/*
 * Copyright 2011-2013 Blender Foundation
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License
 */

CCL_NAMESPACE_BEGIN

#ifdef __SHADOW_RECORD_ALL__

/* Shadow function to compute how much light is blocked, CPU variation.
 *
 * We trace a single ray. If it hits any opaque surface, or more than a given
 * number of transparent surfaces is hit, then we consider the geometry to be
 * entirely blocked. If not, all transparent surfaces will be recorded and we
 * will shade them one by one to determine how much light is blocked. This all
 * happens in one scene intersection function.
 *
 * Recording all hits works well in some cases but may be slower in others. If
 * we have many semi-transparent hairs, one intersection may be faster because
 * you'd be reinteresecting the same hairs a lot with each step otherwise. If
 * however there is mostly binary transparency then we may be recording many
 * unnecessary intersections when one of the first surfaces blocks all light.
 *
 * From tests in real scenes it seems the performance loss is either minimal,
 * or there is a performance increase anyway due to avoiding the need to send
 * two rays with transparent shadows.
 *
 * This is CPU only because of qsort, and malloc or high stack space usage to
 * record all these intersections. */

ccl_device_noinline int shadow_intersections_compare(const void *a, const void *b)
{
	const Intersection *isect_a = (const Intersection*)a;
	const Intersection *isect_b = (const Intersection*)b;

	if(isect_a->t < isect_b->t)
		return -1;
	else if(isect_a->t > isect_b->t)
		return 1;
	else
		return 0;
}

#define STACK_MAX_HITS 64

ccl_device_inline bool shadow_blocked(KernelGlobals *kg, PathState *state, Ray *ray, float3 *shadow)
{
	*shadow = make_float3(1.0f, 1.0f, 1.0f);

	if(ray->t == 0.0f)
		return false;
	
	bool blocked;

	if(kernel_data.integrator.transparent_shadows) {
		/* check transparent bounces here, for volume scatter which can do
		 * lighting before surface path termination is checked */
		if(state->transparent_bounce >= kernel_data.integrator.transparent_max_bounce)
			return true;

		/* intersect to find an opaque surface, or record all transparent surface hits */
		Intersection hits_stack[STACK_MAX_HITS];
		Intersection *hits = hits_stack;
		uint max_hits = kernel_data.integrator.transparent_max_bounce - state->transparent_bounce - 1;

		/* prefer to use stack but use dynamic allocation if too deep max hits
		 * we need max_hits + 1 storage space due to the logic in
		 * scene_intersect_shadow_all which will first store and then check if
		 * the limit is exceeded */
		if(max_hits + 1 > STACK_MAX_HITS)
			hits = (Intersection*)malloc(sizeof(Intersection)*(max_hits + 1));

		uint num_hits;
		blocked = scene_intersect_shadow_all(kg, ray, hits, max_hits, &num_hits);

		/* if no opaque surface found but we did find transparent hits, shade them */
		if(!blocked && num_hits > 0) {
			float3 throughput = make_float3(1.0f, 1.0f, 1.0f);
			float3 Pend = ray->P + ray->D*ray->t;
			float last_t = 0.0f;
			int bounce = state->transparent_bounce;
			Intersection *isect = hits;
#ifdef __VOLUME__
			PathState ps = *state;
#endif

			qsort(hits, num_hits, sizeof(Intersection), shadow_intersections_compare);

			for(int hit = 0; hit < num_hits; hit++, isect++) {
				/* adjust intersection distance for moving ray forward */
				float new_t = isect->t;
				isect->t -= last_t;

				/* skip hit if we did not move forward, step by step raytracing
				 * would have skipped it as well then */
				if(last_t == new_t)
					continue;

				last_t = new_t;

#ifdef __VOLUME__
				/* attenuation between last surface and next surface */
				if(ps.volume_stack[0].shader != SHADER_NONE) {
					Ray segment_ray = *ray;
					segment_ray.t = isect->t;
					kernel_volume_shadow(kg, &ps, &segment_ray, &throughput);
				}
#endif

				/* setup shader data at surface */
				ShaderData sd;
				shader_setup_from_ray(kg, &sd, isect, ray, state->bounce+1, bounce);

				/* attenuation from transparent surface */
				if(!(sd.flag & SD_HAS_ONLY_VOLUME)) {
					shader_eval_surface(kg, &sd, 0.0f, PATH_RAY_SHADOW, SHADER_CONTEXT_SHADOW);
					throughput *= shader_bsdf_transparency(kg, &sd);
				}

				/* stop if all light is blocked */
				if(is_zero(throughput)) {
					/* free dynamic storage */
					if(hits != hits_stack)
						free(hits);
					return true;
				}

