static const uint32_t projectileCategory     =  0x1 << 1;

static const uint32_t projectileCategory     =  0x2;

static const uint32_t projectileCategory     =  0x1 << 1;// bit 1
static const uint32_t doodadCategory         =  0x1 << 2;// bit 2
static const uint32_t playerCategory         =  0x1 << 3;// bit 3
static const uint32_t doorCategory           =  0x1 << 4;// bit 4

0x1: 00000001
0x2: 00000010   <- product of 0x1 << 1
0x4: 00000100   <- product of 0x1 << 2

(0x1 << 1)

0x02