volumetric.c

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/*
 * ***** BEGIN GPL LICENSE BLOCK *****
 *
 * 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.
 *
 * The Original Code is Copyright (C) 2001-2002 by NaN Holding BV.
 * All rights reserved.
 *
 * The Original Code is: all of this file.
 *
 * Contributor(s): Matt Ebb, Raul Fernandez Hernandez (Farsthary)
 *
 * ***** END GPL LICENSE BLOCK *****
 */

#include <math.h>
#include <stdlib.h>
#include <string.h>
#include <float.h>

#include "MEM_guardedalloc.h"

#include "BLI_blenlib.h"
#include "BLI_math.h"
#include "BLI_rand.h"
#include "BLI_voxel.h"
#include "BLI_utildefines.h"

#include "RE_shader_ext.h"

#include "DNA_material_types.h"
#include "DNA_group_types.h"
#include "DNA_lamp_types.h"
#include "DNA_meta_types.h"

#include "BKE_global.h"

#include "render_types.h"
#include "pixelshading.h"
#include "rayintersection.h"
#include "rayobject.h"
#include "shading.h"
#include "shadbuf.h"
#include "texture.h"
#include "volumetric.h"
#include "volume_precache.h"

/* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ */
/* defined in pipeline.c, is hardcopy of active dynamic allocated Render */
/* only to be used here in this file, it's for speed */
extern struct Render R;
/* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ */

/* luminance rec. 709 */
BM_INLINE float luminance(const float col[3])
{
    return (0.212671f*col[0] + 0.71516f*col[1] + 0.072169f*col[2]);
}

/* tracing */
static float vol_get_shadow(ShadeInput *shi, LampRen *lar, const float co[3])
{
    float visibility = 1.f;
    
    if(lar->shb) {
        float dxco[3]={0.f, 0.f, 0.f}, dyco[3]={0.f, 0.f, 0.f};
        
        visibility = testshadowbuf(&R, lar->shb, co, dxco, dyco, 1.0, 0.0);     
    } else if (lar->mode & LA_SHAD_RAY) {
        /* trace shadow manually, no good lamp api atm */
        Isect is;
        
        copy_v3_v3(is.start, co);
        if(lar->type==LA_SUN || lar->type==LA_HEMI) {
            is.dir[0] = -lar->vec[0];
            is.dir[1] = -lar->vec[1];
            is.dir[2] = -lar->vec[2];
            is.dist = R.maxdist;
        } else {
            sub_v3_v3v3(is.dir, lar->co, is.start);
            is.dist = normalize_v3( is.dir );
        }

        is.mode = RE_RAY_MIRROR;
        is.check = RE_CHECK_VLR_NON_SOLID_MATERIAL;
        is.skip = 0;
        
        if(lar->mode & (LA_LAYER|LA_LAYER_SHADOW))
            is.lay= lar->lay;   
        else
            is.lay= -1;
            
        is.orig.ob = NULL;
        is.orig.face = NULL;
        is.last_hit = lar->last_hit[shi->thread];
        
        if(RE_rayobject_raycast(R.raytree,&is)) {
            visibility = 0.f;
        }
        
        lar->last_hit[shi->thread]= is.last_hit;
    }
    return visibility;
}

static int vol_get_bounds(ShadeInput *shi, const float co[3], const float vec[3], float hitco[3], Isect *isect, int intersect_type)
{
    
    copy_v3_v3(isect->start, co);
    copy_v3_v3(isect->dir, vec);
    isect->dist = FLT_MAX;
    isect->mode= RE_RAY_MIRROR;
    isect->last_hit = NULL;
    isect->lay= -1;
    isect->check= RE_CHECK_VLR_NONE;
    
    if (intersect_type == VOL_BOUNDS_DEPTH) {
        isect->skip = RE_SKIP_VLR_NEIGHBOUR;
        isect->orig.face = (void*)shi->vlr;
        isect->orig.ob = (void*)shi->obi;
    } else { // if (intersect_type == VOL_BOUNDS_SS) {
        isect->skip= 0;
        isect->orig.face= NULL;
        isect->orig.ob = NULL;
    }
    
