Converting pixels in this code

Question

I seem to have hit a stump in a small programming assignment. (Please don't tell me how to do the entire problem; I just want to know how to set every byte to red).

The code I have is the following:

#include <stdio.h>
#include <stdlib.h>
#include <cs50.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];
    char* infile = GetString();
    char* outfile = GetString();

    // 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);
            triple.rgbtRed = 'ff'
            // 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;
}

and bmp.h is

#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(PROT.10).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;

Note: I don't have the code for GetString, but please just assume that it will return a string as a char*.

The problem I have is trying to convert EVERY pixel's rgbtRed value to extreme red; I have no idea how to do it. Please provide me only how I can convert rgbtRed?

UPDATE: Puzzle

Welcome to Tudor Mansion. Your host, Mr. John Boddy, has met an untimely end—he's the victim of foul play. To win this game, you must determine whodunit.

Unfortunately for you (though even more unfortunately for Mr. Boddy), the only evidence you have is a 24-bit BMP file called clue.bmp, pictured below, that Mr. Boddy whipped up on his computer in his final moments. Hidden among this file's red "noise" is a drawing of whodunit.

You long ago threw away that piece of red plastic from childhood that would solve this mystery for you, and so you must attack it as a computer scientist instead.

Answer 1

Remove the single quotes. 'ff' is an implementation-defined (or undefined) integer, 0xff is a hexadecimal constant.

The line is question should read

            triple.rgbtRed = 0xff;

Answer 2

To get a nice sky blue on white image:

triple.rgbtBlue = 0xff;

triple.rgbtGreen = 0xff;

If you want it red scale:

triple.rgbtBlue =  0x00;

triple.rgbtGreen = 0x00;

And if you want purple scale:

triple.rgbtBlue = 0xff;

triple.rgbtGreen = 0x00;