Thread: Extra junk at the end of Hex after creation of 8bit BMP

Hello, I found some code to read a write 24 bit BMP here:
Reading a bitmap image

I want however to read only 8 bit BMPs and so I have been trying to understand the above code as to modify it for 8 bit processing. The code is:

Code:

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


/* Global */
int height, width;


//***************** Structures  *******************//
typedef struct{
    char type[2];             // file type
    unsigned int size;         // file size in bytes
    unsigned short int reserved1,reserved2;
    unsigned int offset;     // offset to image data
}HEADER;




typedef struct{
    unsigned char RGB[3]; 
}RGB;


typedef struct{
    unsigned int size;
    int width,height;
    unsigned short int planes;
    unsigned short int bpp;
    unsigned int compression;
    unsigned int imagesize;
    int xresolution,yresolution;
    unsigned int colours;
    unsigned int impcolours;
}INFOHEADER;




// ********** Verify if the file exist **********
FILE* exist(char *name){
    FILE *tmp;
    tmp = fopen(name,"rb");
    if (!tmp){
        printf("\nERROR: Incorrect file or not exist!\n");
        exit(0);
    }
    fseek(tmp,0,0);
    return(tmp);
}


// ********** Verify if the file is BMP *********
void isBMP(FILE* arq, HEADER head, INFOHEADER info){
    char type[3];
    unsigned short int bpp;
    fseek(arq,0,0);
    fread(type,1,2,arq);
    type[2] = '\0';
    
    fseek(arq,28,0);
    fread(&bpp,1,2,arq);
    
    if (strcmp(type,"BM") || (bpp != 8)){
            printf("\nThe file is not BMP format or is not 8 bits\n");
            exit(0);
    }
}


// ********** Read BMP info from file **********
INFOHEADER readInfo(FILE* arq){
    INFOHEADER info;
    
    // Image Width in pixels
    fseek(arq,18,0);
    fread(&info.width,1,4,arq);
    //printf("infowidth: %d\n", info.width);


    // Image Height in pixels
    fseek(arq,22,0);
    fread(&info.height,1,4,arq);
    //printf("infoheight: %d\n", info.height);


    // Color depth, BPP (bits per pixel)
    fseek(arq,28,0);
    fread(&info.bpp,1,2,arq);
    
    // Compression type
    // 0 = Normmally
    // 1 = 8 bits per pixel
    // 2 = 4 bits per pixel
    fseek(arq,30,0);
    fread(&info.compression,1,4,arq);
    //printf("compression: %d\n", info.compression);
    
    // Image size in bytes
    fseek(arq,34,0);
    fread(&info.imagesize,1,4,arq);
    
    // Number of color used (NCL)
    // value = 0 for full color set
    fseek(arq,46,0);
    fread(&info.colours,1,4,arq);
    
    // Number of important color (NIC)
    // value = 0 means all collors important
    fseek(arq,50,0);
    fread(&info.impcolours,1,4,arq);
    
    return(info);
}


// ********** Create Matrix **********
RGB** createMatrix(){
    RGB** Matrix;
    int i;
    Matrix = (RGB**) malloc(height * sizeof(RGB*));
    if (Matrix == NULL){
        perror("***** No memory available *****");
        exit(0);
    }
    for (i=0;i<height;i++){
        Matrix[i] = (RGB*) malloc(width * sizeof(RGB));
        if (Matrix[i] == NULL){
        perror("***** No memory available *****");
                exit(0);
        }
    }
    return(Matrix);
}


void loadImage(FILE* arq, RGB** Matrix){
    int i,j;
    RGB tmp;
    long pos = 51;
    
    fseek(arq,0,0);


    for (i=0; i<height; i++){
        for (j=0; j<width; j++){
               pos+= 3;
            fseek(arq,pos,0);
            fread(&tmp,(sizeof(RGB)),1,arq);
            Matrix[i][j] = tmp;
         }
    }
//printf("size of Matrix: %li\n",(sizeof(Matrix)));
//printf("size of rgb: %li\n",(sizeof(RGB)));
}




// ********** Image Output **********
void writeBMP(RGB **Matrix, HEADER head, FILE* arq){
    FILE* out;
    int i,j;
    RGB tmp;
    long pos = 51;


    char header[54];
    fseek(arq,0,0);
    fread(header,54,1,arq);
    out = fopen("out.bmp","wb");


    fseek(out,0,0);
    fwrite(header,54,1,out);