    if(RE_rayobject_raycast(R.raytree, isect))
    {
        hitco[0] = isect->start[0] + isect->dist*isect->dir[0];
        hitco[1] = isect->start[1] + isect->dist*isect->dir[1];
        hitco[2] = isect->start[2] + isect->dist*isect->dir[2];
        return 1;
    } else {
        return 0;
    }
}

static void shade_intersection(ShadeInput *shi, float col_r[4], Isect *is)
{
    ShadeInput shi_new;
    ShadeResult shr_new;
    
    memset(&shi_new, 0, sizeof(ShadeInput)); 
    
    shi_new.mask= shi->mask;
    shi_new.osatex= shi->osatex;
    shi_new.thread= shi->thread;
    shi_new.depth = shi->depth + 1;
    shi_new.volume_depth= shi->volume_depth + 1;
    shi_new.xs= shi->xs;
    shi_new.ys= shi->ys;
    shi_new.lay= shi->lay;
    shi_new.passflag= SCE_PASS_COMBINED; /* result of tracing needs no pass info */
    shi_new.combinedflag= 0xFFFFFF;      /* ray trace does all options */
    shi_new.light_override= shi->light_override;
    shi_new.mat_override= shi->mat_override;
    
    copy_v3_v3(shi_new.camera_co, is->start);
    
    memset(&shr_new, 0, sizeof(ShadeResult));
    
    /* hardcoded limit of 100 for now - prevents problems in weird geometry */
    if (shi->volume_depth < 100) {
        shade_ray(is, &shi_new, &shr_new);
    }
    
    copy_v3_v3(col_r, shr_new.combined);
    col_r[3] = shr_new.alpha;
}

static void vol_trace_behind(ShadeInput *shi, VlakRen *vlr, const float co[3], float col_r[4])
{
    Isect isect;
    
    copy_v3_v3(isect.start, co);
    copy_v3_v3(isect.dir, shi->view);
    isect.dist = FLT_MAX;
    
    isect.mode= RE_RAY_MIRROR;
    isect.check = RE_CHECK_VLR_NONE;
    isect.skip = RE_SKIP_VLR_NEIGHBOUR;
    isect.orig.ob = (void*) shi->obi;
    isect.orig.face = (void*)vlr;
    isect.last_hit = NULL;
    isect.lay= -1;
    
    /* check to see if there's anything behind the volume, otherwise shade the sky */
    if(RE_rayobject_raycast(R.raytree, &isect)) {
        shade_intersection(shi, col_r, &isect);
    } else {
        shadeSkyView(col_r, co, shi->view, NULL, shi->thread);
        shadeSunView(col_r, shi->view);
    } 
}


/* trilinear interpolation */
static void vol_get_precached_scattering(Render *re, ShadeInput *shi, float scatter_col[3], const float co[3])
{
    VolumePrecache *vp = shi->obi->volume_precache;
    float bbmin[3], bbmax[3], dim[3];
    float world_co[3], sample_co[3];
    
    if (!vp) return;
    
    /* find sample point in global space bounding box 0.0-1.0 */
    global_bounds_obi(re, shi->obi, bbmin, bbmax);
    sub_v3_v3v3(dim, bbmax, bbmin);
    mul_v3_m4v3(world_co, re->viewinv, co); 

    /* sample_co in 0.0-1.0 */
    sample_co[0] = (world_co[0] - bbmin[0]) / dim[0];
    sample_co[1] = (world_co[1] - bbmin[1]) / dim[1];
    sample_co[2] = (world_co[2] - bbmin[2]) / dim[2];

    scatter_col[0] = voxel_sample_triquadratic(vp->data_r, vp->res, sample_co);
    scatter_col[1] = voxel_sample_triquadratic(vp->data_g, vp->res, sample_co);
    scatter_col[2] = voxel_sample_triquadratic(vp->data_b, vp->res, sample_co);
}

