Hello!

I am doing a free online course, and one of the exercises offers
a code that copies a given file and then writes it into another
file. The original file have a picture of a face covered by red
pixels. The task is to remove the red color by changing all pixels
with that are 0000ff into ffffff (into white ones). I have managed
to achieve this because it is a very straightforward task.

But the image I see is very blurred and is in light blue color. Hence
I have tried to make the blue color more saturated by changing the value of pixels that have the value of red != ff. I have decided to do that after checking the information on pixels by xxd.

Please, take a look at the program (it is written by the course staff). I am highlighting the piece of code that is supposed to change only certain
blue pixels to saturated blue. Obviously I am doing something wrong, most likely I am interpreting the result of xxd command correctly.

This piece of code make the whole picture a mixture of blue and white.
Please, explain where I can read the information on how to handle these
issues correctly, and please do not simply solve the problem for me - but
I will be grateful for helpful hints and explanations.

if (triple.rgbtRed != 0xff)
{
triple.rgbtRed = 0x0a;
triple.rgbtGreen = 0x84;
}

Program:
/*
* Copies a BMP piece by piece.
*/

#include <stdio.h>
#include <stdlib.h>

#include "bmp.h"

int main(int argc, char* argv[])
{
// ensure proper usage
if (argc != 3)
{
printf("Usage: ./copy infile outfile\n");
return 1;
}

// remember filenames
char* infile = argv[1];
char* outfile = argv[2];

// open input file
FILE* inptr = fopen(infile, "r");
if (inptr == NULL)
{
printf("Could not open %s.\n", infile);
return 2;
}

// open output file
FILE* outptr = fopen(outfile, "w");
if (outptr == NULL)
{
fclose(inptr);
fprintf(stderr, "Could not create %s.\n", outfile);
return 3;
}

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

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

// 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 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;

// iterate over infile's scanlines
for (int i = 0, biHeight = abs(bi.biHeight); i < biHeight; i++)
{
// 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);

if (triple.rgbtRed == 0xff && triple.rgbtBlue == 0x00 && triple.rgbtGreen == 0x00)
{
triple.rgbtBlue = 0xff;
triple.rgbtGreen = 0xff;
}

/*** THIS PIECE DOESN'T WORK CORRECTLY ***/
if (triple.rgbtRed != 0xff)
{
triple.rgbtRed = 0x0a;
triple.rgbtGreen = 0x84;
}

// write RGB triple to outfile
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);
}
}

// close infile
fclose(inptr);

// close outfile
fclose(outptr);

// that's all folks
return 0;
}

bmp.h
#include <stdint.h>

/**
* Common Data Types
*
* The data types in this section are essentially aliases for C/C++
* primitive data types.
*
* Adapted from http://msdn.microsoft.com/en-us/library/cc230309.aspx.
* See http://en.wikipedia.org/wiki/Stdint.h for more on stdint.h.
*/
typedef uint8_t BYTE;
typedef uint32_t DWORD;
typedef int32_t LONG;
typedef uint16_t WORD;

/**
* BITMAPFILEHEADER
*
* The BITMAPFILEHEADER structure contains information about the type, size,
* and layout of a file that contains a DIB [device-independent bitmap].
*
* Adapted from http://msdn.microsoft.com/en-us/library/dd183374(VS.85).aspx.
*/
typedef struct
{
WORD bfType;
DWORD bfSize;
WORD bfReserved1;
WORD bfReserved2;
DWORD bfOffBits;
} __attribute__((__packed__))
BITMAPFILEHEADER;

/**
* BITMAPINFOHEADER
*
* The BITMAPINFOHEADER structure contains information about the
* dimensions and color format of a DIB [device-independent bitmap].
*
* Adapted from http://msdn.microsoft.com/en-us/library/dd183376(VS.85).aspx.
*/
typedef struct
{
DWORD biSize;
LONG biWidth;
LONG biHeight;
WORD biPlanes;
WORD biBitCount;
DWORD biCompression;
DWORD biSizeImage;
LONG biXPelsPerMeter;
LONG biYPelsPerMeter;
DWORD biClrUsed;
DWORD biClrImportant;
} __attribute__((__packed__))
BITMAPINFOHEADER;

/**
* RGBTRIPLE
*
* This structure describes a color consisting of relative intensities of
* red, green, and blue.
*
* Adapted from http://msdn.microsoft.com/en-us/library/aa922590.aspx.
*/
typedef struct
{
BYTE rgbtBlue;
BYTE rgbtGreen;
BYTE rgbtRed;
} __attribute__((__packed__))
RGBTRIPLE;

Results of xxd:

1) after I have taken away red "noise"
$ xxd -l240 -c24 -g3 -s54 verdict_fin0.bmp
0000036: ffffff ffff88 ffff82 ffff83 ffffff ffff81 ffff83 ffff82 ........................
000004e: feff7c ffff83 ffffff ffffff ffffff ffffff ffff83 ffffff ..|.....................
0000066: ffffff ffff83 ffff88 feff8a ffff83 feff7d ffffff ffffff .................}......
000007e: ffffff ffff83 ffffff ffff88 ffff88 ffffff ffffff ffffff ........................
0000096: ffff88 ffff83 ffff88 ffff83 ffff88 ffff7f feff8a ffffff ........................
00000ae: ffffff ffffff ffff88 ffffff ffff8c ffffff ffff88 feff8a ........................
00000c6: ffffff ffffff ffff88 ffffff feff8a ffffff feff8e ffff88 ........................
00000de: ffffff ffffff ffffff ffffff ffff88 ffffff ffffff ffff7b .......................{
00000f6: feff82 fefe82 ffffff ffffff ffffff ffff83 ffffff ffffff ........................
000010e: ffffff ffffff ffff88 ffffff ffffff ffff88 ffff88 ffffff ........................

2) after the attempt to make blue more saturated

0000036: ffffff ff840a ff840a ff840a ffffff ff840a ff840a ff840a ........................
000004e: fe840a ff840a ffffff ffffff ffffff ffffff ff840a ffffff ........................
0000066: ffffff ff840a ff840a fe840a ff840a fe840a ffffff ffffff ........................
000007e: ffffff ff840a ffffff ff840a ff840a ffffff ffffff ffffff ........................
0000096: ff840a ff840a ff840a ff840a ff840a ff840a fe840a ffffff ........................
00000ae: ffffff ffffff ff840a ffffff ff840a ffffff ff840a fe840a ........................
00000c6: ffffff ffffff ff840a ffffff fe840a ffffff fe840a ff840a ........................
00000de: ffffff ffffff ffffff ffffff ff840a ffffff ffffff ff840a ........................
00000f6: fe840a fe840a ffffff ffffff ffffff ff840a ffffff ffffff ........................
000010e: ffffff ffffff ff840a ffffff ffffff ff840a ff840a ffffff ........................

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