Cycles: Use curve approximation for blackbody instead of lookup table

Now we calculate color in range 800..12000 using an approximation a/x+bx+c for R and G and ((at + b)t + c)t + d) for B.
Max absolute error for RGB for non-lut function is less than 0.0001, which is enough to get the same 8 bit/channel color as for OSL with a noticeable performance difference.
However there is a slight visible difference between previous non-OSL implementation because of lookup table interpolation and offset-by-one mistake.
The previous implementation gave black color outside of soft range (t > 12000), now it gives the same color as for 12000.

Also blackbody node without input connected is being converted to value input at shader compile time.

Reviewers: dingto, sergey

Reviewed By: dingto

Subscribers: nutel, brecht, juicyfruit

Differential Revision: https://developer.blender.org/D1280

1 files changed, 10 insertions, 2 deletions

diff --git a/intern/cycles/render/nodes.cpp b/intern/cycles/render/nodes.cpp
index 8529855ff2e..d17d531aa77 100644
--- a/intern/cycles/render/nodes.cpp
+++ b/intern/cycles/render/nodes.cpp
@@ -3652,9 +3652,17 @@ void BlackbodyNode::compile(SVMCompiler& compiler)
 	ShaderInput *temperature_in = input("Temperature");
 	ShaderOutput *color_out = output("Color");
 
-	compiler.stack_assign(temperature_in);
 	compiler.stack_assign(color_out);
-	compiler.add_node(NODE_BLACKBODY, temperature_in->stack_offset, color_out->stack_offset);
+
+	if(temperature_in->link == NULL) {
+		float3 color = svm_math_blackbody_color(temperature_in->value.x);
+		compiler.add_node(NODE_VALUE_V, color_out->stack_offset);
+		compiler.add_node(NODE_VALUE_V, color);
+	}
+	else {
+		compiler.stack_assign(temperature_in);
+		compiler.add_node(NODE_BLACKBODY, temperature_in->stack_offset, color_out->stack_offset);
+	}
 }
 
 void BlackbodyNode::compile(OSLCompiler& compiler)