/* Meta object density, brute force for now 
 * (might be good enough anyway, don't need huge number of metaobs to model volumetric objects */
static float metadensity(Object* ob, const float co[3])
{
    float mat[4][4], imat[4][4], dens = 0.f;
    MetaBall* mb = (MetaBall*)ob->data;
    MetaElem* ml;
    
    /* transform co to meta-element */
    float tco[3] = {co[0], co[1], co[2]};
    mult_m4_m4m4(mat, R.viewmat, ob->obmat);
    invert_m4_m4(imat, mat);
    mul_m4_v3(imat, tco);
    
    for (ml = mb->elems.first; ml; ml=ml->next) {
        float bmat[3][3], dist2;
        
        /* element rotation transform */
        float tp[3] = {ml->x - tco[0], ml->y - tco[1], ml->z - tco[2]};
        quat_to_mat3( bmat,ml->quat);
        transpose_m3(bmat); // rot.only, so inverse == transpose
        mul_m3_v3(bmat, tp);
        
        /* MB_BALL default */
        switch (ml->type) {
            case MB_ELIPSOID:
                tp[0] /= ml->expx, tp[1] /= ml->expy, tp[2] /= ml->expz;
                break;
            case MB_CUBE:
                tp[2] = (tp[2] > ml->expz) ? (tp[2] - ml->expz) : ((tp[2] < -ml->expz) ? (tp[2] + ml->expz) : 0.f);
                // no break, xy as plane
            case MB_PLANE:
                tp[1] = (tp[1] > ml->expy) ? (tp[1] - ml->expy) : ((tp[1] < -ml->expy) ? (tp[1] + ml->expy) : 0.f);
                // no break, x as tube
            case MB_TUBE:
                tp[0] = (tp[0] > ml->expx) ? (tp[0] - ml->expx) : ((tp[0] < -ml->expx) ? (tp[0] + ml->expx) : 0.f);
        }
        
        /* ml->rad2 is not set */
        dist2 = 1.f - ((tp[0]*tp[0] + tp[1]*tp[1] + tp[2]*tp[2]) / (ml->rad*ml->rad));
        if (dist2 > 0.f)
            dens += (ml->flag & MB_NEGATIVE) ? -ml->s*dist2*dist2*dist2 : ml->s*dist2*dist2*dist2;
    }
    
    dens -= mb->thresh;
    return (dens < 0.f) ? 0.f : dens;
}

float vol_get_density(struct ShadeInput *shi, const float co[3])
{
    float density = shi->mat->vol.density;
    float density_scale = shi->mat->vol.density_scale;
        
    if (shi->mat->mapto_textured & MAP_DENSITY)
        do_volume_tex(shi, co, MAP_DENSITY, NULL, &density, &R);
    
    // if meta-object, modulate by metadensity without increasing it
    if (shi->obi->obr->ob->type == OB_MBALL) {
        const float md = metadensity(shi->obi->obr->ob, co);
        if (md < 1.f) density *= md;
     }
    
    return density * density_scale;
}


/* Color of light that gets scattered out by the volume */
/* Uses same physically based scattering parameter as in transmission calculations, 
 * along with artificial reflection scale/reflection color tint */
static void vol_get_reflection_color(ShadeInput *shi, float ref_col[3], const float co[3])
{
    float scatter = shi->mat->vol.scattering;
    float reflection= shi->mat->vol.reflection;
    copy_v3_v3(ref_col, shi->mat->vol.reflection_col);
    
    if (shi->mat->mapto_textured & (MAP_SCATTERING+MAP_REFLECTION_COL))
        do_volume_tex(shi, co, MAP_SCATTERING+MAP_REFLECTION_COL, ref_col, &scatter, &R);
    
    /* only one single float parameter at a time... :s */
    if (shi->mat->mapto_textured & (MAP_REFLECTION))
        do_volume_tex(shi, co, MAP_REFLECTION, NULL, &reflection, &R);
    
    ref_col[0] = reflection * ref_col[0] * scatter;
    ref_col[1] = reflection * ref_col[1] * scatter;
    ref_col[2] = reflection * ref_col[2] * scatter;
}

