Geometry Nodes: use virtual arrays in internal attribute api

A virtual array is a data structure that is similar to a normal array
in that its elements can be accessed by an index. However, a virtual
array does not have to be a contiguous array internally. Instead, its
elements can be layed out arbitrarily while element access happens
through a virtual function call. However, the virtual array data
structures are designed so that the virtual function call can be avoided
in cases where it could become a bottleneck.

Most commonly, a virtual array is backed by an actual array/span or
is a single value internally, that is the same for every index.
Besides those, there are many more specialized virtual arrays like the
ones that provides vertex positions based on the `MVert` struct or
vertex group weights.

Not all attributes used by geometry nodes are stored in simple contiguous
arrays. To provide uniform access to all kinds of attributes, the attribute
API has to provide virtual array functionality that hides the implementation
details of attributes.

Before this refactor, the attribute API provided its own virtual array
implementation as part of the `ReadAttribute` and `WriteAttribute` types.
That resulted in unnecessary code duplication with the virtual array system.
Even worse, it bound many algorithms used by geometry nodes to the specifics
of the attribute API, even though they could also use different data sources
(such as data from sockets, default values, later results of expressions, ...).

This refactor removes the `ReadAttribute` and `WriteAttribute` types and
replaces them with `GVArray` and `GVMutableArray` respectively. The `GV`
stands for "generic virtual". The "generic" means that the data type contained
in those virtual arrays is only known at run-time. There are the corresponding
statically typed types `VArray<T>` and `VMutableArray<T>` as well.

No regressions are expected from this refactor. It does come with one
improvement for users. The attribute API can convert the data type
on write now. This is especially useful when writing to builtin attributes
like `material_index` with e.g. the Attribute Math node (which usually
just writes to float attributes, while `material_index` is an integer attribute).

Differential Revision: https://developer.blender.org/D10994

1 files changed, 23 insertions, 40 deletions

diff --git a/source/blender/nodes/geometry/nodes/node_geo_attribute_math.cc b/source/blender/nodes/geometry/nodes/node_geo_attribute_math.cc
index 5ee31e78be2..99ba6acd94d 100644
--- a/source/blender/nodes/geometry/nodes/node_geo_attribute_math.cc
+++ b/source/blender/nodes/geometry/nodes/node_geo_attribute_math.cc
@@ -149,9 +149,9 @@ static void geo_node_attribute_math_update(bNodeTree *UNUSED(ntree), bNode *node
       operation_use_input_c(operation));
 }
 
-static void do_math_operation(Span<float> span_a,
-                              Span<float> span_b,
-                              Span<float> span_c,
+static void do_math_operation(const VArray<float> &span_a,
+                              const VArray<float> &span_b,
+                              const VArray<float> &span_c,
                               MutableSpan<float> span_result,
                               const NodeMathOperation operation)
 {
@@ -165,8 +165,8 @@ static void do_math_operation(Span<float> span_a,
   UNUSED_VARS_NDEBUG(success);
 }
 
-static void do_math_operation(Span<float> span_a,
-                              Span<float> span_b,
+static void do_math_operation(const VArray<float> &span_a,
+                              const VArray<float> &span_b,
                               MutableSpan<float> span_result,
                               const NodeMathOperation operation)
 {
@@ -180,7 +180,7 @@ static void do_math_operation(Span<float> span_a,
   UNUSED_VARS_NDEBUG(success);
 }
 
-static void do_math_operation(Span<float> span_input,
+static void do_math_operation(const VArray<float> &span_input,
                               MutableSpan<float> span_result,
                               const NodeMathOperation operation)
 {
@@ -200,9 +200,9 @@ static AttributeDomain get_result_domain(const GeometryComponent &component,
                                          StringRef result_name)
 {
   /* Use the domain of the result attribute if it already exists. */
-  ReadAttributePtr result_attribute = component.attribute_try_get_for_read(result_name);
+  ReadAttributeLookup result_attribute = component.attribute_try_get_for_read(result_name);
   if (result_attribute) {
-    return result_attribute->domain();
+    return result_attribute.domain;
   }
 
   /* Otherwise use the highest priority domain from existing input attributes, or the default. */
@@ -224,56 +224,39 @@ static void attribute_math_calc(GeometryComponent &component, const GeoNodeExecP
   const std::string result_name = params.get_input<std::string>("Result");
 
   /* The result type of this node is always float. */
-  const CustomDataType result_type = CD_PROP_FLOAT;
   const AttributeDomain result_domain = get_result_domain(
       component, params, operation, result_name);
 
-  OutputAttributePtr attribute_result = component.attribute_try_get_for_output(
-      result_name, result_domain, result_type);
+  OutputAttribute_Typed<float> attribute_result =
+      component.attribute_try_get_for_output_only<float>(result_name, result_domain);
   if (!attribute_result) {
     return;
   }
 
-  ReadAttributePtr attribute_a = params.get_input_attribute(
-      "A", component, result_domain, result_type, nullptr);
-  if (!attribute_a) {
-    return;
-  }
+  GVArray_Typed<float> attribute_a = params.get_input_attribute<float>(
+      "A", component, result_domain, 0.0f);
 
-  /* Note that passing the data with `get_span<float>()` works
+  MutableSpan<float> result_span = attribute_result.as_span();
+
+  /* Note that passing the data with `get_internal_span<float>()` works
    * because the attributes were accessed with #CD_PROP_FLOAT. */
   if (operation_use_input_b(operation)) {
-    ReadAttributePtr attribute_b = params.get_input_attribute(
-        "B", component, result_domain, result_type, nullptr);
-    if (!attribute_b) {
-      return;
-    }
+    GVArray_Typed<float> attribute_b = params.get_input_attribute<float>(
+        "B", component, result_domain, 0.0f);
     if (operation_use_input_c(operation)) {
-      ReadAttributePtr attribute_c = params.get_input_attribute(
-          "C", component, result_domain, result_type, nullptr);
-      if (!attribute_c) {
-        return;
-      }
-      do_math_operation(attribute_a->get_span<float>(),
-                        attribute_b->get_span<float>(),
-                        attribute_c->get_span<float>(),
-                        attribute_result->get_span_for_write_only<float>(),
-                        operation);
+      GVArray_Typed<float> attribute_c = params.get_input_attribute<float>(
+          "C", component, result_domain, 0.0f);
+      do_math_operation(attribute_a, attribute_b, attribute_c, result_span, operation);
     }
     else {
-      do_math_operation(attribute_a->get_span<float>(),
-                        attribute_b->get_span<float>(),
-                        attribute_result->get_span_for_write_only<float>(),
-                        operation);
+      do_math_operation(attribute_a, attribute_b, result_span, operation);
     }
   }
   else {
-    do_math_operation(attribute_a->get_span<float>(),
-                      attribute_result->get_span_for_write_only<float>(),
-                      operation);
+    do_math_operation(attribute_a, result_span, operation);
   }
 
-  attribute_result.apply_span_and_save();
+  attribute_result.save();
 }
 
 static void geo_node_attribute_math_exec(GeoNodeExecParams params)