user2985363 user2985363 - 1 month ago 12
C Question

Trouble reading .bmp header file properly

I am trying to take in a .bmp file and eventually edit the pixels one by one but I am coming up with a problem with the width and height returned to me in the INFOHEADER struct. The width returned is 13107200 and the height is 65536. However, whenever I run through the program a total of only 60003 total pixels are counted. I have no idea why this is. Any help would be greatly appreciated.

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

int main( int argc, char *argv[] ){
//define structures
typedef struct
{ unsigned short int Type; /* Magic identifier */
unsigned int Size; /* File size in bytes */
unsigned short int Reserved1, Reserved2;
unsigned int Offset; /* Offset to data (in B)*/
}HEADER; /* -- 14 Bytes -- */

typedef struct
{ unsigned int Size; /* Header size in bytes */
int Width, Height; /* Width / Height of image */
unsigned short int Planes; /* Number of colour planes */
unsigned short int Bits; /* Bits per pixel */
unsigned int Compression; /* Compression type */
unsigned int ImageSize; /* Image size in bytes */
int xResolution, yResolution;/* Pixels per meter */
unsigned int Colors; /* Number of colors */
unsigned int ImportantColors;/* Important colors */
}INFOHEADER; /* -- 40 Bytes -- */

typedef struct
{ unsigned char Red, Green, Blue;
}PIXEL;

//make instance of all three structures
HEADER data;
INFOHEADER data2;
PIXEL pixel;

//declare file read pointer
FILE *file;

//declare fileout read pointer
//FILE *fileout; //declare file printed file pointer

// open file 1 of argument counter and return 0 apon error
if( !(file = fopen( "CU.bmp","rb")))return 0;
//read HEADER data into data
fread(&data,sizeof(HEADER),1,file);
//read IB+NFOHEADER data into data2
fread(&data2,sizeof(INFOHEADER),1,file);
//Print PIXEL data

//Allocate space for pixelarray
PIXEL **pixelarray;
int r=0,c=0,rows=data2.Height,collumns=data2.Width;
pixelarray= malloc(rows*sizeof(PIXEL *));
for(r=0; r<rows; r++){
pixelarray[r]=malloc(collumns*sizeof(PIXEL));
}

//fill pixel array with pixel structs
r=0;c=0;
int pixelnum=1;
while( fread(&pixel,sizeof(PIXEL),1,file) ){
if(c == collumns){
c=0;
r++;
}
pixelarray[r][c] = pixel;
printf("\nPixel %10d: %02X%02X%02X",pixelnum,pixelarray[r][c].Red,pixelarray[r][c].Blue,pixelarray[r][c].Green);
fflush(stdout);
c++;pixelnum++;

}

free(pixelarray);

fclose(file); //close the files prior to exiting

Answer Source

I guess your problem is structure alignment. You can refer to it here and here. To eliminate it use the #pragma directive. So your structure declaration would be something like this:

#pragma pack(push)  // push current alignment to stack
#pragma pack(1)     // set alignment to 1 byte boundary
typedef struct
{  
    unsigned short int Type; /* Magic identifier */
    unsigned int Size; /* File size in bytes */
    unsigned short int Reserved1;
    unsigned short int Reserved2;
    unsigned int Offset; /* Offset to data (in B)*/
}HEADER; /* -- 14 Bytes -- */

typedef struct
{
    unsigned int Size; /* Header size in bytes */
    int Width;
    int Height; /* Width / Height of image */
    unsigned short int Planes; /* Number of colour planes */
    unsigned short int Bits; /* Bits per pixel */
    unsigned int Compression; /* Compression type */
    unsigned int ImageSize; /* Image size in bytes */
    int xResolution;
    int yResolution;/* Pixels per meter */
    unsigned int Colors; /* Number of colors */
    unsigned int ImportantColors;/* Important colors */
}INFOHEADER; /* -- 40 Bytes -- */

typedef struct
{
    unsigned char Red;
    unsigned char Green;
    unsigned char Blue;
}PIXEL;
#pragma pack(pop)   // restore original alignment from stack

This correctly reads the width and height of BMP image. Further while reading image data, do it directly:

for( r=0; r<rows; r++ )
{
    for( c=0; c<collumns; c++ )     // read pixel data from image
    {
        fread(&pixelarray[r][c] , 1, sizeof(PIXEL), file);
        pixelnum++;
    }
}