Eevee: Overhaul the volumetric system.

The system now uses several 3D textures in order to decouple every steps of the volumetric rendering.

See https://www.ea.com/frostbite/news/physically-based-unified-volumetric-rendering-in-frostbite for more details.

On the technical side, instead of using a compute shader to populate the 3D textures we use layered rendering with a geometry shader to render 1 fullscreen triangle per 3D texture slice.

1 files changed, 19 insertions, 377 deletions

diff --git a/source/blender/draw/engines/eevee/shaders/volumetric_frag.glsl b/source/blender/draw/engines/eevee/shaders/volumetric_frag.glsl
index 7f44dd53163..a3e2979a9b4 100644
--- a/source/blender/draw/engines/eevee/shaders/volumetric_frag.glsl
+++ b/source/blender/draw/engines/eevee/shaders/volumetric_frag.glsl
@@ -1,396 +1,38 @@
 
-#ifdef VOLUMETRICS
+/* Based on Frosbite Unified Volumetric.
+ * https://www.ea.com/frostbite/news/physically-based-unified-volumetric-rendering-in-frostbite */
 
 #define NODETREE_EXEC
 
-#define VOLUMETRIC_INTEGRATION_MAX_STEP 256
-#define VOLUMETRIC_SHADOW_MAX_STEP 128
-
-uniform int light_count;
-uniform vec2 volume_start_end;
-uniform vec4 volume_samples_clamp;
-
-#define volume_start                   volume_start_end.x
-#define volume_end                     volume_start_end.y
+uniform ivec3 volumeTextureSize;
+uniform vec3 volume_jitter;
 
-#define volume_integration_steps       volume_samples_clamp.x
-#define volume_shadows_steps           volume_samples_clamp.y
-#define volume_sample_distribution     volume_samples_clamp.z
-#define volume_light_clamp             volume_samples_clamp.w
-
-#ifdef COLOR_TRANSMITTANCE
-layout(location = 0) out vec4 outScattering;
-layout(location = 1) out vec4 outTransmittance;
-#else
-out vec4 outScatteringTransmittance;
-#endif
+flat in int slice;
 
 /* Warning: theses are not attributes, theses are global vars. */
 vec3 worldPosition = vec3(0.0);
 vec3 viewPosition = vec3(0.0);
 vec3 viewNormal = vec3(0.0);
 
-uniform sampler2D depthFull;
-
-void participating_media_properties(vec3 wpos, out vec3 extinction, out vec3 scattering, out vec3 emission, out float anisotropy)
-{
-#ifndef VOLUME_HOMOGENEOUS
-	worldPosition = wpos;
-	viewPosition = (ViewMatrix * vec4(wpos, 1.0)).xyz; /* warning, Perf. */
-#endif
-
-	Closure cl = nodetree_exec();
-
-	scattering = cl.scatter;
-	emission = cl.emission;
-	anisotropy = cl.anisotropy;
-	extinction = max(vec3(1e-4), cl.absorption + cl.scatter);
-}
-
-vec3 participating_media_extinction(vec3 wpos)
-{
-#ifndef VOLUME_HOMOGENEOUS
-	worldPosition = wpos;
-	viewPosition = (ViewMatrix * vec4(wpos, 1.0)).xyz; /* warning, Perf. */
-#endif
-
-	Closure cl = nodetree_exec();
+layout(location = 0) out vec4 volumeScattering;
+layout(location = 1) out vec4 volumeExtinction;
+layout(location = 2) out vec4 volumeEmissive;
+layout(location = 3) out vec4 volumePhase;
 
-	return max(vec3(1e-4), cl.absorption + cl.scatter);
-}
+/* Store volumetric properties into the froxel textures. */
 
