Convert NSData to primitive variable with ieee-754 or twos-complement ?

Posted by William GILLARD on Stack Overflow See other posts from Stack Overflow or by William GILLARD
Published on 2010-06-11T20:44:33Z Indexed on 2010/06/11 21:12 UTC
Read the original article Hit count: 520

Hi every one.

I am new programmer in Obj-C and cocoa. Im a trying to write a framework which will be used to read a binary files (Flexible Image Transport System or FITS binary files, usually used by astronomers). The binary data, that I am interested to extract, can have various formats and I get its properties by reading the header of the FITS file.

Up to now, I manage to create a class to store the content of the FITS file and to isolate the header into a NSString object and the binary data into a NSData object. I also manage to write method which allow me to extract the key values from the header that are very valuable to interpret the binary data.

I am now trying to convert the NSData object into a primitive array (array of double, int, short ...). But, here, I get stuck and would appreciate any help.

According to the documentation I have about the FITS file, I have 5 possibilities to interpret the binary data depending on the value of the BITPIX key:

BITPIX value | Data represented
  8          | Char or unsigned binary int
 16          | 16-bit two's complement binary integer
 32          | 32-bit two's complement binary integer
 64          | 64-bit two's complement binary integer
-32          | IEEE single precision floating-point
-64          | IEEE double precision floating-point

I already write the peace of code, shown bellow, to try to convert the NSData into a primitive array.

// self reefer to my FITS class which contain a NSString object  
// with the content of the header and a NSData object with the binary data. 

-(void*) GetArray
switch (BITPIX)
    case 8:
        return [self GetArrayOfUInt];
    case 16:
        return [self GetArrayOfInt];
    case 32:
        return [self GetArrayOfLongInt];
    case 64:
        return [self GetArrayOfLongLong];
    case -32:
        return [self GetArrayOfFloat];
    case -64:
        return [self GetArrayOfDouble];
        return NULL;

// then I show you the method to convert the NSData into a primitive array.
// I restrict my example to the case of 'double'. Code is similar for other methods
// just change double by 'unsigned int' (BITPIX 8), 'short' (BITPIX 16)
// 'int' (BITPIX 32) 'long lon' (BITPIX 64), 'float' (BITPIX -32). 

-(double*) GetArrayOfDouble
int Nelements=[self NPIXEL]; // Metod to extract, from the header 
                             // the number of element into the array
NSLog(@"TOTAL NUMBER OF ELEMENTS [%i]\n",Nelements);

double (*array)[Nelements];

// Get the total number of bits in the binary data
int Nbit = abs(BITPIX)*GCOUNT*(PCOUNT + Nelements); // GCOUNT and PCOUNT are defined
                                                        // into the header
NSLog(@"TOTAL NUMBER OF BIT [%i]\n",Nbit);
int i=0;

double Value;

for(int bit=0; bit < Nbit; bit+=sizeof(double))
    [Img getBytes:&Value range:NSMakeRange(bit,sizeof(double))];


return (*array);


However, the value I print in the loop are very different from the expected values (compared using official FITS software). Therefore, I think that the Obj-C double does not use the IEEE-754 convention as well as the Obj-C int are not twos-complement. I am really not familiar with this two convention (IEEE and twos-complement) and would like to know how I can do this conversion with Obj-C.

In advance many thanks for any help or information.

© Stack Overflow or respective owner

Related posts about objective-c

Related posts about cocoa