Cycles: implement partial constant folding for exponentiation.

This is also an important mathematical operation that can be folded
if it is known that one argument is a certain constant. For colors
the operation is provided as a Gamma node.

The SVM Gamma node needs a small fix to make it follow the 0 ^ 0 == 1
rule, same as the Power node, or the Gamma node itself in OSL mode.

Reviewers: #cycles

Differential Revision: https://developer.blender.org/D2263

5 files changed, 139 insertions, 0 deletions

diff --git a/intern/cycles/kernel/svm/svm_math_util.h b/intern/cycles/kernel/svm/svm_math_util.h
index 6d13a0d8e02..01547b60014 100644
--- a/intern/cycles/kernel/svm/svm_math_util.h
+++ b/intern/cycles/kernel/svm/svm_math_util.h
@@ -164,6 +164,9 @@ ccl_device float3 svm_math_blackbody_color(float t) {
 
 ccl_device_inline float3 svm_math_gamma_color(float3 color, float gamma)
 {
+	if(gamma == 0.0f)
+		return make_float3(1.0f, 1.0f, 1.0f);
+
 	if(color.x > 0.0f)
 		color.x = powf(color.x, gamma);
 	if(color.y > 0.0f)
diff --git a/intern/cycles/render/constant_fold.cpp b/intern/cycles/render/constant_fold.cpp
index 200a4c497cd..b7f25663bc3 100644
--- a/intern/cycles/render/constant_fold.cpp
+++ b/intern/cycles/render/constant_fold.cpp
@@ -89,6 +89,19 @@ void ConstantFolder::make_zero() const
 	}
 }
 
+void ConstantFolder::make_one() const
+{
+	if(output->type() == SocketType::FLOAT) {
+		make_constant(1.0f);
+	}
+	else if(SocketType::is_float3(output->type())) {
+		make_constant(make_float3(1.0f, 1.0f, 1.0f));
+	}
+	else {
+		assert(0);
+	}
+}
+
 void ConstantFolder::bypass(ShaderOutput *new_output) const
 {
 	assert(new_output);
@@ -321,6 +334,15 @@ void ConstantFolder::fold_math(NodeMath type, bool clamp) const
 				make_zero();
 			}
 			break;
+		case NODE_MATH_POWER:
+			/* 1 ^ X == X ^ 0 == 1 */
+			if(is_one(value1_in) || is_zero(value2_in)) {
+				make_one();
+			}
+			/* X ^ 1 == X */
+			else if(is_one(value2_in)) {
+				try_bypass_or_make_constant(value1_in, clamp);
+			}
 		default:
 			break;
 	}
diff --git a/intern/cycles/render/constant_fold.h b/intern/cycles/render/constant_fold.h
index 2b31c2a5887..7962698319f 100644
--- a/intern/cycles/render/constant_fold.h
+++ b/intern/cycles/render/constant_fold.h
@@ -43,6 +43,7 @@ public:
 	void make_constant_clamp(float value, bool clamp) const;
 	void make_constant_clamp(float3 value, bool clamp) const;
 	void make_zero() const;
+	void make_one() const;
 
 	/* Bypass node, relinking to another output socket. */
 	void bypass(ShaderOutput *output) const;
diff --git a/intern/cycles/render/nodes.cpp b/intern/cycles/render/nodes.cpp
index df43863ea7a..7ea52b28b9c 100644
--- a/intern/cycles/render/nodes.cpp
+++ b/intern/cycles/render/nodes.cpp
@@ -3896,6 +3896,19 @@ void GammaNode::constant_fold(const ConstantFolder& folder)
 	if(folder.all_inputs_constant()) {
 		folder.make_constant(svm_math_gamma_color(color, gamma));
 	}
+	else {
+		ShaderInput *color_in = input("Color");
+		ShaderInput *gamma_in = input("Gamma");
+
+		/* 1 ^ X == X ^ 0 == 1 */
+		if(folder.is_one(color_in) || folder.is_zero(gamma_in)) {
+			folder.make_one();
+		}
+		/* X ^ 1 == X */
+		else if(folder.is_one(gamma_in)) {
+			folder.try_bypass_or_make_constant(color_in, false);
+		}
+	}
 }
 
 void GammaNode::compile(SVMCompiler& compiler)
diff --git a/intern/cycles/test/render_graph_finalize_test.cpp b/intern/cycles/test/render_graph_finalize_test.cpp
index 60e41be16aa..6f1c0b88b51 100644
--- a/intern/cycles/test/render_graph_finalize_test.cpp
+++ b/intern/cycles/test/render_graph_finalize_test.cpp
@@ -931,6 +931,72 @@ TEST(render_graph, constant_fold_gamma)
 }
 
