forked from bartvdbraak/blender
Brecht Van Lommel
01df756bd1
This is done by adding a Volume Scatter node. In many cases you will want to add together a Volume Absorption and Volume Scatter node with the same color and density to get the expected results. This should work with branched path tracing, mixing closures, overlapping volumes, etc. However there's still various optimizations needed for sampling. The main missing thing from the volume branch is the equiangular sampling for homogeneous volumes. The heterogeneous scattering code was arranged such that we can use a single stratified random number for distance sampling, which gives less noise than pseudo random numbers for each step. For volumes where the color is textured there still seems to be something off, needs to be investigated.
561 lines
17 KiB
C
561 lines
17 KiB
C
/*
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* Copyright 2011-2013 Blender Foundation
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License
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*/
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CCL_NAMESPACE_BEGIN
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typedef enum VolumeIntegrateResult {
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VOLUME_PATH_SCATTERED = 0,
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VOLUME_PATH_ATTENUATED = 1,
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VOLUME_PATH_MISSED = 2
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} VolumeIntegrateResult;
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/* Volume shader properties
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*
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* extinction coefficient = absorption coefficient + scattering coefficient
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* sigma_t = sigma_a + sigma_s */
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typedef struct VolumeShaderCoefficients {
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float3 sigma_a;
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float3 sigma_s;
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float3 emission;
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} VolumeShaderCoefficients;
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/* evaluate shader to get extinction coefficient at P */
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ccl_device bool volume_shader_extinction_sample(KernelGlobals *kg, ShaderData *sd, PathState *state, float3 P, float3 *extinction)
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{
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sd->P = P;
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shader_eval_volume(kg, sd, state->volume_stack, PATH_RAY_SHADOW, SHADER_CONTEXT_SHADOW);
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if(!(sd->flag & (SD_ABSORPTION|SD_SCATTER)))
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return false;
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float3 sigma_t = make_float3(0.0f, 0.0f, 0.0f);
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for(int i = 0; i < sd->num_closure; i++) {
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const ShaderClosure *sc = &sd->closure[i];
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if(CLOSURE_IS_VOLUME(sc->type))
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sigma_t += sc->weight;
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}
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*extinction = sigma_t;
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return true;
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}
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/* evaluate shader to get absorption, scattering and emission at P */
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ccl_device bool volume_shader_sample(KernelGlobals *kg, ShaderData *sd, PathState *state, float3 P, VolumeShaderCoefficients *coeff)
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{
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sd->P = P;
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shader_eval_volume(kg, sd, state->volume_stack, state->flag, SHADER_CONTEXT_VOLUME);
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if(!(sd->flag & (SD_ABSORPTION|SD_SCATTER|SD_EMISSION)))
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return false;
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coeff->sigma_a = make_float3(0.0f, 0.0f, 0.0f);
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coeff->sigma_s = make_float3(0.0f, 0.0f, 0.0f);
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coeff->emission = make_float3(0.0f, 0.0f, 0.0f);
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for(int i = 0; i < sd->num_closure; i++) {
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const ShaderClosure *sc = &sd->closure[i];
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if(sc->type == CLOSURE_VOLUME_ABSORPTION_ID)
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coeff->sigma_a += sc->weight;
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else if(sc->type == CLOSURE_EMISSION_ID)
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coeff->emission += sc->weight;
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else if(CLOSURE_IS_VOLUME(sc->type))
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coeff->sigma_s += sc->weight;
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}
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/* when at the max number of bounces, treat scattering as absorption */
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if(sd->flag & SD_SCATTER) {
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if(state->volume_bounce >= kernel_data.integrator.max_volume_bounce) {
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coeff->sigma_a += coeff->sigma_s;
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coeff->sigma_s = make_float3(0.0f, 0.0f, 0.0f);
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sd->flag &= ~SD_SCATTER;
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sd->flag |= SD_ABSORPTION;
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}
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}
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return true;
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}
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ccl_device float3 volume_color_attenuation(float3 sigma, float t)
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{
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return make_float3(expf(-sigma.x * t), expf(-sigma.y * t), expf(-sigma.z * t));
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}
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ccl_device bool volume_stack_is_heterogeneous(KernelGlobals *kg, VolumeStack *stack)
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{
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for(int i = 0; stack[i].shader != SHADER_NO_ID; i++) {