    //printf("\nMatrix = %c\n", Matrix[0][0].RGB[0]);
    for(i=0;i<height;i++){
        for(j=0;j<width;j++){
            pos+= 3;
            //printf("pos %li \n",pos);
            fseek(out,pos,0);
            tmp = Matrix[i][j];
            fwrite(&tmp,(sizeof(RGB)),1,out);
        }
    }
    fflush(out);
    fclose(out);
}


// ********** Free memory allocated for Matrix **********
void freeMatrix(RGB** Matrix,INFOHEADER info)
{
    int i;
    int lines = info.height;


    for (i=0;i<lines;i++){
        free(Matrix[i]);
    }
    free(Matrix);
}




/* in your main program you just call */
//    FILE* arq; /* the bitmap file 24 bits */
/*    RGB** Matrix_aux, Matrix;
    INFOHEADER info;
    info = readInfo(FILE* arq);
    height = info.height;
    width = info.width;
     
    Matrix_aux = createMatrix();
    Matrix = loadImage(arq,Matrix_aux);
*/


main(){


    FILE *arq; 
    RGB  **Matrix;
    INFOHEADER info;
    HEADER head;
    char name[15];


    printf("Type the image's name : ");
    scanf("%s",name);


    arq = exist(name);
    isBMP(arq,head,info);
    info = readInfo(arq);
    height = info.height;
//    printf("height : %d\n", height);


    width = info.width;
//    printf("width: %d \n", width);


    Matrix = createMatrix();
    loadImage(arq,Matrix);
    writeBMP(Matrix,head,arq);
    freeMatrix(Matrix,info);
}

The above code runs with no errors and reads in an 8bit BMP and then saves it as out.bmp however it is much larger then in input file (1MB in -->3.1MB out). Opening the output file in an HEX editor it is clear that the output image is the same however is 3 times as long! the last 2 thirds of the file is the same sequence repeated over and over. If this section is deleted in the hex editor the file then drops to 1MB. (NOTE in the image still opens if this section is not deleted.)

What part of the code is causing these extra junk values??

Thanks

GReaper told you already that the 8 bit BMP uses an additional color table/palette.
According to the file format description, each entry in the color table uses 4 bytes (there are exceptions, you need to check the header information).
256 colors, 4 bytes each -> 256 * 4 = 1024.

In other words, you can't just copy the code you've found but need to study the file format description.

Bye, Andreas

Thank you to both GReaper and AndiPersti. This was the fist time I have worked with images and am still new to C programming but you both provided the answer in as many words.

The working code is as follows (Note the bulk code came from the link above for 24bit images. Only minor changes were made to get the 8bit config with the help of both GReaper and AndiPersti)

Code:

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


/////////////// Structures
typedef struct{
	char type[2]; 			// file type
	unsigned int size; 		// file size in bytes
	unsigned short int reserved1,reserved2;
	unsigned int offset; 	// offset to image data
}HEADER;




typedef struct{
	unsigned int size;
	int width,height;
	unsigned short int planes;
	unsigned short int bpp;
	unsigned int compression;
	unsigned int imagesize;
	int xresolution,yresolution;
	unsigned int colours;
	unsigned int impcolours;
	unsigned char colourtable[1024]; // 256*4 bytes long
}INFOHEADER;




/////////////// Verify if the file exist
FILE* exist(char *name){
	FILE *tmp;
	tmp = fopen(name,"rb");
	if (!tmp){
		printf("\nERROR: Incorrect file or not exist!\n");
		exit(0);
	}
	fseek(tmp,0,0);
	return(tmp);
}


/////////////// Verify if the file is 8 bit BMP 
void isBMP(FILE* arq){
	char type[3];
	unsigned short int bpp;
	fseek(arq,0,0);
	fread(type,1,2,arq);
	type[2] = '\0';
	
	fseek(arq,28,0);
	fread(&bpp,1,2,arq);
	
	if (strcmp(type,"BM") || (bpp != 8)){
	        printf("\nThe file is not BMP format or is not 8 bits\n");
	        exit(0);
	}
}




/////////////// Read BMP info from file 
INFOHEADER readInfo(FILE* arq){
	INFOHEADER info;
	
	// Image Width in pixels
	fseek(arq,18,0);
	fread(&info.width,1,4,arq);
	printf("info.width: %d\n", info.width);


	// Image Height in pixels
	fseek(arq,22,0);
	fread(&info.height,1,4,arq);
	printf("info.height: %d\n", info.height);


