Add motion vector support for GPU 3D Particles.

Add the capability of resizing the transforms buffer for particles to be double its size and alternate where the current output is written to. Only works for particles that use index as their draw order.

2 files changed, 84 insertions, 16 deletions

diff --git a/servers/rendering/renderer_rd/storage_rd/particles_storage.cpp b/servers/rendering/renderer_rd/storage_rd/particles_storage.cpp
index 0a91672544..3a415e97e0 100644
--- a/servers/rendering/renderer_rd/storage_rd/particles_storage.cpp
+++ b/servers/rendering/renderer_rd/storage_rd/particles_storage.cpp
@@ -666,6 +666,19 @@ RID ParticlesStorage::particles_get_draw_pass_mesh(RID p_particles, int p_pass)
 	return particles->draw_passes[p_pass];
 }
 
+void ParticlesStorage::particles_update_dependency(RID p_particles, DependencyTracker *p_instance) {
+	Particles *particles = particles_owner.get_or_null(p_particles);
+	ERR_FAIL_COND(!particles);
+	p_instance->update_dependency(&particles->dependency);
+}
+
+void ParticlesStorage::particles_get_instance_buffer_motion_vectors_offsets(RID p_particles, uint32_t &r_current_offset, uint32_t &r_prev_offset) {
+	Particles *particles = particles_owner.get_or_null(p_particles);
+	ERR_FAIL_COND(!particles);
+	r_current_offset = particles->instance_motion_vectors_current_offset;
+	r_prev_offset = particles->instance_motion_vectors_previous_offset;
+}
+
 void ParticlesStorage::particles_add_collision(RID p_particles, RID p_particles_collision_instance) {
 	Particles *particles = particles_owner.get_or_null(p_particles);
 	ERR_FAIL_COND(!particles);
@@ -1185,6 +1198,7 @@ void ParticlesStorage::particles_set_view_axis(RID p_particles, const Vector3 &p
 	copy_push_constant.order_by_lifetime = (particles->draw_order == RS::PARTICLES_DRAW_ORDER_LIFETIME || particles->draw_order == RS::PARTICLES_DRAW_ORDER_REVERSE_LIFETIME);
 	copy_push_constant.lifetime_split = (MIN(int(particles->amount * particles->phase), particles->amount - 1) + 1) % particles->amount;
 	copy_push_constant.lifetime_reverse = particles->draw_order == RS::PARTICLES_DRAW_ORDER_REVERSE_LIFETIME;
+	copy_push_constant.motion_vectors_current_offset = particles->instance_motion_vectors_current_offset;
 
 	copy_push_constant.frame_remainder = particles->interpolate ? particles->frame_remainder : 0.0;
 	copy_push_constant.total_particles = particles->amount;
@@ -1252,28 +1266,50 @@ void ParticlesStorage::_particles_update_buffers(Particles *particles) {
 		userdata_count = particle_shader_data->userdata_count;
 	}
 
+	bool uses_motion_vectors = RSG::viewport->get_num_viewports_with_motion_vectors() > 0;
+	bool index_draw_order = particles->draw_order == RS::ParticlesDrawOrder::PARTICLES_DRAW_ORDER_INDEX;
+	bool enable_motion_vectors = uses_motion_vectors && index_draw_order && !particles->instance_motion_vectors_enabled;
+	bool only_instances_changed = false;
+
 	if (userdata_count != particles->userdata_count) {
-		// Mismatch userdata, re-create buffers.
+		// Mismatch userdata, re-create all buffers.
 		_particles_free_data(particles);
+	} else if (enable_motion_vectors) {
+		// Only motion vectors are required, release the transforms buffer and uniform set.
+		if (particles->particle_instance_buffer.is_valid()) {
+			RD::get_singleton()->free(particles->particle_instance_buffer);
+			particles->particle_instance_buffer = RID();
+		}
+
+		particles->particles_transforms_buffer_uniform_set = RID();
+		only_instances_changed = true;
+	} else if (!particles->particle_buffer.is_null()) {
+		// No operation is required because a buffer already exists, return early.
+		return;
 	}
 