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int shader_flag = kernel_tex_fetch(__shader_flag, (stack[i].shader & SHADER_MASK)*2);
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if(shader_flag & SD_HETEROGENEOUS_VOLUME)
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return true;
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}
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return false;
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}
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/* Volume Shadows
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*
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* These functions are used to attenuate shadow rays to lights. Both absorption
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* and scattering will block light, represented by the extinction coefficient. */
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/* homogenous volume: assume shader evaluation at the starts gives
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* the extinction coefficient for the entire line segment */
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ccl_device void kernel_volume_shadow_homogeneous(KernelGlobals *kg, PathState *state, Ray *ray, ShaderData *sd, float3 *throughput)
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{
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float3 sigma_t;
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if(volume_shader_extinction_sample(kg, sd, state, ray->P, &sigma_t))
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*throughput *= volume_color_attenuation(sigma_t, ray->t);
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}
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/* heterogeneous volume: integrate stepping through the volume until we
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* reach the end, get absorbed entirely, or run out of iterations */
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ccl_device void kernel_volume_shadow_heterogeneous(KernelGlobals *kg, PathState *state, Ray *ray, ShaderData *sd, float3 *throughput)
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{
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float3 tp = *throughput;
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const float tp_eps = 1e-10f; /* todo: this is likely not the right value */
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/* prepare for stepping */
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int max_steps = kernel_data.integrator.volume_max_steps;
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float step = kernel_data.integrator.volume_step_size;
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float random_jitter_offset = lcg_step_float(&state->rng_congruential) * step;
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/* compute extinction at the start */
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float t = 0.0f;
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float3 P = ray->P;
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float3 sigma_t;
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if(!volume_shader_extinction_sample(kg, sd, state, P, &sigma_t))
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sigma_t = make_float3(0.0f, 0.0f, 0.0f);
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for(int i = 0; i < max_steps; i++) {
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/* advance to new position */
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float new_t = min(ray->t, t + random_jitter_offset + i * step);
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float3 new_P = ray->P + ray->D * new_t;
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float3 new_sigma_t;
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/* compute attenuation over segment */
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if(volume_shader_extinction_sample(kg, sd, state, new_P, &new_sigma_t)) {
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/* todo: we could avoid computing expf() for each step by summing,
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* because exp(a)*exp(b) = exp(a+b), but we still want a quick
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* tp_eps check too */
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tp *= volume_color_attenuation(0.5f*(sigma_t + new_sigma_t), new_t - t);
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/* stop if nearly all light blocked */
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if(tp.x < tp_eps && tp.y < tp_eps && tp.z < tp_eps)
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break;
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sigma_t = new_sigma_t;
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}
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else {
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/* skip empty space */
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sigma_t = make_float3(0.0f, 0.0f, 0.0f);
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}
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/* stop if at the end of the volume */
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t = new_t;
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if(t == ray->t)
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break;
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}
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*throughput = tp;
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}
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/* get the volume attenuation over line segment defined by ray, with the
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* assumption that there are no surfaces blocking light between the endpoints */
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ccl_device_noinline void kernel_volume_shadow(KernelGlobals *kg, PathState *state, Ray *ray, float3 *throughput)
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{
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ShaderData sd;
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shader_setup_from_volume(kg, &sd, ray, state->bounce);
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if(volume_stack_is_heterogeneous(kg, state->volume_stack))
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kernel_volume_shadow_heterogeneous(kg, state, ray, &sd, throughput);
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else
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kernel_volume_shadow_homogeneous(kg, state, ray, &sd, throughput);
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}
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/* Volume Path */
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/* homogenous volume: assume shader evaluation at the starts gives
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* the volume shading coefficient for the entire line segment */
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ccl_device VolumeIntegrateResult kernel_volume_integrate_homogeneous(KernelGlobals *kg,