				/* move ray forward */
				ray->P = sd.P;
				if(ray->t != FLT_MAX)
					ray->D = normalize_len(Pend - ray->P, &ray->t);

#ifdef __VOLUME__
				/* exit/enter volume */
				kernel_volume_stack_enter_exit(kg, &sd, ps.volume_stack);
#endif

				bounce++;
			}

#ifdef __VOLUME__
			/* attenuation for last line segment towards light */
			if(ps.volume_stack[0].shader != SHADER_NONE)
				kernel_volume_shadow(kg, &ps, ray, &throughput);
#endif

			*shadow = throughput;

			if(hits != hits_stack)
				free(hits);
			return is_zero(throughput);
		}

		/* free dynamic storage */
		if(hits != hits_stack)
			free(hits);
	}
	else {
		Intersection isect;
		blocked = scene_intersect(kg, ray, PATH_RAY_SHADOW_OPAQUE, &isect, NULL, 0.0f, 0.0f);
	}

#ifdef __VOLUME__
	if(!blocked && state->volume_stack[0].shader != SHADER_NONE) {
		/* apply attenuation from current volume shader */
		kernel_volume_shadow(kg, state, ray, shadow);
	}
#endif

	return blocked;
}

#undef STACK_MAX_HITS

#else

/* Shadow function to compute how much light is blocked, GPU variation.
 *
 * Here we raytrace from one transparent surface to the next step by step.
 * To minimize overhead in cases where we don't need transparent shadows, we
 * first trace a regular shadow ray. We check if the hit primitive was
 * potentially transparent, and only in that case start marching. this gives
 * one extra ray cast for the cases were we do want transparency. */

ccl_device_inline bool shadow_blocked(KernelGlobals *kg, PathState *state, Ray *ray, float3 *shadow)
{
	*shadow = make_float3(1.0f, 1.0f, 1.0f);

	if(ray->t == 0.0f)
		return false;

	Intersection isect;
	bool blocked = scene_intersect(kg, ray, PATH_RAY_SHADOW_OPAQUE, &isect, NULL, 0.0f, 0.0f);

#ifdef __TRANSPARENT_SHADOWS__
	if(blocked && kernel_data.integrator.transparent_shadows) {
		if(shader_transparent_shadow(kg, &isect)) {
			float3 throughput = make_float3(1.0f, 1.0f, 1.0f);
			float3 Pend = ray->P + ray->D*ray->t;
			int bounce = state->transparent_bounce;
#ifdef __VOLUME__
			PathState ps = *state;
#endif

			for(;;) {
				if(bounce >= kernel_data.integrator.transparent_max_bounce)
					return true;

				if(!scene_intersect(kg, ray, PATH_RAY_SHADOW_TRANSPARENT, &isect, NULL, 0.0f, 0.0f))
				{

#ifdef __VOLUME__
					/* attenuation for last line segment towards light */
					if(ps.volume_stack[0].shader != SHADER_NONE)
						kernel_volume_shadow(kg, &ps, ray, &throughput);
#endif

					*shadow *= throughput;

					return false;
				}

				if(!shader_transparent_shadow(kg, &isect))
					return true;

#ifdef __VOLUME__
				/* attenuation between last surface and next surface */
				if(ps.volume_stack[0].shader != SHADER_NONE) {
					Ray segment_ray = *ray;
					segment_ray.t = isect.t;
					kernel_volume_shadow(kg, &ps, &segment_ray, &throughput);
				}
#endif

				/* setup shader data at surface */
				ShaderData sd;
				shader_setup_from_ray(kg, &sd, &isect, ray, state->bounce+1, bounce);

				/* attenuation from transparent surface */
				if(!(sd.flag & SD_HAS_ONLY_VOLUME)) {
					shader_eval_surface(kg, &sd, 0.0f, PATH_RAY_SHADOW, SHADER_CONTEXT_SHADOW);
					throughput *= shader_bsdf_transparency(kg, &sd);
				}

				if(is_zero(throughput))
					return true;

				/* move ray forward */
				ray->P = ray_offset(sd.P, -sd.Ng);
				if(ray->t != FLT_MAX)
					ray->D = normalize_len(Pend - ray->P, &ray->t);

#ifdef __VOLUME__
				/* exit/enter volume */
				kernel_volume_stack_enter_exit(kg, &sd, ps.volume_stack);
#endif

				bounce++;
			}
		}
	}
#ifdef __VOLUME__
	else if(!blocked && state->volume_stack[0].shader != SHADER_NONE) {
		/* apply attenuation from current volume shader */
		kernel_volume_shadow(kg, state, ray, shadow);
	}
#endif
#endif

	return blocked;
}

#endif

CCL_NAMESPACE_END