-	if (particles->amount > 0 && particles->particle_buffer.is_null()) {
+	if (particles->amount > 0) {
 		int total_amount = particles->amount;
 		if (particles->trails_enabled && particles->trail_bind_poses.size() > 1) {
 			total_amount *= particles->trail_bind_poses.size();
 		}
 
 		uint32_t xform_size = particles->mode == RS::PARTICLES_MODE_2D ? 2 : 3;
-
-		particles->particle_buffer = RD::get_singleton()->storage_buffer_create((sizeof(ParticleData) + userdata_count * sizeof(float) * 4) * total_amount);
-
-		particles->userdata_count = userdata_count;
+		if (particles->particle_buffer.is_null()) {
+			particles->particle_buffer = RD::get_singleton()->storage_buffer_create((sizeof(ParticleData) + userdata_count * sizeof(float) * 4) * total_amount);
+			particles->userdata_count = userdata_count;
+		}
 
 		PackedByteArray data;
-		data.resize_zeroed(sizeof(float) * 4 * (xform_size + 1 + 1) * total_amount);
+		uint32_t particle_instance_buffer_size = total_amount * (xform_size + 1 + 1) * sizeof(float) * 4;
+		if (uses_motion_vectors) {
+			particle_instance_buffer_size *= 2;
+			particles->instance_motion_vectors_enabled = true;
+		}
 
-		particles->particle_instance_buffer = RD::get_singleton()->storage_buffer_create(sizeof(float) * 4 * (xform_size + 1 + 1) * total_amount, data);
-		//needs to clear it
+		data.resize_zeroed(particle_instance_buffer_size);
+
+		particles->particle_instance_buffer = RD::get_singleton()->storage_buffer_create(particle_instance_buffer_size, data);
 
 		{
 			Vector<RD::Uniform> uniforms;
@@ -1295,9 +1331,20 @@ void ParticlesStorage::_particles_update_buffers(Particles *particles) {
 
 			particles->particles_copy_uniform_set = RD::get_singleton()->uniform_set_create(uniforms, particles_shader.copy_shader.version_get_shader(particles_shader.copy_shader_version, 0), 0);
 		}
+
+		particles->instance_motion_vectors_current_offset = 0;
+		particles->instance_motion_vectors_previous_offset = 0;
+		particles->instance_motion_vectors_last_change = -1;
+
+		if (only_instances_changed) {
+			// Notify the renderer the instances uniform must be retrieved again, as it's the only element that has been changed because motion vectors were enabled.
+			particles->dependency.changed_notify(Dependency::DEPENDENCY_CHANGED_PARTICLES_INSTANCES);
+		}
 	}
 }
 void ParticlesStorage::update_particles() {
+	uint32_t frame = RSG::rasterizer->get_frame_number();
+	bool uses_motion_vectors = RSG::viewport->get_num_viewports_with_motion_vectors() > 0;
 	while (particle_update_list) {
 		//use transform feedback to process particles
 
@@ -1461,16 +1508,25 @@ void ParticlesStorage::update_particles() {
 		// Ensure that memory is initialized (the code above should ensure that _particles_process is always called at least once upon clearing).
 		DEV_ASSERT(!particles->clear);
 
+		int total_amount = particles->amount;
+		if (particles->trails_enabled && particles->trail_bind_poses.size() > 1) {
+			total_amount *= particles->trail_bind_poses.size();
+		}
+
+		// Swap offsets for motion vectors. Motion vectors can only be used when the draw order keeps the indices consistent across frames.
+		bool index_draw_order = particles->draw_order == RS::ParticlesDrawOrder::PARTICLES_DRAW_ORDER_INDEX;
+		particles->instance_motion_vectors_previous_offset = particles->instance_motion_vectors_current_offset;
+		if (uses_motion_vectors && index_draw_order && particles->instance_motion_vectors_enabled && (frame - particles->instance_motion_vectors_last_change) == 1) {
+			particles->instance_motion_vectors_current_offset = total_amount - particles->instance_motion_vectors_current_offset;
+		}
+
+		particles->instance_motion_vectors_last_change = frame;
+
 		// Copy particles to instance buffer.
 		if (particles->draw_order != RS::PARTICLES_DRAW_ORDER_VIEW_DEPTH && particles->transform_align != RS::PARTICLES_TRANSFORM_ALIGN_Z_BILLBOARD && particles->transform_align != RS::PARTICLES_TRANSFORM_ALIGN_Z_BILLBOARD_Y_TO_VELOCITY) {
 			//does not need view dependent operation, do copy here
 			ParticlesShader::CopyPushConstant copy_push_constant;
 
