Image read in MATLAB and C

I have program to read an image in MATLAB

% open image file
  fd = fopen("i00.dng");
  % read order
  order = fread(fd, 1, 'uint16');
  

I converted this to C as follows:

int order[1234];
FILE *fd=fopen("i00.dng","wb");
fread(order,2,1,fd);
printf("%d",order);	

But the value of order in both cases is different. Did I convert it wrong?What is the exact replica of this code in C?

Answer 1

[James Tursa]

An int is likely 4 bytes on your machine, not 2 bytes. Also the expression "order" by itself evaluates as a pointer to the first element, not the first element itself. And you should be opening the file for reading, not writing. So,

unsigned short order[1234];
FILE *fd = fopen("i00.dng","rb"); /* you should check that this result is not NULL */
fread(order,2,1,fd);
printf("%u\n",order[0]);

This is almost a replica of the MATLAB code. Your MATLAB code converts the input to double. If you don't want that, then use an asterisk on the class name:

order = fread(fd, 1, '*uint16');

[Pravitha A]

This 'order' is to check whether the file is big endian supported. So this 'order' value is compared with hex2dec('4949')

if ~(order == hex2dec('4949'))   
    disp('Error: big endian is not supported');
  end

Why this number 4949. Is it applicable to all cases??Because when i tried your code, I got the value '4992'.

[James Tursa]

Can you post the exact wording of the doc where it is describing this '4949' number so I know for sure what we are comparing to?

[Pravitha A]

function [I idescr] = dng_read(fname)
% open image file
  fd = fopen(fname);
  % read order
  order = fread(fd, 1, 'uint16');
  if ~(order == hex2dec('4949'))   
    disp('Error: big endian is not supported');
  end
  % skip 2 bytes
  fread(fd, 1, 'uint16');
  .
  .
  .
  .

This is a funtion to read an image file in dng format. The section where the above mentioned is described directly after the file is read. I don't have any document except the code regarding this. Thanks in advance

[Walter Roberson]

If you want big endian then you need to pass in the third parameter to fopen as 'b' or 'ieee-be'. Or you can instead pass that information as the fifth parameter to fread(). Or you can swapbytes() the value that you fread.

[Pravitha A]

like this??

fd=fopen("filename","rb",'ieee-be')