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PathState *state, Ray *ray, ShaderData *sd, PathRadiance *L, float3 *throughput,
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RNG *rng)
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{
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VolumeShaderCoefficients coeff;
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if(!volume_shader_sample(kg, sd, state, ray->P, &coeff))
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return VOLUME_PATH_MISSED;
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int closure_flag = sd->flag;
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float t = ray->t;
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float3 new_tp;
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float3 transmittance;
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/* randomly scatter, and if we do t is shortened */
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if(closure_flag & SD_SCATTER) {
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float3 sigma_t = coeff.sigma_a + coeff.sigma_s;
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/* set up variables for sampling */
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float rphase = path_state_rng_1D(kg, rng, state, PRNG_PHASE);
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int channel = (int)(rphase*3.0f);
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sd->randb_closure = rphase*3.0f - channel;
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/* pick random color channel, we use the Veach one-sample
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* model with balance heuristic for the channels */
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float sample_sigma_t;
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if(channel == 0)
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sample_sigma_t = sigma_t.x;
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else if(channel == 1)
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sample_sigma_t = sigma_t.y;
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else
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sample_sigma_t = sigma_t.z;
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/* xi is [0, 1[ so log(0) should never happen, division by zero is
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* avoided because sample_sigma_t > 0 when SD_SCATTER is set */
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float xi = path_state_rng_1D(kg, rng, state, PRNG_SCATTER_DISTANCE);
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float sample_t = min(t, -logf(1.0f - xi)/sample_sigma_t);
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transmittance = volume_color_attenuation(sigma_t, sample_t);
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if(sample_t < t) {
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float pdf = dot(sigma_t, transmittance);
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new_tp = *throughput * coeff.sigma_s * transmittance * (3.0f / pdf);
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t = sample_t;
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}
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else {
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float pdf = (transmittance.x + transmittance.y + transmittance.z);
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new_tp = *throughput * transmittance * (3.0f / pdf);
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}
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}
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else if(closure_flag & SD_ABSORPTION) {
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/* absorption only, no sampling needed */
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transmittance = volume_color_attenuation(coeff.sigma_a, t);
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new_tp = *throughput * transmittance;
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}
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/* integrate emission attenuated by extinction
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* integral E * exp(-sigma_t * t) from 0 to t = E * (1 - exp(-sigma_t * t))/sigma_t
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* this goes to E * t as sigma_t goes to zero
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*
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* todo: we should use an epsilon to avoid precision issues near zero sigma_t */
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if(closure_flag & SD_EMISSION) {
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float3 emission = coeff.emission;
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if(closure_flag & SD_ABSORPTION) {
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float3 sigma_t = coeff.sigma_a + coeff.sigma_s;
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emission.x *= (sigma_t.x > 0.0f)? (1.0f - transmittance.x)/sigma_t.x: t;
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emission.y *= (sigma_t.y > 0.0f)? (1.0f - transmittance.y)/sigma_t.y: t;
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emission.z *= (sigma_t.z > 0.0f)? (1.0f - transmittance.z)/sigma_t.z: t;
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}
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else
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emission *= t;
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path_radiance_accum_emission(L, *throughput, emission, state->bounce);
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}
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/* modify throughput */
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if(closure_flag & (SD_ABSORPTION|SD_SCATTER)) {
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*throughput = new_tp;
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/* prepare to scatter to new direction */
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if(t < ray->t) {
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/* adjust throughput and move to new location */
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sd->P = ray->P + t*ray->D;
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return VOLUME_PATH_SCATTERED;
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}
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}
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return VOLUME_PATH_ATTENUATED;
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}
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/* heterogeneous volume: integrate stepping through the volume until we
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* reach the end, get absorbed entirely, or run out of iterations */
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ccl_device VolumeIntegrateResult kernel_volume_integrate_heterogeneous(KernelGlobals *kg,
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PathState *state, Ray *ray, ShaderData *sd, PathRadiance *L, float3 *throughput, RNG *rng)