-float phase_function_isotropic()
-{
-	return 1.0 / (4.0 * M_PI);
-}
-
-float phase_function(vec3 v, vec3 l, float g)
-{
-#ifndef VOLUME_ISOTROPIC /* TODO Use this flag when only isotropic closures are used */
-	/* Henyey-Greenstein */
-	float cos_theta = dot(v, l);
-	g = clamp(g, -1.0 + 1e-3, 1.0 - 1e-3);
-	float sqr_g = g * g;
-	return (1- sqr_g) / (4.0 * M_PI * pow(1 + sqr_g - 2 * g * cos_theta, 3.0 / 2.0));
-#else
-	return phase_function_isotropic();
-#endif
-}
-
-float light_volume(LightData ld, vec4 l_vector)
-{
-	float power;
-	float dist = max(1e-4, abs(l_vector.w - ld.l_radius));
-	/* TODO : Area lighting ? */
-	/* Removing Area Power. */
-	/* TODO : put this out of the shader. */
-	if (ld.l_type == AREA) {
-		power = 0.0962 * (ld.l_sizex * ld.l_sizey * 4.0f * M_PI);
-	}
-	else {
-		power = 0.0248 * (4.0 * ld.l_radius * ld.l_radius * M_PI * M_PI);
-	}
-	return min(power / (l_vector.w * l_vector.w), volume_light_clamp);
-}
-
-vec3 irradiance_volumetric(vec3 wpos)
-{
-	IrradianceData ir_data = load_irradiance_cell(0, vec3(1.0));
-	vec3 irradiance = ir_data.cubesides[0] + ir_data.cubesides[1] + ir_data.cubesides[2];
-	ir_data = load_irradiance_cell(0, vec3(-1.0));
-	irradiance += ir_data.cubesides[0] + ir_data.cubesides[1] + ir_data.cubesides[2];
-	irradiance *= 0.16666666; /* 1/6 */
-	return irradiance;
-}
-
-vec3 light_volume_shadow(LightData ld, vec3 ray_wpos, vec4 l_vector, vec3 s_extinction)
-{
-#ifdef VOLUME_SHADOW
-
-#ifdef VOLUME_HOMOGENEOUS
-	/* Simple extinction */
-	return exp(-s_extinction * l_vector.w);
-#else
-	/* Heterogeneous volume shadows */
-	float dd = l_vector.w / volume_shadows_steps;
-	vec3 L = l_vector.xyz * l_vector.w;
-	vec3 shadow = vec3(1.0);
-	for (float s = 0.5; s < VOLUMETRIC_SHADOW_MAX_STEP && s < (volume_shadows_steps - 0.1); s += 1.0) {
-		vec3 pos = ray_wpos + L * (s / volume_shadows_steps);
-		vec3 s_extinction = participating_media_extinction(pos);
-		shadow *= exp(-s_extinction * dd);
-	}
-	return shadow;
-#endif /* VOLUME_HOMOGENEOUS */
-
-#else
-	return vec3(1.0);
-#endif /* VOLUME_SHADOW */
-}
-
-float find_next_step(float iter, float noise)
-{
-	float progress = (iter + noise) / volume_integration_steps;
-
-	float linear_split = mix(volume_start, volume_end, progress);
-
-	if (ProjectionMatrix[3][3] == 0.0) {
-		float exp_split = volume_start * pow(volume_end / volume_start, progress);
-		return mix(linear_split, exp_split, volume_sample_distribution);
-	}
-	else {
-		return linear_split;
-	}
-}
-
-/* Based on Frosbite Unified Volumetric.
