CODEWITHSUNDEEP
cbinarybinaryfilesmonochrome
user2967899
user2967899

Reputation: 85

image data of bmp in C

How to convert a monochrome bmp image file (in my case 16*16 pixels) into binary format? This code reads the bitmap information. I have to store the pixel information into an array & it's not stored properly. I have shared the code

#pragma pack(push, 1)

typedef struct BitMap
   {
short Signature;
long Reserved1;
long Reserved2;
long DataOffSet;
long Size;
long Width;
long Height;
short Planes;
short BitsPerPixel;
long Compression;
long SizeImage;
long XPixelsPreMeter;
long YPixelsPreMeter;
long ColorsUsed;
long ColorsImportant;
long data[16];
}BitMap;
#pragma pack(pop)

reading image file:

struct BitMap source_info;
struct Pix source_pix;

FILE *fp;
FILE *Dfp;
Dfp=fopen("filename.bin","wb")

if(!(fp=fopen("filename.bmp","rb")))
 {
    printf(" can not open file");
    exit(-1);
 }

fread(&source_info, sizeof(source_info),1,fp);
printf("%d\n",source_info.DataOffSet);
printf("%d\n",source_info.Width*source_info.Height);
for(i=0;i<16;i++)
fprintf(Dfp,"%d\t",source_info.data[i]);

Observed output using hex editor is hex output of monochrome bmp image

Highlighted data i want to get stored in data array so that i can use it further in the code.

However output in filename.bin is

0  16777215 63 63 63 95 95 95
31 31       31 31 31 31 31 31

I'm new to this field. Can someone help me out where i'm going wrong?

Upvotes: 2

Views: 3028

Answers (1)

starrify
starrify

Reputation: 14731

There's actually no problem with the data.
The problem is you're using the wrong way to print them.

Try replacing your code:

printf("%d\n",source_info.DataOffSet);
printf("%d\n",source_info.Width*source_info.Height);
for(i=0;i<16;i++)
    fprintf(Dfp,"%d\t",source_info.data[i]);

with this:

printf("%x\n",source_info.DataOffSet);
printf("%x\n",source_info.Width*source_info.Height);
for(i=0;i<16;i++)
    fprintf(Dfp,"%x\t",source_info.data[i]);

As %d is for signed decimals while %x is for hexadecimals. See the section of The conversion specifier in the manual page of printf

EDITED:

As you've posted your new questions in the comments:

output in hex is 0x00 0xffffff 0x3f 0x3f 0x3f 0x5f 0x5f 0x5f 0x1f 0x1f 0x1f 0x1f 0x1f 0x1f 0x1f 0x1f Can u explain how the output is getting stored? I'm unable to get the same output – user2967899 7 mins ago

here's my edited answer.

Assumptions: your working platform is just as normal, on which size of short is 2 bytes and of long it's 4.
From definition of struct BitMap we know the field data is at its offset of 0x36. Comparing of the image we know the data shall be (in hexadecimal):

data[0]: 0000 0000
data[1]: ffff ff00
......

Then the result you got seems strange since data[1] is 0x00ffffffff instead of 0xffffff00. However that's correct. This is cause by endianess, for which please read this wiki page first: http://en.wikipedia.org/wiki/Endianness
As the hex-editor represents data in the real order of bytes, and I assume you're working with a little-endian machine (which most PC on this planet has), this order is just reversed of the real order of your data in long:

/* data in C */
unsigned long x = 305419896; /* 305419896 == 0x12345678 */

/* arithmetically the four bytes in x: */
/* 0x12 0x34 0x56 0x78 */

/* the real order to be observed in a hex-editor due to endianess: */
/* 0x78 0x56 0x34 0x12 */

/* so this holds true in C: */
unsigned char *a = &x;
assert(a[0] == 0x78);
assert(a[1] == 0x56);
assert(a[2] == 0x34);
assert(a[3] == 0x12);

Upvotes: 5

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