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{
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VolumeShaderCoefficients coeff;
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float3 tp = *throughput;
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const float tp_eps = 1e-10f; /* todo: this is likely not the right value */
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/* prepare for stepping */
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int max_steps = kernel_data.integrator.volume_max_steps;
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float step = kernel_data.integrator.volume_step_size;
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float random_jitter_offset = lcg_step_float(&state->rng_congruential) * step;
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/* compute coefficients at the start */
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float t = 0.0f;
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float3 P = ray->P;
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if(!volume_shader_sample(kg, sd, state, P, &coeff)) {
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coeff.sigma_a = make_float3(0.0f, 0.0f, 0.0f);
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coeff.sigma_s = make_float3(0.0f, 0.0f, 0.0f);
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coeff.emission = make_float3(0.0f, 0.0f, 0.0f);
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}
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/* accumulate these values so we can use a single stratified number to sample */
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float3 accum_transmittance = make_float3(1.0f, 1.0f, 1.0f);
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float3 accum_sigma_t = make_float3(0.0f, 0.0f, 0.0f);
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float3 accum_sigma_s = make_float3(0.0f, 0.0f, 0.0f);
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/* cache some constant variables */
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float nlogxi;
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int channel = -1;
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bool has_scatter = false;
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for(int i = 0; i < max_steps; i++) {
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/* advance to new position */
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float new_t = min(ray->t, t + random_jitter_offset + i * step);
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float3 new_P = ray->P + ray->D * new_t;
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VolumeShaderCoefficients new_coeff;
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/* compute segment */
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if(volume_shader_sample(kg, sd, state, new_P, &new_coeff)) {
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int closure_flag = sd->flag;
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float dt = new_t - t;
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float3 new_tp;
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float3 transmittance;
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bool scatter = false;
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/* randomly scatter, and if we do dt and new_t are shortened */
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if((closure_flag & SD_SCATTER) || (has_scatter && (closure_flag & SD_ABSORPTION))) {
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has_scatter = true;
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/* average sigma_t and sigma_s over segment */
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float3 last_sigma_t = coeff.sigma_a + coeff.sigma_s;
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float3 new_sigma_t = new_coeff.sigma_a + new_coeff.sigma_s;
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float3 sigma_t = 0.5f*(last_sigma_t + new_sigma_t);
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float3 sigma_s = 0.5f*(coeff.sigma_s + new_coeff.sigma_s);
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/* lazily set up variables for sampling */
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if(channel == -1) {
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float xi = path_state_rng_1D(kg, rng, state, PRNG_SCATTER_DISTANCE);
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nlogxi = -logf(1.0f - xi);
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float rphase = path_state_rng_1D(kg, rng, state, PRNG_PHASE);
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channel = (int)(rphase*3.0f);
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sd->randb_closure = rphase*3.0f - channel;
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}
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/* pick random color channel, we use the Veach one-sample
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* model with balance heuristic for the channels */
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float sample_sigma_t;
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if(channel == 0)
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sample_sigma_t = accum_sigma_t.x + dt*sigma_t.x;
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else if(channel == 1)
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sample_sigma_t = accum_sigma_t.y + dt*sigma_t.y;
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else
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sample_sigma_t = accum_sigma_t.z + dt*sigma_t.z;
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if(nlogxi < sample_sigma_t) {
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/* compute sampling distance */
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sample_sigma_t /= new_t;
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new_t = nlogxi/sample_sigma_t;
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dt = new_t - t;
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transmittance = volume_color_attenuation(sigma_t, dt);
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accum_transmittance *= transmittance;
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accum_sigma_t = (accum_sigma_t + dt*sigma_t)/new_t;
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accum_sigma_s = (accum_sigma_s + dt*sigma_s)/new_t;
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/* todo: it's not clear to me that this is correct if we move
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* through a color volumed, needs verification */
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float pdf = dot(accum_sigma_t, accum_transmittance);
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new_tp = tp * accum_sigma_s * transmittance * (3.0f / pdf);
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scatter = true;
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}
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else {
