Aayush Agrawal Aayush Agrawal - 2 months ago 9x
C Question

Converting pixels in this code

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)
fprintf(stderr, "Could not create %s.\n", outfile);
return 3;

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

// read infile's BITMAPINFOHEADER
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)
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

// 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

// close outfile

// 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;

* 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__))

* 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__))

* 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__))

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

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.

enter image description here

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.


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;