/* compute emission component, amount of radiance to add per segment
 * can be textured with 'emit' */
static void vol_get_emission(ShadeInput *shi, float emission_col[3], const float co[3])
{
    float emission = shi->mat->vol.emission;
    copy_v3_v3(emission_col, shi->mat->vol.emission_col);
    
    if (shi->mat->mapto_textured & (MAP_EMISSION+MAP_EMISSION_COL))
        do_volume_tex(shi, co, MAP_EMISSION+MAP_EMISSION_COL, emission_col, &emission, &R);
    
    emission_col[0] = emission_col[0] * emission;
    emission_col[1] = emission_col[1] * emission;
    emission_col[2] = emission_col[2] * emission;
}


/* A combination of scattering and absorption -> known as sigma T.
 * This can possibly use a specific scattering color, 
 * and absorption multiplier factor too, but these parameters are left out for simplicity.
 * It's easy enough to get a good wide range of results with just these two parameters. */
static void vol_get_sigma_t(ShadeInput *shi, float sigma_t[3], const float co[3])
{
    /* technically absorption, but named transmission color 
     * since it describes the effect of the coloring *after* absorption */
    float transmission_col[3] = {shi->mat->vol.transmission_col[0], shi->mat->vol.transmission_col[1], shi->mat->vol.transmission_col[2]};
    float scattering = shi->mat->vol.scattering;
    
    if (shi->mat->mapto_textured & (MAP_SCATTERING+MAP_TRANSMISSION_COL))
        do_volume_tex(shi, co, MAP_SCATTERING+MAP_TRANSMISSION_COL, transmission_col, &scattering, &R);
    
    sigma_t[0] = (1.0f - transmission_col[0]) + scattering;
    sigma_t[1] = (1.0f - transmission_col[1]) + scattering;
    sigma_t[2] = (1.0f - transmission_col[2]) + scattering;
}

/* phase function - determines in which directions the light 
 * is scattered in the volume relative to incoming direction 
 * and view direction */
static float vol_get_phasefunc(ShadeInput *UNUSED(shi), float g, const float w[3], const float wp[3])
{
    const float normalize = 0.25f; // = 1.f/4.f = M_PI/(4.f*M_PI)
    
    /* normalization constant is 1/4 rather than 1/4pi, since
     * Blender's shading system doesn't normalise for
     * energy conservation - eg. multiplying by pdf ( 1/pi for a lambert brdf ).
     * This means that lambert surfaces in Blender are pi times brighter than they 'should be'
     * and therefore, with correct energy conservation, volumes will darker than other solid objects,
     * for the same lighting intensity.
     * To correct this, scale up the phase function values by pi
     * until Blender's shading system supports this better. --matt
     */
    
    if (g == 0.f) { /* isotropic */
        return normalize * 1.f;
    } else {        /* schlick */
        const float k = 1.55f * g - .55f * g * g * g;
        const float kcostheta = k * dot_v3v3(w, wp);
        return normalize * (1.f - k*k) / ((1.f - kcostheta) * (1.f - kcostheta));
    }
    
    /*
     * not used, but here for reference:
    switch (phasefunc_type) {
        case MA_VOL_PH_MIEHAZY:
            return normalize * (0.5f + 4.5f * powf(0.5 * (1.f + costheta), 8.f));
        case MA_VOL_PH_MIEMURKY:
            return normalize * (0.5f + 16.5f * powf(0.5 * (1.f + costheta), 32.f));
        case MA_VOL_PH_RAYLEIGH:
            return normalize * 3.f/4.f * (1 + costheta * costheta);
        case MA_VOL_PH_HG:
            return normalize * (1.f - g*g) / powf(1.f + g*g - 2.f * g * costheta, 1.5f));
        case MA_VOL_PH_SCHLICK:
        {
            const float k = 1.55f * g - .55f * g * g * g;
            const float kcostheta = k * costheta;
            return normalize * (1.f - k*k) / ((1.f - kcostheta) * (1.f - kcostheta));
        }
        case MA_VOL_PH_ISOTROPIC:
        default:
            return normalize * 1.f;
    }
    */
}