 /*
+ * Tests: Gamma with one constant 0 input.
+ */
+TEST(render_graph, constant_fold_gamma_part_0)
+{
+	DEFINE_COMMON_VARIABLES(builder, log);
+
+	EXPECT_ANY_MESSAGE(log);
+	INVALID_INFO_MESSAGE(log, "Folding Gamma_Cx::");
+	CORRECT_INFO_MESSAGE(log, "Folding Gamma_xC::Color to constant (1, 1, 1).");
+
+	builder
+		.add_attribute("Attribute")
+		/* constant on the left */
+		.add_node(ShaderNodeBuilder<GammaNode>("Gamma_Cx")
+		          .set("Color", make_float3(0.0f, 0.0f, 0.0f)))
+		.add_connection("Attribute::Fac", "Gamma_Cx::Gamma")
+		/* constant on the right */
+		.add_node(ShaderNodeBuilder<GammaNode>("Gamma_xC")
+		          .set("Gamma", 0.0f))
+		.add_connection("Attribute::Color", "Gamma_xC::Color")
+		/* output sum */
+		.add_node(ShaderNodeBuilder<MixNode>("Out")
+		          .set(&MixNode::type, NODE_MIX_ADD)
+		          .set(&MixNode::use_clamp, true)
+		          .set("Fac", 1.0f))
+		.add_connection("Gamma_Cx::Color", "Out::Color1")
+		.add_connection("Gamma_xC::Color", "Out::Color2")
+		.output_color("Out::Color");
+
+	graph.finalize(&scene);
+}
+
+/*
+ * Tests: Gamma with one constant 1 input.
+ */
+TEST(render_graph, constant_fold_gamma_part_1)
+{
+	DEFINE_COMMON_VARIABLES(builder, log);
+
+	EXPECT_ANY_MESSAGE(log);
+	CORRECT_INFO_MESSAGE(log, "Folding Gamma_Cx::Color to constant (1, 1, 1).");
+	CORRECT_INFO_MESSAGE(log, "Folding Gamma_xC::Color to socket Attribute::Color.");
+
+	builder
+		.add_attribute("Attribute")
+		/* constant on the left */
+		.add_node(ShaderNodeBuilder<GammaNode>("Gamma_Cx")
+		          .set("Color", make_float3(1.0f, 1.0f, 1.0f)))
+		.add_connection("Attribute::Fac", "Gamma_Cx::Gamma")
+		/* constant on the right */
+		.add_node(ShaderNodeBuilder<GammaNode>("Gamma_xC")
+		          .set("Gamma", 1.0f))
+		.add_connection("Attribute::Color", "Gamma_xC::Color")
+		/* output sum */
+		.add_node(ShaderNodeBuilder<MixNode>("Out")
+		          .set(&MixNode::type, NODE_MIX_ADD)
+		          .set(&MixNode::use_clamp, true)
+		          .set("Fac", 1.0f))
+		.add_connection("Gamma_Cx::Color", "Out::Color1")
+		.add_connection("Gamma_xC::Color", "Out::Color2")
+		.output_color("Out::Color");
+
+	graph.finalize(&scene);
+}
+
+/*
  * Tests: BrightnessContrast with all constant inputs.
  */
 TEST(render_graph, constant_fold_bright_contrast)
@@ -1143,6 +1209,40 @@ TEST(render_graph, constant_fold_part_math_div_0)
 }
 
 /*
+ * Tests: partial folding for Math Power with known 0.
+ */
+TEST(render_graph, constant_fold_part_math_pow_0)
+{
+	DEFINE_COMMON_VARIABLES(builder, log);
+
+	EXPECT_ANY_MESSAGE(log);
+	/* X ^ 0 == 1 */
+	INVALID_INFO_MESSAGE(log, "Folding Math_Cx::");
+	CORRECT_INFO_MESSAGE(log, "Folding Math_xC::Value to constant (1).");
+	INVALID_INFO_MESSAGE(log, "Folding Out::");
+
+	build_math_partial_test_graph(builder, NODE_MATH_POWER, 0.0f);
+	graph.finalize(&scene);
+}
+
+/*
+ * Tests: partial folding for Math Power with known 1.
+ */
+TEST(render_graph, constant_fold_part_math_pow_1)
+{
+	DEFINE_COMMON_VARIABLES(builder, log);
+
+	EXPECT_ANY_MESSAGE(log);
+	/* 1 ^ X == 1; X ^ 1 == X */
+	CORRECT_INFO_MESSAGE(log, "Folding Math_Cx::Value to constant (1)");
+	CORRECT_INFO_MESSAGE(log, "Folding Math_xC::Value to socket Attribute::Fac.");
+	INVALID_INFO_MESSAGE(log, "Folding Out::");
+
+	build_math_partial_test_graph(builder, NODE_MATH_POWER, 1.0f);
+	graph.finalize(&scene);
+}
+
+/*
  * Tests: Vector Math with all constant inputs.
  */
 TEST(render_graph, constant_fold_vector_math)