From b79e5cf1a3c07cad4edcad2c909318a5a5b6115e Mon Sep 17 00:00:00 2001
From: "Claudius \"keldu\" Holeksa" <mail@keldu.de>
Date: Thu, 5 Feb 2026 18:05:18 +0100
Subject: Fixing boundary conditions

---
 examples/poiseulle_particles_2d_gpu/sim.cpp |  6 +++---
 lib/core/c++/boundary.hpp                   |  2 +-
 lib/core/c++/write_vtk.hpp                  | 12 +++++++-----
 3 files changed, 11 insertions(+), 9 deletions(-)

diff --git a/examples/poiseulle_particles_2d_gpu/sim.cpp b/examples/poiseulle_particles_2d_gpu/sim.cpp
index bdd93b0..305ef80 100644
--- a/examples/poiseulle_particles_2d_gpu/sim.cpp
+++ b/examples/poiseulle_particles_2d_gpu/sim.cpp
@@ -7,8 +7,8 @@
 namespace kel {
 namespace lbm {
 
-constexpr uint64_t dim_x = 1024u;
-constexpr uint64_t dim_y = 512u;
+constexpr uint64_t dim_x = 2048u;
+constexpr uint64_t dim_y = 1024u;
 
 namespace sch {
 using namespace saw::schema;
@@ -119,7 +119,7 @@ saw::error_or<void> step(
 		// Need nicer things to handle the flow. I see improvement here
 		component<T,Desc,cmpt::BGK,encode::Sycl<saw::encode::Native>> collision{0.6};
 		component<T,Desc,cmpt::BounceBack,encode::Sycl<saw::encode::Native>> bb;
-		component<T,Desc,cmpt::ZouHeHorizontal<true>,encode::Sycl<saw::encode::Native>> flow_in{1.001};
+		component<T,Desc,cmpt::ZouHeHorizontal<true>,encode::Sycl<saw::encode::Native>> flow_in{1.01};
 		component<T,Desc,cmpt::ZouHeHorizontal<false>,encode::Sycl<saw::encode::Native>> flow_out{1.0};
 
 		h.parallel_for(acpp::sycl::range<Desc::D>{dim_x,dim_y}, [=](acpp::sycl::id<Desc::D> idx){
diff --git a/lib/core/c++/boundary.hpp b/lib/core/c++/boundary.hpp
index ec16edf..6d83c79 100644
--- a/lib/core/c++/boundary.hpp
+++ b/lib/core/c++/boundary.hpp
@@ -117,7 +117,7 @@ public:
 
 		for(saw::data<sch::UInt64> k{0u}; k < saw::data<sch::UInt64>{Descriptor::Q}; ++k){
 			auto c_k = dfi::directions[k.get()];
-			if(c_k[0u]*known_dir > 0){
+			if(c_k[0u]*known_dir < 0){
 				sum_unknown_dfs += dfs_old.at({k}) * c_k[0u];
 			}
 		}
diff --git a/lib/core/c++/write_vtk.hpp b/lib/core/c++/write_vtk.hpp
index 3c60a66..751ec50 100644
--- a/lib/core/c++/write_vtk.hpp
+++ b/lib/core/c++/write_vtk.hpp
@@ -67,12 +67,14 @@ struct lbm_vtk_writer<sch::FixedArray<T,D>> {
 
 template<typename T, uint64_t Ghost, uint64_t... D>
 struct lbm_vtk_writer<sch::Chunk<T,Ghost,D...>> {
+
 	template<uint64_t d>
 	static saw::error_or<void> apply_d(std::ostream& vtk_file, const saw::data<sch::Chunk<T,Ghost,D...>>& field, saw::data<sch::FixedArray<sch::UInt64,sizeof...(D)>>& index){
-
-		if constexpr ( d < sizeof...(D)){
-			for(index.at({d}) = 0u; index.at({d}) < field.get_dims().at({d}); ++index.at({d})){
-				auto eov = apply_d<d+1u>(vtk_file, field, index);
+		// VTK wants to iterate over z,y,x instead of x,y,z
+		// We could reorder the dimensions, but eh
+		if constexpr ( d > 0u){
+			for(index.at({d-1u}) = 0u; index.at({d-1u}) < field.get_dims().at({d-1u}); ++index.at({d-1u})){
+				auto eov = apply_d<d-1u>(vtk_file, field, index);
 			}
 		}else{
 			auto eov = lbm_vtk_writer<T>::apply(vtk_file, field.at(index));
@@ -94,7 +96,7 @@ struct lbm_vtk_writer<sch::Chunk<T,Ghost,D...>> {
 		}
 
 		{
-			auto eov = apply_d<0u>(vtk_file, field, index);
+			auto eov = apply_d<sizeof...(D)>(vtk_file, field, index);
 			if(eov.is_error()){
 				return eov;
 			}
-- 
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