/* Compute transmittance = e^(-attenuation) */
static void vol_get_transmittance_seg(ShadeInput *shi, float tr[3], float stepsize, const float co[3], float density)
{
    /* input density = density at co */
    float tau[3] = {0.f, 0.f, 0.f};
    const float stepd = density * stepsize;
    float sigma_t[3];
    
    vol_get_sigma_t(shi, sigma_t, co);
    
    /* homogenous volume within the sampled distance */
    tau[0] += stepd * sigma_t[0];
    tau[1] += stepd * sigma_t[1];
    tau[2] += stepd * sigma_t[2];
    
    tr[0] *= expf(-tau[0]);
    tr[1] *= expf(-tau[1]);
    tr[2] *= expf(-tau[2]);
}

/* Compute transmittance = e^(-attenuation) */
static void vol_get_transmittance(ShadeInput *shi, float tr[3], const float co[3], const float endco[3])
{
    float p[3] = {co[0], co[1], co[2]};
    float step_vec[3] = {endco[0] - co[0], endco[1] - co[1], endco[2] - co[2]};
    float tau[3] = {0.f, 0.f, 0.f};

    float t0 = 0.f;
    float t1 = normalize_v3(step_vec);
    float pt0 = t0;
    
    t0 += shi->mat->vol.stepsize * ((shi->mat->vol.stepsize_type == MA_VOL_STEP_CONSTANT) ? 0.5f : BLI_thread_frand(shi->thread));
    p[0] += t0 * step_vec[0];
    p[1] += t0 * step_vec[1];
    p[2] += t0 * step_vec[2];
    mul_v3_fl(step_vec, shi->mat->vol.stepsize);

    for (; t0 < t1; pt0 = t0, t0 += shi->mat->vol.stepsize) {
        const float d = vol_get_density(shi, p);
        const float stepd = (t0 - pt0) * d;
        float sigma_t[3];
        
        vol_get_sigma_t(shi, sigma_t, p);
        
        tau[0] += stepd * sigma_t[0];
        tau[1] += stepd * sigma_t[1];
        tau[2] += stepd * sigma_t[2];
        
        add_v3_v3(p, step_vec);
    }
    
    /* return transmittance */
    tr[0] = expf(-tau[0]);
    tr[1] = expf(-tau[1]);
    tr[2] = expf(-tau[2]);
}

static void vol_shade_one_lamp(struct ShadeInput *shi, const float co[3], const float view[3], LampRen *lar, float lacol[3])
{
    float visifac, lv[3], lampdist;
    float tr[3]={1.0,1.0,1.0};
    float hitco[3], *atten_co;
    float p, ref_col[3];
    
    if (lar->mode & LA_LAYER) if((lar->lay & shi->obi->lay)==0) return;
    if ((lar->lay & shi->lay)==0) return;
    if (lar->energy == 0.0f) return;
    
    if ((visifac= lamp_get_visibility(lar, co, lv, &lampdist)) == 0.f) return;
    
    copy_v3_v3(lacol, &lar->r);
    
    if(lar->mode & LA_TEXTURE) {
        shi->osatex= 0;
        do_lamp_tex(lar, lv, shi, lacol, LA_TEXTURE);
    }

    mul_v3_fl(lacol, visifac);

    if (ELEM(lar->type, LA_SUN, LA_HEMI))
        copy_v3_v3(lv, lar->vec);
    negate_v3(lv);
    
    if (shi->mat->vol.shade_type == MA_VOL_SHADE_SHADOWED) {
        mul_v3_fl(lacol, vol_get_shadow(shi, lar, co));
    }
    else if (ELEM3(shi->mat->vol.shade_type, MA_VOL_SHADE_SHADED, MA_VOL_SHADE_MULTIPLE, MA_VOL_SHADE_SHADEDPLUSMULTIPLE))
    {
        Isect is;
        
        if (shi->mat->vol.shadeflag & MA_VOL_RECV_EXT_SHADOW) {
            mul_v3_fl(lacol, vol_get_shadow(shi, lar, co));
            if (luminance(lacol) < 0.001f) return;
        }
        
