Unofficial PBM format for HDR images, PFM (Portable Float Map)

PFM (Portable Float Map) is an unofficial extension to the pbm image format collection that supports HDR imaging, namely a floating point value per r,g,b [or a single floating point value for grey scale HDR images].

Identifier

The identifier for a PBM HDR image is either "PF" or "Pf" depending on whether it is r,g,b or grey scale.

Dimensions

The next three values correspond to the width, height, and aspect ratio of the pixels. These can be on one, two or three lines ... there seems to be no agreed standard except that there is a newline character at the end of the third value. The aspect ratio is floating point (usually 1) but it also encodes the endian status of the following floating point binary numbers. A negative aspect ratio indicates little endian, otherwise big endian. So depending on the endian of the machine being used to read the file a byte may ormay need to be performed.

Pixel data

The pixel data is stored as width * height floating point r,g,b or single grey scale values, encoded as IEEE floating points values with the specified endian status.

typedef struct {
   float r,g,b;
} HDRPIXEL;

/*
   Read a possibly byte swapped floating point number
   Assume IEEE format
*/
int ReadFloat(FILE *fptr,float *n,int swap)
{
   unsigned char *cptr,tmp;

   if (fread(n,4,1,fptr) != 1)
      return(FALSE);
   if (swap) {
      cptr = (unsigned char *)n;
      tmp = cptr[0];
      cptr[0] = cptr[3];
      cptr[3] =tmp;
      tmp = cptr[1];
      cptr[1] = cptr[2];
      cptr[2] = tmp;
   }
   return(TRUE);
}