Difficulties converting 8-bit greyscale .BMP images to 1-bit .BMP Images - c

Question

I'm looking for a way to convert a BMP image of 256 grey levels to a 1-bit BMP image. The conversion rule is that the bit should be 0 for grey levels from 0 - 127, and 1 from 128 - 255. So far I have attempted to make a program in C. The problem is even though I changed the bits per pixel in the information header to 1 BBP, the output is a black image (I'm using Lena as the input file btw) as if the colour depth is still 8-bit (while in Properties it says 1-bit).
I tried to open the BMP file in a hex editor and saw that the pixel data had been changed to 0 and 1 as expected and the headers of the input and output files were the same except the colour depth. Am I missing anything? What changes do I need to make for the image to show what is expected?
Here is my program:
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#pragma pack(1) //avoid padding in a struct
typedef uint8_t BYTE;
typedef uint32_t DWORD;
typedef int32_t LONG;
typedef uint16_t WORD;
typedef struct
{
WORD Signature;
DWORD fSize;
WORD fReserved_1;
WORD fReserved_2;
DWORD fOffset;
} BITMAP_HEADER;
typedef struct
{
DWORD Size;
LONG Width;
LONG Height;
WORD Planes;
WORD BitsPerPixel;
DWORD Compression;
DWORD imgSize;
LONG ResX;
LONG ResY;
DWORD Color;
DWORD ImpColor;
} INFO_HEADER;
typedef struct
{
BYTE rgbtBlue; //since I'm only dealing with 8-bit colour depth
//so only one value is needed I guess?
// BYTE rgbtGreen;
// BYTE rgbtRed;
} RGBTRIPLE;
void BinaryConvert(DWORD height, DWORD width, RGBTRIPLE img[height][width]);
int main()
{
FILE* fp, *cp;
fp = fopen("Resources/test.bmp", "rb"); //bmp is binary file, therefore use "rb" permission
cp = fopen("Resources/output.bmp", "wb");
BITMAP_HEADER fHeader;
INFO_HEADER fInfo;
//read headers
fread(&fHeader, sizeof(BITMAP_HEADER), 1, fp);
fread(&fInfo, sizeof(INFO_HEADER), 1, fp);
int width = fInfo.Width;
int height = fInfo.Height;
int padding = (4 - (width * sizeof(RGBTRIPLE)) % 4) % 4;
RGBTRIPLE (*image)[width] = calloc(height, width * sizeof(RGBTRIPLE));
//read pixel data
for (int i = 0; i < height; i++)
{
fread(image[i], sizeof(RGBTRIPLE), width, fp);
fseek(fp, padding, SEEK_CUR); //skip over padding
}
//coonvert grey levels into 0 and 1
BinaryConvert(height, width, image);
//modify file header
fInfo.noBitsPerPixel = 1;
//write headers into output file
fseek(cp, 0, SEEK_SET);
fwrite(&fHeader, sizeof(BITMAP_HEADER), 1, cp);
fwrite(&fInfo, sizeof(INFO_HEADER), 1, cp);
//write pixel data into output file
for (int i = 0; i < height; i++)
{
fwrite(image[i], sizeof(RGBTRIPLE), width, cp);
for (int k = 0; k < padding; k++)
{
fputc(0x00, cp);
}
}
fclose(cp);
fclose(fp);
}
void BinaryConvert(DWORD height, DWORD width, RGBTRIPLE img[height][width])
{
for (int i = 0; i < height; i++)
{
for (int j = 0; j < width; j++)
{
if (img[i][j].rgbtBlue <= 127 && img[i][j].rgbtBlue >=0)
{
img[i][j].rgbtBlue = 0;
}
else if (img[i][j].rgbtBlue <= 255 && img[i][j].rgbtBlue >= 128)
img[i][j].rgbtBlue = 1;
}
}
}
Input file (lena.bmp). (257 KB)
Result from program (257 KB)

Answer 1

You need to use 255 instead of 1. That is the color for white