        /* find minimum of volume bounds, or lamp coord */
        if (vol_get_bounds(shi, co, lv, hitco, &is, VOL_BOUNDS_SS)) {
            float dist = len_v3v3(co, hitco);
            VlakRen *vlr = (VlakRen *)is.hit.face;
            
            /* simple internal shadowing */
            if (vlr->mat->material_type == MA_TYPE_SURFACE) {
                lacol[0] = lacol[1] = lacol[2] = 0.0f;
                return;
            }

            if (ELEM(lar->type, LA_SUN, LA_HEMI))
                /* infinite lights, can never be inside volume */
                atten_co = hitco;
            else if ( lampdist < dist ) {
                atten_co = lar->co;
            } else
                atten_co = hitco;
            
            vol_get_transmittance(shi, tr, co, atten_co);
            
            mul_v3_v3v3(lacol, lacol, tr);
        }
        else {
            /* Point is on the outside edge of the volume,
             * therefore no attenuation, full transmission.
             * Radiance from lamp remains unchanged */
        }
    }
    
    if (luminance(lacol) < 0.001f) return;
    
    normalize_v3(lv);
    p = vol_get_phasefunc(shi, shi->mat->vol.asymmetry, view, lv);
    
    /* physically based scattering with non-physically based RGB gain */
    vol_get_reflection_color(shi, ref_col, co);
    
    lacol[0] *= p * ref_col[0];
    lacol[1] *= p * ref_col[1];
    lacol[2] *= p * ref_col[2];
}

/* single scattering only for now */
void vol_get_scattering(ShadeInput *shi, float scatter_col[3], const float co[3], const float view[3])
{
    ListBase *lights;
    GroupObject *go;
    LampRen *lar;

    zero_v3(scatter_col);

    lights= get_lights(shi);
    for(go=lights->first; go; go= go->next)
    {
        float lacol[3] = {0.f, 0.f, 0.f};
        lar= go->lampren;
        
        if (lar) {
            vol_shade_one_lamp(shi, co, view, lar, lacol);
            add_v3_v3(scatter_col, lacol);
        }
    }
}

    
/*
The main volumetric integrator, using an emission/absorption/scattering model.

Incoming radiance = 

outgoing radiance from behind surface * beam transmittance/attenuation
+ added radiance from all points along the ray due to participating media
    --> radiance for each segment = 
        (radiance added by scattering + radiance added by emission) * beam transmittance/attenuation
*/

/* For ease of use, I've also introduced a 'reflection' and 'reflection color' parameter, which isn't 
 * physically correct. This works as an RGB tint/gain on out-scattered light, but doesn't affect the light 
 * that is transmitted through the volume. While having wavelength dependent absorption/scattering is more correct,
 * it also makes it harder to control the overall look of the volume since coloring the outscattered light results
 * in the inverse color being transmitted through the rest of the volume.
 */
static void volumeintegrate(struct ShadeInput *shi, float col[4], const float co[3], const float endco[3])
{
    float radiance[3] = {0.f, 0.f, 0.f};
    float tr[3] = {1.f, 1.f, 1.f};
    float p[3] = {co[0], co[1], co[2]};
    float step_vec[3] = {endco[0] - co[0], endco[1] - co[1], endco[2] - co[2]};
    const float stepsize = shi->mat->vol.stepsize;
    
    float t0 = 0.f;
    float pt0 = t0;
    float t1 = normalize_v3(step_vec);  /* returns vector length */
    
    t0 += stepsize * ((shi->mat->vol.stepsize_type == MA_VOL_STEP_CONSTANT) ? 0.5f : BLI_thread_frand(shi->thread));
    p[0] += t0 * step_vec[0];
    p[1] += t0 * step_vec[1];
    p[2] += t0 * step_vec[2];
    mul_v3_fl(step_vec, stepsize);
    
    for (; t0 < t1; pt0 = t0, t0 += stepsize) {
        const float density = vol_get_density(shi, p);
        
        if (density > 0.00001f) {
            float scatter_col[3] = {0.f, 0.f, 0.f}, emit_col[3];
            const float stepd = (t0 - pt0) * density;
            