- * https://www.ea.com/frostbite/news/physically-based-unified-volumetric-rendering-in-frostbite */
 void main()
 {
-	vec2 uv = (gl_FragCoord.xy * 2.0) / ivec2(textureSize(depthFull, 0));
-	float scene_depth = texelFetch(depthFull, ivec2(gl_FragCoord.xy) * 2, 0).r; /* use the same depth as in the upsample step */
-	vec3 vpos = get_view_space_from_depth(uv, scene_depth);
-	vec3 wpos = (ViewMatrixInverse * vec4(vpos, 1.0)).xyz;
-	vec3 wdir = (ProjectionMatrix[3][3] == 0.0) ? normalize(cameraPos - wpos) : cameraForward;
-
-	/* Note: this is NOT the distance to the camera. */
-	float max_z = vpos.z;
-
-	/* project ray to clip plane so we can integrate in even steps in clip space. */
-	vec3 wdir_proj = wdir / abs(dot(cameraForward, wdir));
-	float wlen = length(wdir_proj);
-
-	/* Transmittance: How much light can get through. */
-	vec3 transmittance = vec3(1.0);
-
-	/* Scattering: Light that has been accumulated from scattered light sources. */
-	vec3 scattering = vec3(0.0);
-
-	vec3 ray_origin = (ProjectionMatrix[3][3] == 0.0)
-		? cameraPos
-		: (ViewMatrixInverse * vec4(get_view_space_from_depth(uv, 0.5), 1.0)).xyz;
-
-#ifdef VOLUME_HOMOGENEOUS
-	/* Put it out of the loop for homogeneous media. */
-	vec3 s_extinction, s_scattering, s_emission;
-	float s_anisotropy;
-	participating_media_properties(vec3(0.0), s_extinction, s_scattering, s_emission, s_anisotropy);
-#endif
-
-	/* Start from near clip. TODO make start distance an option. */
-	float rand = texture(utilTex, vec3(gl_FragCoord.xy / LUT_SIZE, 2.0)).r;
-	/* Less noisy but noticeable patterns, could work better with temporal AA. */
-	// float rand = (1.0 / 16.0) * float(((int(gl_FragCoord.x + gl_FragCoord.y) & 0x3) << 2) + (int(gl_FragCoord.x) & 0x3));
-	float dist = volume_start;
-	for (float i = 0.5; i < VOLUMETRIC_INTEGRATION_MAX_STEP && i < (volume_integration_steps - 0.1); ++i) {
-		float new_dist = max(max_z, find_next_step(rand, i));
-		float step = dist - new_dist; /* Marching step */
-		dist = new_dist;
-
-		vec3 ray_wpos = ray_origin + wdir_proj * dist;
-
-#ifndef VOLUME_HOMOGENEOUS
-		vec3 s_extinction, s_scattering, s_emission;
-		float s_anisotropy;
-		participating_media_properties(ray_wpos, s_extinction, s_scattering, s_emission, s_anisotropy);
-#endif
-
-		/* Evaluate each light */
-		vec3 Lscat = s_emission;
-
-#ifdef VOLUME_LIGHTING /* Lights */
-		for (int i = 0; i < MAX_LIGHT && i < light_count; ++i) {
-			LightData ld = lights_data[i];
+	ivec3 volume_cell = ivec3(gl_FragCoord.xy, slice);
+	vec3 ndc_cell = volume_to_ndc((vec3(volume_cell) + volume_jitter) / volumeTextureSize);
 