-			int total_amount = particles->amount;
-			if (particles->trails_enabled && particles->trail_bind_poses.size() > 1) {
-				total_amount *= particles->trail_bind_poses.size();
-			}
-
 			// Affect 2D only.
 			if (particles->use_local_coords) {
 				// In local mode, particle positions are calculated locally (relative to the node position)
@@ -1506,6 +1562,7 @@ void ParticlesStorage::update_particles() {
 			copy_push_constant.order_by_lifetime = (particles->draw_order == RS::PARTICLES_DRAW_ORDER_LIFETIME || particles->draw_order == RS::PARTICLES_DRAW_ORDER_REVERSE_LIFETIME);
 			copy_push_constant.lifetime_split = (MIN(int(particles->amount * particles->phase), particles->amount - 1) + 1) % particles->amount;
 			copy_push_constant.lifetime_reverse = particles->draw_order == RS::PARTICLES_DRAW_ORDER_REVERSE_LIFETIME;
+			copy_push_constant.motion_vectors_current_offset = particles->instance_motion_vectors_current_offset;
 
 			RD::ComputeListID compute_list = RD::get_singleton()->compute_list_begin();
 			copy_push_constant.copy_mode_2d = particles->mode == RS::PARTICLES_MODE_2D ? 1 : 0;
diff --git a/servers/rendering/renderer_rd/storage_rd/particles_storage.h b/servers/rendering/renderer_rd/storage_rd/particles_storage.h
index 612420a36c..7cea706a8c 100644
--- a/servers/rendering/renderer_rd/storage_rd/particles_storage.h
+++ b/servers/rendering/renderer_rd/storage_rd/particles_storage.h
@@ -225,6 +225,11 @@ private:
 		double frame_remainder = 0;
 		real_t collision_base_size = 0.01;
 
+		uint32_t instance_motion_vectors_current_offset = 0;
+		uint32_t instance_motion_vectors_previous_offset = 0;
+		uint64_t instance_motion_vectors_last_change = -1;
+		bool instance_motion_vectors_enabled = false;
+
 		bool clear = true;
 
 		bool force_sub_emit = false;
@@ -288,10 +293,13 @@ private:
 			float align_up[3];
 			uint32_t align_mode;
 
-			uint32_t order_by_lifetime;
 			uint32_t lifetime_split;
 			uint32_t lifetime_reverse;
-			uint32_t copy_mode_2d;
+			uint32_t motion_vectors_current_offset;
+			struct {
+				uint32_t order_by_lifetime : 1;
+				uint32_t copy_mode_2d : 1;
+			};
 
 			float inv_emission_transform[16];
 		};
@@ -521,12 +529,15 @@ public:
 		return particles->particles_transforms_buffer_uniform_set;
 	}
 
+	void particles_get_instance_buffer_motion_vectors_offsets(RID p_particles, uint32_t &r_current_offset, uint32_t &r_prev_offset);
+
 	virtual void particles_add_collision(RID p_particles, RID p_particles_collision_instance) override;
 	virtual void particles_remove_collision(RID p_particles, RID p_particles_collision_instance) override;
 	void particles_set_canvas_sdf_collision(RID p_particles, bool p_enable, const Transform2D &p_xform, const Rect2 &p_to_screen, RID p_texture);
 
 	virtual void update_particles() override;
 
+	void particles_update_dependency(RID p_particles, DependencyTracker *p_instance);
 	Dependency *particles_get_dependency(RID p_particles) const;
 
 	/* Particles Collision */