            /* transmittance component (alpha) */
            vol_get_transmittance_seg(shi, tr, stepsize, co, density);
            
            if (t0 > t1 * 0.25f) {
                /* only use depth cutoff after we've traced a little way into the volume */
                if (luminance(tr) < shi->mat->vol.depth_cutoff) break;
            }
            
            vol_get_emission(shi, emit_col, p);
            
            if (shi->obi->volume_precache) {
                float p2[3];
                
                p2[0] = p[0] + (step_vec[0] * 0.5f);
                p2[1] = p[1] + (step_vec[1] * 0.5f);
                p2[2] = p[2] + (step_vec[2] * 0.5f);
                
                vol_get_precached_scattering(&R, shi, scatter_col, p2);
            } else
                vol_get_scattering(shi, scatter_col, p, shi->view);
            
            radiance[0] += stepd * tr[0] * (emit_col[0] + scatter_col[0]);
            radiance[1] += stepd * tr[1] * (emit_col[1] + scatter_col[1]);
            radiance[2] += stepd * tr[2] * (emit_col[2] + scatter_col[2]);
        }
        add_v3_v3(p, step_vec);
    }
    
    /* multiply original color (from behind volume) with transmittance over entire distance */
    mul_v3_v3v3(col, tr, col);
    add_v3_v3(col, radiance);
    
    /* alpha <-- transmission luminance */
    col[3] = 1.0f - luminance(tr);
}

/* the main entry point for volume shading */
static void volume_trace(struct ShadeInput *shi, struct ShadeResult *shr, int inside_volume)
{
    float hitco[3], col[4] = {0.f,0.f,0.f,0.f};
    float *startco, *endco;
    int trace_behind = 1;
    const int ztransp= ((shi->depth==0) && (shi->mat->mode & MA_TRANSP) && (shi->mat->mode & MA_ZTRANSP));
    Isect is;

    /* check for shading an internal face a volume object directly */
    if (inside_volume == VOL_SHADE_INSIDE)
        trace_behind = 0;
    else if (inside_volume == VOL_SHADE_OUTSIDE) {
        if (shi->flippednor)
            inside_volume = VOL_SHADE_INSIDE;
    }
    
    if (ztransp && inside_volume == VOL_SHADE_INSIDE) {
        MatInside *mi;
        int render_this=0;
        
        /* don't render the backfaces of ztransp volume materials.
         
         * volume shading renders the internal volume from between the
         * ' view intersection of the solid volume to the
         * intersection on the other side, as part of the shading of
         * the front face.
         
         * Because ztransp renders both front and back faces independently
         * this will double up, so here we prevent rendering the backface as well, 
         * which would otherwise render the volume in between the camera and the backface
         * --matt */
        
        for (mi=R.render_volumes_inside.first; mi; mi=mi->next) {
            /* weak... */
            if (mi->ma == shi->mat) render_this=1;
        }
        if (!render_this) return;
    }
    

    if (inside_volume == VOL_SHADE_INSIDE)
    {
        startco = shi->camera_co;
        endco = shi->co;
        
        if (trace_behind) {
            if (!ztransp)
                /* trace behind the volume object */
                vol_trace_behind(shi, shi->vlr, endco, col);
        } else {
            /* we're tracing through the volume between the camera 
             * and a solid surface, so use that pre-shaded radiance */
            copy_v4_v4(col, shr->combined);
        }
        