-			vec4 l_vector;
-			l_vector.xyz = ld.l_position - ray_wpos;
-			l_vector.w = length(l_vector.xyz);
+	viewPosition = get_view_space_from_depth(ndc_cell.xy, ndc_cell.z);
+	worldPosition = transform_point(ViewMatrixInverse, viewPosition);
 
-			float Vis = light_visibility(ld, ray_wpos, l_vector);
-
-			vec3 Li = ld.l_color * light_volume(ld, l_vector) * light_volume_shadow(ld, ray_wpos, l_vector, s_extinction);
-
-			Lscat += Li * Vis * s_scattering * phase_function(-wdir, l_vector.xyz / l_vector.w, s_anisotropy);
-		}
-#endif
-
-		/* Environment : Average color. */
-		Lscat += irradiance_volumetric(wpos) * s_scattering * phase_function_isotropic();
-
-		/* Evaluate Scattering */
-		float s_len = wlen * step;
-		vec3 Tr = exp(-s_extinction * s_len);
-
-		/* integrate along the current step segment */
-		Lscat = (Lscat - Lscat * Tr) / s_extinction;
-		/* accumulate and also take into account the transmittance from previous steps */
-		scattering += transmittance * Lscat;
-
-		/* Evaluate transmittance to view independantely */
-		transmittance *= Tr;
-
-		if (dist <= max_z)
-			break;
-	}
-
-#ifdef COLOR_TRANSMITTANCE
-	outScattering = vec4(scattering, 1.0);
-	outTransmittance = vec4(transmittance, 1.0);
-#else
-	float mono_transmittance = dot(transmittance, vec3(1.0)) / 3.0;
-
-	outScatteringTransmittance = vec4(scattering, mono_transmittance);
-#endif
-}
-
-#else /* STEP_UPSAMPLE */
-
-out vec4 FragColor;
-
-uniform sampler2D depthFull;
-uniform sampler2D volumetricBuffer;
-
-uniform mat4 ProjectionMatrix;
-
-vec4 get_view_z_from_depth(vec4 depth)
-{
-	vec4 d = 2.0 * depth - 1.0;
-	return -ProjectionMatrix[3][2] / (d + ProjectionMatrix[2][2]);
-}
-
-void main()
-{
-#if 0 /* 2 x 2 with bilinear */
-
-	const vec4 bilinear_weights[4] = vec4[4](
-		vec4(9.0 / 16.0,  3.0 / 16.0, 3.0 / 16.0, 1.0 / 16.0 ),
-		vec4(3.0 / 16.0,  9.0 / 16.0, 1.0 / 16.0, 3.0 / 16.0 ),
-		vec4(3.0 / 16.0,  1.0 / 16.0, 9.0 / 16.0, 3.0 / 16.0 ),
-		vec4(1.0 / 16.0,  3.0 / 16.0, 3.0 / 16.0, 9.0 / 16.0 )
-	);
-
-	/* Depth aware upsampling */
-	vec4 depths;
-	ivec2 texel_co = ivec2(gl_FragCoord.xy * 0.5) * 2;
-
-	/* TODO use textureGather on glsl 4.0 */
-	depths.x = texelFetchOffset(depthFull, texel_co, 0, ivec2(0, 0)).r;
-	depths.y = texelFetchOffset(depthFull, texel_co, 0, ivec2(2, 0)).r;
-	depths.z = texelFetchOffset(depthFull, texel_co, 0, ivec2(0, 2)).r;
-	depths.w = texelFetchOffset(depthFull, texel_co, 0, ivec2(2, 2)).r;
-
-	vec4 target_depth = texelFetch(depthFull, ivec2(gl_FragCoord.xy), 0).rrrr;
-
-	depths = get_view_z_from_depth(depths);
-	target_depth = get_view_z_from_depth(target_depth);
-
-	vec4 weights = 1.0 - step(0.05, abs(depths - target_depth));
-
-	/* Index in range [0-3] */
-	int pix_id = int(dot(mod(ivec2(gl_FragCoord.xy), 2), ivec2(1, 2)));
-	weights *= bilinear_weights[pix_id];
-
-	float weight_sum = dot(weights, vec4(1.0));
-
-	if (weight_sum == 0.0) {
-		weights.x = 1.0;
-		weight_sum = 1.0;
-	}
-
-	texel_co = ivec2(gl_FragCoord.xy * 0.5);
-
-	vec4 integration_result;
-	integration_result  = texelFetchOffset(volumetricBuffer, texel_co, 0, ivec2(0, 0)) * weights.x;
-	integration_result += texelFetchOffset(volumetricBuffer, texel_co, 0, ivec2(1, 0)) * weights.y;
-	integration_result += texelFetchOffset(volumetricBuffer, texel_co, 0, ivec2(0, 1)) * weights.z;
-	integration_result += texelFetchOffset(volumetricBuffer, texel_co, 0, ivec2(1, 1)) * weights.w;
-
-#else /* 4 x 4 */
-
-	/* Depth aware upsampling */
-	vec4 depths[4];
-	ivec2 texel_co = ivec2(gl_FragCoord.xy * 0.5) * 2;
-
-	/* TODO use textureGather on glsl 4.0 */
-	texel_co += ivec2(-2, -2);
-	depths[0].x = texelFetchOffset(depthFull, texel_co, 0, ivec2(0, 0)).r;
-	depths[0].y = texelFetchOffset(depthFull, texel_co, 0, ivec2(2, 0)).r;
-	depths[0].z = texelFetchOffset(depthFull, texel_co, 0, ivec2(0, 2)).r;
-	depths[0].w = texelFetchOffset(depthFull, texel_co, 0, ivec2(2, 2)).r;
-
-	texel_co += ivec2(4, 0);