	// Color depth, BPP (bits per pixel)
	fseek(arq,28,0);
	fread(&info.bpp,1,2,arq);
	printf("info.bbp: %d\n", info.bpp);
	
	// Compression type
	// 0 = Normmally
	// 1 = 8 bits per pixel
	// 2 = 4 bits per pixel
	fseek(arq,30,0);
	fread(&info.compression,1,4,arq);
	printf("info.compression: %d\n", info.compression);
	
	// Image size in bytes
	fseek(arq,34,0);
	fread(&info.imagesize,1,4,arq);
	printf("info.imagesize: %d\n", info.imagesize);


	
	// Number of color used (NCL)
	// value = 0 for full color set
	fseek(arq,46,0);
	fread(&info.colours,1,4,arq);
	printf("info.colours: %d\n", info.colours);


	
	// Number of important color (NIC)
	// value = 0 means all collors important
	fseek(arq,50,0);
	fread(&info.impcolours,1,4,arq);


	// Colour table
	fseek(arq,54,0);
	fread(&info.colourtable,256,4,arq);
	
	return(info);
}


/////////////// Create Matrix 
unsigned char** createMatrix(INFOHEADER info){
	unsigned char** Matrix;
	int i;


	Matrix = (unsigned char**) malloc(info.height * sizeof(unsigned char*));
	if (Matrix == NULL){
    	perror("***** No memory available *****");
	    exit(0);
	}
	for (i=0;i<info.height;i++){
    	Matrix[i] = (unsigned char*) malloc(info.width * sizeof(unsigned char));
	    if (Matrix[i] == NULL){
	    perror("***** No memory available *****");
	            exit(0);
	    }
	}
	return(Matrix);
}


/////////////// Load image data to matrix
void loadImage(FILE* arq, INFOHEADER info, unsigned char** Matrix){
	int i,j;
	unsigned char tmp;
	int test;
	long pos = 1077; // header + colourtable length is 1078 start reading pixel info from here
	
	fseek(arq,0,0);


	for (i=0; i<info.height; i++){
		for (j=0; j<info.width; j++){
	       	pos+= 1;
	        fseek(arq,pos,0);
	        fread(&tmp,(sizeof(unsigned char)),1,arq);
	        Matrix[i][j] = tmp;
			//printf("Matrix[%i][%i]= %i \n", i,j,tmp);
	 	}
	}
}




// ********** Image Output **********
void writeBMP(unsigned char **Matrix, HEADER head, INFOHEADER info, FILE* arq){
	FILE* out;
	int i,j;
	unsigned char tmp;
	long pos = 1077;


	char header[54];
	char colourtable[1024];


	fseek(arq,0,0);
	fread(header,54,1,arq); 		// read header 54 bytes


	out = fopen("out.bmp","wb");


	fseek(out,0,0);
	fwrite(header,54,1,out); 		// write header to output


	fseek(out,54,0);
	fwrite(info.colourtable,1024,1,out); // write colour table to output


	for(i=0;i<info.height;i++){
		for(j=0;j<info.width;j++){
			pos+= 1;
			fseek(out,pos,0);
			tmp = Matrix[i][j];
			fwrite(&tmp,(sizeof(unsigned char)),1,out);
		}
	}
	fflush(out);
	fclose(out);
}


// ********** Free memory allocated for Matrix **********
void freeMatrix(unsigned char** Matrix,INFOHEADER info)
{
	int i;
	int lines = info.height;


	for (i=0;i<lines;i++){
		free(Matrix[i]);
	}
	free(Matrix);
}




/* in your main program you just call */
//	FILE* arq; /* the bitmap file 24 bits */
/*	RGB** Matrix_aux, Matrix;
	INFOHEADER info;
	info = readInfo(FILE* arq);
	height = info.height;
	width = info.width;
	 
	Matrix_aux = createMatrix();
	Matrix = loadImage(arq,Matrix_aux);
*/


main(){


	FILE *arq; 
	unsigned char  **Matrix;
	INFOHEADER info;
	HEADER head;
	char name[15];


	printf("Type the image's name : ");
	scanf("%s",name);


	arq = exist(name);
	isBMP(arq);
	info = readInfo(arq);






	Matrix = createMatrix(info);
	loadImage(arq,info,Matrix);
	writeBMP(Matrix,head,info,arq);
	freeMatrix(Matrix,info);
}