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transmittance = volume_color_attenuation(sigma_t, dt);
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accum_transmittance *= transmittance;
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accum_sigma_t += dt*sigma_t;
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accum_sigma_s += dt*sigma_s;
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new_tp = tp * transmittance;
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}
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}
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else if(closure_flag & SD_ABSORPTION) {
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/* absorption only, no sampling needed */
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float3 sigma_a = 0.5f*(coeff.sigma_a + new_coeff.sigma_a);
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transmittance = volume_color_attenuation(sigma_a, dt);
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accum_transmittance *= transmittance;
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accum_sigma_t += dt*sigma_a;
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new_tp = tp * transmittance;
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/* todo: we could avoid computing expf() for each step by summing,
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* because exp(a)*exp(b) = exp(a+b), but we still want a quick
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* tp_eps check too */
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}
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/* integrate emission attenuated by absorption
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* integral E * exp(-sigma_t * t) from 0 to t = E * (1 - exp(-sigma_t * t))/sigma_t
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* this goes to E * t as sigma_t goes to zero
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*
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* todo: we should use an epsilon to avoid precision issues near zero sigma_t */
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if(closure_flag & SD_EMISSION) {
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float3 emission = 0.5f*(coeff.emission + new_coeff.emission);
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if(closure_flag & SD_ABSORPTION) {
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float3 sigma_t = 0.5f*(coeff.sigma_a + coeff.sigma_s + new_coeff.sigma_a + new_coeff.sigma_s);
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emission.x *= (sigma_t.x > 0.0f)? (1.0f - transmittance.x)/sigma_t.x: dt;
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emission.y *= (sigma_t.y > 0.0f)? (1.0f - transmittance.y)/sigma_t.y: dt;
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emission.z *= (sigma_t.z > 0.0f)? (1.0f - transmittance.z)/sigma_t.z: dt;
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}
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else
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emission *= dt;
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path_radiance_accum_emission(L, tp, emission, state->bounce);
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}
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/* modify throughput */
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if(closure_flag & (SD_ABSORPTION|SD_SCATTER)) {
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tp = new_tp;
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/* stop if nearly all light blocked */
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if(tp.x < tp_eps && tp.y < tp_eps && tp.z < tp_eps) {
|
|
tp = make_float3(0.0f, 0.0f, 0.0f);
|
|
break;
|
|
}
|
|
|
|
/* prepare to scatter to new direction */
|
|
if(scatter) {
|
|
/* adjust throughput and move to new location */
|
|
sd->P = ray->P + new_t*ray->D;
|
|
*throughput = tp;
|
|
|
|
return VOLUME_PATH_SCATTERED;
|
|
}
|
|
}
|
|
|
|
coeff = new_coeff;
|
|
}
|
|
else {
|
|
/* skip empty space */
|
|
coeff.sigma_a = make_float3(0.0f, 0.0f, 0.0f);
|
|
coeff.sigma_s = make_float3(0.0f, 0.0f, 0.0f);
|
|
coeff.emission = make_float3(0.0f, 0.0f, 0.0f);
|
|
}
|
|
|
|
/* stop if at the end of the volume */
|
|
t = new_t;
|
|
if(t == ray->t)
|
|
break;
|
|
}
|
|
|
|
/* include pdf for volumes with scattering */
|
|
if(has_scatter) {
|
|
float pdf = (accum_transmittance.x + accum_transmittance.y + accum_transmittance.z);
|
|
if(pdf > 0.0f)
|
|
tp *= (3.0f/pdf);
|
|
}
|
|
|
|
*throughput = tp;
|
|
|
|
return VOLUME_PATH_ATTENUATED;
|
|
}
|
|
|
|
/* get the volume attenuation and emission over line segment defined by
|
|
* ray, with the assumption that there are no surfaces blocking light
|
|
* between the endpoints */
|
|
ccl_device_noinline VolumeIntegrateResult kernel_volume_integrate(KernelGlobals *kg,
|
|
PathState *state, ShaderData *sd, Ray *ray, PathRadiance *L, float3 *throughput, RNG *rng)
|
|
{
|
|
shader_setup_from_volume(kg, sd, ray, state->bounce);
|
|
|
|
if(volume_stack_is_heterogeneous(kg, state->volume_stack))
|
|
return kernel_volume_integrate_heterogeneous(kg, state, ray, sd, L, throughput, rng);
|
|
else
|
|
return kernel_volume_integrate_homogeneous(kg, state, ray, sd, L, throughput, rng);
|
|
}
|
|
|
|
/* Volume Stack
|
|
*
|
|
* This is an array of object/shared ID's that the current segment of the path
|
|
* is inside of. */
|
|
|
|
ccl_device void kernel_volume_stack_init(KernelGlobals *kg, VolumeStack *stack)
|
|
{
|
|
/* todo: this assumes camera is always in air, need to detect when it isn't */
|
|
if(kernel_data.background.volume_shader == SHADER_NO_ID) {
|
|
stack[0].shader = SHADER_NO_ID;
|
|
}
|
|
else {
|
|
stack[0].shader = kernel_data.background.volume_shader;
|
|
stack[0].object = ~0;
|
|
stack[1].shader = SHADER_NO_ID;
|
|
}
|
|
}
|
|
|
|
ccl_device void kernel_volume_stack_enter_exit(KernelGlobals *kg, ShaderData *sd, VolumeStack *stack)
|
|
{
|
|
/* todo: we should have some way for objects to indicate if they want the
|
|
* world shader to work inside them. excluding it by default is problematic
|
|
* because non-volume objects can't be assumed to be closed manifolds */
|
|
|
|
if(!(sd->flag & SD_HAS_VOLUME))
|
|
return;
|
|
|
|
if(sd->flag & SD_BACKFACING) {
|
|
/* exit volume object: remove from stack */
|
|
for(int i = 0; stack[i].shader != SHADER_NO_ID; i++) {
|
|
if(stack[i].object == sd->object) {
|
|
/* shift back next stack entries */
|
|
do {
|
|
stack[i] = stack[i+1];
|
|
i++;
|
|
}
|
|
while(stack[i].shader != SHADER_NO_ID);
|
|
|
|
return;
|
|
}
|
|
}
|
|
}
|
|
else {
|
|
/* enter volume object: add to stack */
|
|
int i;
|
|
|
|
for(i = 0; stack[i].shader != SHADER_NO_ID; i++) {
|
|
/* already in the stack? then we have nothing to do */
|
|
if(stack[i].object == sd->object)
|
|
return;
|
|
}
|
|
|
|
/* if we exceed the stack limit, ignore */
|
|
if(i >= VOLUME_STACK_SIZE-1)
|
|
return;
|
|
|
|
/* add to the end of the stack */
|
|
stack[i].shader = sd->shader;
|
|
stack[i].object = sd->object;
|
|
stack[i+1].shader = SHADER_NO_ID;
|
|
}
|
|
}
|
|
|
|
CCL_NAMESPACE_END
|
|
|