later to prepare for next iteration
            int back = ftell(outptr);

            for(int k = 0; k < n; k++){
                for(int l = 0; l < n; l++){
                    // write RGB triple to outfile
                    fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr);
                }
                // jumps line
                fseek(outptr, size + outPadding - n * sizeof(RGBTRIPLE), SEEK_CUR);
            }
            // backs to first unfilled pixel 
            fseek(outptr, back + n*sizeof(RGBTRIPLE), SEEK_SET);
        }

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

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

        for(int k = 0; k < n - 1; k++){
            // skip lines in output
            fseek(outptr, size, SEEK_CUR);
            for (int l = 0; l < outPadding; l++)
            {
                fputc(0x00, outptr);
            }    
        }
    }
}

Answer 1

While the image data may be correct, there are two errors in header data due to incorrect calculation.

You need to run the same source file against your code and against the staff version of resize and compare the headers of the output files. If you are running your code only and manually calculating what values should result, you are probably making the same mistake in your manual calculations as the code is making, so it looks right to you.

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