-	depths[1].x = texelFetchOffset(depthFull, texel_co, 0, ivec2(0, 0)).r;
-	depths[1].y = texelFetchOffset(depthFull, texel_co, 0, ivec2(2, 0)).r;
-	depths[1].z = texelFetchOffset(depthFull, texel_co, 0, ivec2(0, 2)).r;
-	depths[1].w = texelFetchOffset(depthFull, texel_co, 0, ivec2(2, 2)).r;
-
-	texel_co += ivec2(-4, 4);
-	depths[2].x = texelFetchOffset(depthFull, texel_co, 0, ivec2(0, 0)).r;
-	depths[2].y = texelFetchOffset(depthFull, texel_co, 0, ivec2(2, 0)).r;
-	depths[2].z = texelFetchOffset(depthFull, texel_co, 0, ivec2(0, 2)).r;
-	depths[2].w = texelFetchOffset(depthFull, texel_co, 0, ivec2(2, 2)).r;
-
-	texel_co += ivec2(4, 0);
-	depths[3].x = texelFetchOffset(depthFull, texel_co, 0, ivec2(0, 0)).r;
-	depths[3].y = texelFetchOffset(depthFull, texel_co, 0, ivec2(2, 0)).r;
-	depths[3].z = texelFetchOffset(depthFull, texel_co, 0, ivec2(0, 2)).r;
-	depths[3].w = texelFetchOffset(depthFull, texel_co, 0, ivec2(2, 2)).r;
-
-	vec4 target_depth = texelFetch(depthFull, ivec2(gl_FragCoord.xy), 0).rrrr;
-
-	depths[0] = get_view_z_from_depth(depths[0]);
-	depths[1] = get_view_z_from_depth(depths[1]);
-	depths[2] = get_view_z_from_depth(depths[2]);
-	depths[3] = get_view_z_from_depth(depths[3]);
-
-	target_depth = get_view_z_from_depth(target_depth);
-
-	vec4 weights[4];
-	weights[0] = 1.0 - step(0.05, abs(depths[0] - target_depth));
-	weights[1] = 1.0 - step(0.05, abs(depths[1] - target_depth));
-	weights[2] = 1.0 - step(0.05, abs(depths[2] - target_depth));
-	weights[3] = 1.0 - step(0.05, abs(depths[3] - target_depth));
-
-	float weight_sum;
-	weight_sum  = dot(weights[0], vec4(1.0));
-	weight_sum += dot(weights[1], vec4(1.0));
-	weight_sum += dot(weights[2], vec4(1.0));
-	weight_sum += dot(weights[3], vec4(1.0));
-
-	if (weight_sum == 0.0) {
-		weights[0].x = 1.0;
-		weight_sum = 1.0;
-	}
-
-	texel_co = ivec2(gl_FragCoord.xy * 0.5);
-
-	vec4 integration_result;
-
-	texel_co += ivec2(-1, -1);
-	integration_result  = texelFetchOffset(volumetricBuffer, texel_co, 0, ivec2(0, 0)) * weights[0].x;
-	integration_result += texelFetchOffset(volumetricBuffer, texel_co, 0, ivec2(1, 0)) * weights[0].y;
-	integration_result += texelFetchOffset(volumetricBuffer, texel_co, 0, ivec2(0, 1)) * weights[0].z;
-	integration_result += texelFetchOffset(volumetricBuffer, texel_co, 0, ivec2(1, 1)) * weights[0].w;
-
-	texel_co += ivec2(2, 0);
-	integration_result += texelFetchOffset(volumetricBuffer, texel_co, 0, ivec2(0, 0)) * weights[1].x;
-	integration_result += texelFetchOffset(volumetricBuffer, texel_co, 0, ivec2(1, 0)) * weights[1].y;
-	integration_result += texelFetchOffset(volumetricBuffer, texel_co, 0, ivec2(0, 1)) * weights[1].z;
-	integration_result += texelFetchOffset(volumetricBuffer, texel_co, 0, ivec2(1, 1)) * weights[1].w;
-
-	texel_co += ivec2(-2, 2);
-	integration_result += texelFetchOffset(volumetricBuffer, texel_co, 0, ivec2(0, 0)) * weights[2].x;
-	integration_result += texelFetchOffset(volumetricBuffer, texel_co, 0, ivec2(1, 0)) * weights[2].y;
-	integration_result += texelFetchOffset(volumetricBuffer, texel_co, 0, ivec2(0, 1)) * weights[2].z;
-	integration_result += texelFetchOffset(volumetricBuffer, texel_co, 0, ivec2(1, 1)) * weights[2].w;
-
-	texel_co += ivec2(2, 0);
-	integration_result += texelFetchOffset(volumetricBuffer, texel_co, 0, ivec2(0, 0)) * weights[3].x;
-	integration_result += texelFetchOffset(volumetricBuffer, texel_co, 0, ivec2(1, 0)) * weights[3].y;
-	integration_result += texelFetchOffset(volumetricBuffer, texel_co, 0, ivec2(0, 1)) * weights[3].z;
-	integration_result += texelFetchOffset(volumetricBuffer, texel_co, 0, ivec2(1, 1)) * weights[3].w;
-#endif
+	Closure cl = nodetree_exec();
 
-	FragColor = integration_result / weight_sum;
+	volumeScattering = vec4(cl.scatter, 1.0);
+	volumeExtinction = vec4(max(vec3(1e-4), cl.absorption + cl.scatter), 1.0);
+	volumeEmissive = vec4(cl.emission, 1.0);
+	volumePhase = vec4(cl.anisotropy, vec3(1.0));
 }
-#endif