converting 24bpp bmp colour image to 8bpp grayscale bmp image in c - c

Question

I am trying to convert 24bpp BMP color image to 8bpp grayscale BMP image. I have done something but i am not getting 100% correct output.
I have converted 24bpp to 8bpp. but in result image colour table is also being considered as pixel data. I have tried setting offset byte in header but problem still persists.
#include <stdio.h>
int main()
{
FILE* fIn = fopen("lena_colored_256.bmp","rb");
FILE* fOut = fopen("lena_gray.bmp", "w+");
int i, j, y;
unsigned char byte[54];
unsigned char colorTable[1024];
if (fIn == NULL)// check if the input file has not been opened
{
printf("File does not exist.\n");
}
for (i = 0; i < 54; i++)//read the 54 byte header from fIn
{
byte[i] = getc(fIn);
}
byte[28] = (unsigned char)8; // write code to modify bitDepth to 8
byte[11] = (unsigned char)04;
//write code to add colorTable
for (i = 0; i < 256; i++)
{
colorTable[i * 4 + 0] = i;
colorTable[i * 4 + 1] = i;
colorTable[i * 4 + 2] = i;
colorTable[i * 4 + 3] = i;
}
for (i = 0; i < 54; i++)
{
putc(byte[i], fOut);
}
for (i = 0; i < 1024; i++)
{
putc(colorTable[i], fOut);
}
// extract image height, width and bitDepth from imageHeader
int *height = (int)& byte[18];
int *width = (int)& byte[22];
int *bitDepth = (int)& byte[28];
printf("bitDepth : %d\n", *bitDepth);
printf("width: %d\n", *width);
printf("height: %d\n", *height);
int size = (*height) * (*width)*3; //calculate image size
unsigned char* buffer;
buffer = (unsigned char*)malloc(sizeof(int) * size);
for (i = 0; i < size; i=i+3) //RGB to gray
{
buffer[i+2] = getc(fIn);//blue
buffer[i+1] = getc(fIn);//green
buffer[i+0] = getc(fIn);//red
y = (buffer[i+0] * 0.33) + (buffer[i+1] * 0.33) + (buffer[i+2] * 0.33);
putc(y, fOut);
}
fclose(fOut);
fclose(fIn);
return 0;
}
Colour table data is also being considered as pixel data by image.
i have checked my colour table data entering into BMP file. I have printed out file pointer location, after entering at 94th byte it is increasing by 2 byte instead of 1 byte, this is happening total 4 times and other 1020 time file pointer is increasing by 1 byte. Any explanation regarding this?

Answer 1

Changing 24-bit to 8-bit bitmap is not as simple as changing the bitcount in header file. 24-bit bitmap doesn't have a color table. You have to build a color table for the 8-bit. Fortunately this is relatively easy for gray scale images.
Several values in bitmap header file need to be modified.
Then change the 24-bit to gray scale, and change to 8-bit bitmap. See bitmap file format for additional information. Also read about "padding" where the width of bitmap in bytes should always be a multiple of 4.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <inttypes.h>
#pragma pack(push, 2)
typedef struct {
int16_t bfType;
int32_t bfSize;
int16_t bfReserved1;
int16_t bfReserved2;
int32_t bfOffBits;
} BITMAPFILEHEADER;
typedef struct {
int32_t biSize;
int32_t biWidth;
int32_t biHeight;
int16_t biPlanes;
int16_t biBitCount;
int32_t biCompression;
int32_t biSizeImage;
int32_t biXPelsPerMeter;
int32_t biYPelsPerMeter;
int32_t biClrUsed;
int32_t biClrImportant;
} BITMAPINFOHEADER;
#pragma pack(pop)
int main()
{
FILE* fin = fopen("fin.bmp", "rb");
FILE* fout = fopen("fout.bmp", "wb");
if(!fin) { printf("fin error\n"); goto error; }
if(!fout) { printf("fout error\n"); goto error; }
BITMAPFILEHEADER bf;
BITMAPINFOHEADER bi;
fread(&bf, sizeof bf, 1, fin);
fread(&bi, sizeof bi, 1, fin);
if(sizeof bf != 14) { printf("Wrong pragma pack\n"); goto error; }
if(sizeof bi != 40) { printf("Wrong pragma pack\n"); goto error; }
if(bf.bfType != 0x4D42) { printf("not bmp, or not LE system\n"); goto error; }
if(bi.biSize != 40) { printf("Can't handle this bitmap format\n"); goto error; }
if(bi.biBitCount != 24) { printf("not 24-bit\n"); goto error; }
int height = bi.biHeight;
if(height < 0) height = -height;
//width in bytes:
int src_wb = ((bi.biWidth * 24 + 31) / 32) * 4;
int dst_wb = ((bi.biWidth * 8 + 31) / 32) * 4;
int src_size = src_wb * height;
int dst_size = dst_wb * height;
//allocate for source and destination
uint8_t *src = malloc(src_size);
uint8_t *dst = malloc(dst_size);
//read pixels
fread(src, 1, src_size, fin);
//make gray scale color-table
uint8_t clr[1024] = { 0 };
for(int i = 0; i < 256; i++)
clr[i * 4 + 0] = clr[i * 4 + 1] = clr[i * 4 + 2] = (uint8_t)i;
for(int y = height - 1; y >= 0; y--)
{
for(int x = 0; x < bi.biWidth; x++)
{
uint8_t blu = src[y * src_wb + x * 3 + 0];
uint8_t grn = src[y * src_wb + x * 3 + 1];
uint8_t red = src[y * src_wb + x * 3 + 2];
uint8_t gry = (uint8_t)(.33 * red + .34 * grn + .33 * blu);
dst[y * dst_wb + x] = gry; //this will match the index in color-table
}
}
//modify bitmap headers
bf.bfSize = 54 + 1024 + dst_size;
bi.biBitCount = 8;
bi.biSizeImage = dst_size;
fwrite(&bf, sizeof bf, 1, fout);
fwrite(&bi, sizeof bi, 1, fout);
fwrite(clr, 1, 1024, fout);
fwrite(dst, 1, dst_size, fout);
free(src);
free(dst);
error:
fclose(fout);
fclose(fin);
return 0;
}

Answer 2

FILE* fOut = fopen("lena_gray.bmp", "w+");//ERROR
FILE* fOut = fopen("lena_gray.bmp", "wb");//TRUE