Geografic-Information-System/imagery/i.modis.qc/mod09A1a.c

/* MODLAND QA Bits 500m long int bits[0-1]
 * 00 -> class 0: Corrected product produced at ideal quality -- all bands
 * 01 -> class 1: Corrected product produced at less than idel quality -- some or all bands
 * 10 -> class 2: Corrected product NOT produced due to cloud effect -- all bands
 * 11 -> class 3: Corrected product NOT produced due to other reasons -- some or all bands mayb be fill value (Note that a value of [11] overrides a value of [01])
 */  

#include <grass/raster.h>

CELL mod09A1a(CELL pixel) 
{
    CELL qctemp;

    /* Select bit 0 and 1 (right-side).
     * hexadecimal "0x03" => binary "11" 
     * this will set all other bits to null */
    qctemp = pixel & 0x03;

    return qctemp;
}