blender/intern/cycles/kernel/kernel_random.h

/*
 * Copyright 2011, Blender Foundation.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 */

CCL_NAMESPACE_BEGIN

typedef uint RNG;

#ifdef __SOBOL__

/* skip initial numbers that are not as well distributed, especially the
   first sequence is just 0 everywhere, which can be problematic for e.g.
   path termination */
#define SOBOL_SKIP 64

/* High Dimensional Sobol */

/* van der corput radical inverse */
__device uint van_der_corput(uint bits)
{
	bits = (bits << 16) | (bits >> 16);
	bits = ((bits & 0x00ff00ff) << 8) | ((bits & 0xff00ff00) >> 8);
	bits = ((bits & 0x0f0f0f0f) << 4) | ((bits & 0xf0f0f0f0) >> 4);
	bits = ((bits & 0x33333333) << 2) | ((bits & 0xcccccccc) >> 2);
	bits = ((bits & 0x55555555) << 1) | ((bits & 0xaaaaaaaa) >> 1);
	return bits;
}

/* sobol radical inverse */
__device uint sobol(uint i)
{
	uint r = 0;

	for(uint v = 1U << 31; i; i >>= 1, v ^= v >> 1)
		if(i & 1)
			r ^= v;

	return r;
}

/* inverse of sobol radical inverse */
__device uint sobol_inverse(uint i)
{
	const uint msb = 1U << 31;
	uint r = 0;

	for(uint v = 1; i; i <<= 1, v ^= v << 1)
		if(i & msb)
			r ^= v;

	return r;
}

/* multidimensional sobol with generator matrices
   dimension 0 and 1 are equal to van_der_corput() and sobol() respectively */
__device uint sobol_dimension(KernelGlobals *kg, int index, int dimension)
{
	uint result = 0;
	uint i = index;

	for(uint j = 0; i; i >>= 1, j++)
		if(i & 1)
			result ^= kernel_tex_fetch(__sobol_directions, 32*dimension + j);
	
	return result;
}

/* lookup index and x/y coordinate, assumes m is a power of two */
__device uint sobol_lookup(const uint m, const uint frame, const uint ex, const uint ey, uint *x, uint *y)
{
	/* shift is constant per frame */
	const uint shift = frame << (m << 1);
	const uint sobol_shift = sobol(shift);
	/* van der Corput is its own inverse */
	const uint lower = van_der_corput(ex << (32 - m));
	/* need to compensate for ey difference and shift */
	const uint sobol_lower = sobol(lower);
	const uint mask = ~-(1 << m) << (32 - m); /* only m upper bits */
	const uint delta = ((ey << (32 - m)) ^ sobol_lower ^ sobol_shift) & mask;
	/* only use m upper bits for the index (m is a power of two) */
	const uint sobol_result = delta | (delta >> m);
	const uint upper = sobol_inverse(sobol_result);
	const uint index = shift | upper | lower;
	*x = van_der_corput(index);
	*y = sobol_shift ^ sobol_result ^ sobol_lower;
	return index;
}

__device_inline float path_rng(KernelGlobals *kg, RNG *rng, int sample, int dimension)
{
#ifdef __SOBOL_FULL_SCREEN__
	uint result = sobol_dimension(kg, *rng, dimension);
	float r = (float)result * (1.0f/(float)0xFFFFFFFF);
	return r;
#else
	/* compute sobol sequence value using direction vectors */
	uint result = sobol_dimension(kg, sample + SOBOL_SKIP, dimension);
	float r = (float)result * (1.0f/(float)0xFFFFFFFF);

	/* Cranly-Patterson rotation using rng seed */
	float shift;

	if(dimension & 1)
		shift = (*rng >> 16)/((float)0xFFFF);
	else
		shift = (*rng & 0xFFFF)/((float)0xFFFF);

	return r + shift - floor(r + shift);
#endif
}

__device_inline void path_rng_init(KernelGlobals *kg, __global uint *rng_state, int sample, RNG *rng, int x, int y, float *fx, float *fy)
{
#ifdef __SOBOL_FULL_SCREEN__
	uint px, py;
	uint bits = 16; /* limits us to 65536x65536 and 65536 samples */
	uint size = 1 << bits;
	uint frame = sample;

	*rng = sobol_lookup(bits, frame, x, y, &px, &py);

	*rng ^= kernel_data.integrator.seed;

	if(sample == 0) {
		*fx = 0.5f;
		*fy = 0.5f;
	}
	else {
		*fx = size * (float)px * (1.0f/(float)0xFFFFFFFF) - x;
		*fy = size * (float)py * (1.0f/(float)0xFFFFFFFF) - y;
	}
#else
	*rng = *rng_state;

	*rng ^= kernel_data.integrator.seed;

	if(sample == 0) {
		*fx = 0.5f;
		*fy = 0.5f;
	}
	else {
		*fx = path_rng(kg, rng, sample, PRNG_FILTER_U);
		*fy = path_rng(kg, rng, sample, PRNG_FILTER_V);
	}
#endif
}

__device void path_rng_end(KernelGlobals *kg, __global uint *rng_state, RNG rng)
{
	/* nothing to do */
}

#else

/* Linear Congruential Generator */

__device float path_rng(KernelGlobals *kg, RNG *rng, int sample, int dimension)
{
	/* implicit mod 2^32 */
	*rng = (1103515245*(*rng) + 12345);
	return (float)*rng * (1.0f/(float)0xFFFFFFFF);
}

__device void path_rng_init(KernelGlobals *kg, __global uint *rng_state, int sample, RNG *rng, int x, int y, float *fx, float *fy)
{
	/* load state */
	*rng = *rng_state;

	*rng ^= kernel_data.integrator.seed;

	if(sample == 0) {
		*fx = 0.5f;
		*fy = 0.5f;
	}
	else {
		*fx = path_rng(kg, rng, sample, PRNG_FILTER_U);
		*fy = path_rng(kg, rng, sample, PRNG_FILTER_V);
	}
}

__device void path_rng_end(KernelGlobals *kg, __global uint *rng_state, RNG rng)
{
	/* store state for next sample */
	*rng_state = rng;
}

#endif

CCL_NAMESPACE_END