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I keep getting an error from the IDE saying "Invalid or Unsupported Image Format" and I haven't been able to figure out why. I have looked at other questions but none seem to help.

bi_new.biWidth = bi.biWidth * n;
bi_new.biHeight = bi.biHeight * n;
int padding = (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;
int new_padding = (4 - (bi_new.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;
bf_new.bfSize = (bi_new.biWidth * abs(bi_new.biHeight )) * sizeof(RGBTRIPLE) + new_padding * abs(bi_new.biHeight) + 54;
bi_new.biSizeImage = bf_new.bfSize - 54;
// write outfile's BITMAPFILEHEADER
fwrite(&bf_new, sizeof(BITMAPFILEHEADER), 1, outptr);

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

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


// iterate over infile's scanlines
for (int i = 0, biHeight = abs(bi.biHeight); i < biHeight; i++)
{
    int current_pos = ftell(inptr); 
    // iterate over pixels in scanline
    for (int j = 0; j < bi.biWidth; j++)
    {
        fseek(inptr, current_pos, SEEK_SET);
        //bi.biHeight = bi.biHeight *= n;
        //bi.biWidth = bi.biWidth *= n;
        // temporary storage
        RGBTRIPLE triple;

        //long int pos = ftell(inptr);
        // read RGB triple from infile
        fread(&triple, sizeof(RGBTRIPLE), 1, inptr);

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

            // write RGB triple to outfile
            fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr);
        }
        //fseek(inptr, pos, SEEK_SET);
    }
    fwrite(outptr, new_padding, SEEK_CUR);
    // skip over padding, if any
    fseek(inptr, padding, SEEK_CUR);

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

// close infile
fclose(inptr);

// close outfile
fclose(outptr);

// success
return 0;

}

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The file and bitmap header need to have all the same values as in the input file, only those four values changed. Do you initialise bi_new and bf_new using a line like

BITMAPINFOHEADER bi_new = bi;
BITMAPFILEHEADER bf_new = bf;

(obviously after reading to bi and bf)?

Also, for(int m = 0; m <= n; m++) loops n+1 instead of n times due to the <= instead of <.

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