        /* shade volume from 'camera' to 1st hit point */
        volumeintegrate(shi, col, startco, endco);
    }
    /* trace to find a backface, the other side bounds of the volume */
    /* (ray intersect ignores front faces here) */
    else if (vol_get_bounds(shi, shi->co, shi->view, hitco, &is, VOL_BOUNDS_DEPTH))
    {
        VlakRen *vlr = (VlakRen *)is.hit.face;
        
        startco = shi->co;
        endco = hitco;
        
        if (!ztransp) {
            /* if it's another face in the same material */
            if (vlr->mat == shi->mat) {
                /* trace behind the 2nd (raytrace) hit point */
                vol_trace_behind(shi, (VlakRen *)is.hit.face, endco, col);
            } else {
                shade_intersection(shi, col, &is);
            }
        }
        
        /* shade volume from 1st hit point to 2nd hit point */
        volumeintegrate(shi, col, startco, endco);
    }
    
    if (ztransp)
        col[3] = col[3]>1.f?1.f:col[3];
    else
        col[3] = 1.f;
    
    copy_v3_v3(shr->combined, col);
    shr->alpha = col[3];
    
    copy_v3_v3(shr->diff, shr->combined);
}

/* Traces a shadow through the object, 
 * pretty much gets the transmission over a ray path */
void shade_volume_shadow(struct ShadeInput *shi, struct ShadeResult *shr, struct Isect *last_is)
{
    float hitco[3];
    float tr[3] = {1.0,1.0,1.0};
    Isect is= {{0}};
    float *startco, *endco;

    memset(shr, 0, sizeof(ShadeResult));
    
    /* if 1st hit normal is facing away from the camera, 
     * then we're inside the volume already. */
    if (shi->flippednor) {
        startco = last_is->start;
        endco = shi->co;
    }
    
    /* trace to find a backface, the other side bounds of the volume */
    /* (ray intersect ignores front faces here) */
    else if (vol_get_bounds(shi, shi->co, shi->view, hitco, &is, VOL_BOUNDS_DEPTH)) {
        startco = shi->co;
        endco = hitco;
    }
    else {
        shr->combined[0] = shr->combined[1] = shr->combined[2] = 0.f;
        shr->alpha = shr->combined[3] = 1.f;
        return;
    }

    vol_get_transmittance(shi, tr, startco, endco);

    
    /* if we hit another face in the same volume bounds */
    /* shift raytrace coordinates to the hit point, to avoid shading volume twice */
    /* due to idiosyncracy in ray_trace_shadow_tra() */
    if (is.hit.ob == shi->obi) {
        copy_v3_v3(shi->co, hitco);
        last_is->dist -= is.dist;
        shi->vlr = (VlakRen *)is.hit.face;
    }

    
    copy_v3_v3(shr->combined, tr);
    shr->combined[3] = 1.0f - luminance(tr);
    shr->alpha = shr->combined[3];
}


/* delivers a fully filled in ShadeResult, for all passes */
void shade_volume_outside(ShadeInput *shi, ShadeResult *shr)
{
    memset(shr, 0, sizeof(ShadeResult));
    volume_trace(shi, shr, VOL_SHADE_OUTSIDE);
}


void shade_volume_inside(ShadeInput *shi, ShadeResult *shr)
{
    MatInside *m;
    Material *mat_backup;
    ObjectInstanceRen *obi_backup;
    float prev_alpha = shr->alpha;

    /* XXX: extend to multiple volumes perhaps later */
    mat_backup = shi->mat;
    obi_backup = shi->obi;
    
    m = R.render_volumes_inside.first;
    shi->mat = m->ma;
    shi->obi = m->obi;
    shi->obr = m->obi->obr;
    
    volume_trace(shi, shr, VOL_SHADE_INSIDE);
    
    shr->alpha = shr->alpha + prev_alpha;
    CLAMP(shr->alpha, 0.0f, 1.0f);

    shi->mat = mat_backup;
    shi->obi = obi_backup;
    shi->obr = obi_backup->obr;
}