Cycles: Implement extrapolation for RGB curves

Previously RGB Curves node will clamp input to 0..1 which is rather useless
when one wants to use HDR image textures and do bit of correction on them.

Now kernel code supports extrapolation of baked LUT based on first/last two
table points and performs linear extrapolation.

The only tricky part is to guess the range to bake the LUT for. Currently
it's using simple approach -- minmax of the input curves. While this behaves
ok for the simple cases it's easy to trick the system up causing incorrect
results.

Not sure we can solve those issues in a general case and since the new code
is giving more expected results it's not that bad actually. In the worst
case artist migh always create explicit point to make sure LUT is created
for the needed HDR range.

Reviewers: brecht, juicyfruit

Subscribers: sebastian_k

Differential Revision: https://developer.blender.org/D1658

1 files changed, 22 insertions, 3 deletions

diff --git a/intern/cycles/kernel/shaders/node_rgb_curves.osl b/intern/cycles/kernel/shaders/node_rgb_curves.osl
index 60cb273ba98..0427c001f6d 100644
--- a/intern/cycles/kernel/shaders/node_rgb_curves.osl
+++ b/intern/cycles/kernel/shaders/node_rgb_curves.osl
@@ -19,6 +19,21 @@
 
 float ramp_lookup(color ramp[RAMP_TABLE_SIZE], float at, int component)
 {
+	if (at < 0.0 || at > 1.0) {
+		float t0, dy;
+		if(at < 0.0) {
+			t0 = ramp[0][component];
+			dy = t0 - ramp[1][component];
+			at = -at;
+		}
+		else {
+			t0 = ramp[RAMP_TABLE_SIZE - 1][component];
+			dy = t0 - ramp[RAMP_TABLE_SIZE - 2][component];
+			at = at - 1.0;
+		}
+		return t0 + dy * at * (RAMP_TABLE_SIZE - 1);
+	}
+
 	float f = clamp(at, 0.0, 1.0) * (RAMP_TABLE_SIZE - 1);
 
 	/* clamp int as well in case of NaN */
@@ -37,14 +52,18 @@ float ramp_lookup(color ramp[RAMP_TABLE_SIZE], float at, int component)
 
 shader node_rgb_curves(
 	color ramp[RAMP_TABLE_SIZE] = {0.0},
+	float min_x = 0.0,
+	float max_x = 1.0,
 
 	color ColorIn = 0.0,
 	float Fac = 0.0,
 	output color ColorOut = 0.0)
 {
-	ColorOut[0] = ramp_lookup(ramp, ColorIn[0], 0);
-	ColorOut[1] = ramp_lookup(ramp, ColorIn[1], 1);
-	ColorOut[2] = ramp_lookup(ramp, ColorIn[2], 2);
+	color c = (ColorIn - color(min_x, min_x, min_x)) / (max_x - min_x);
+
+	ColorOut[0] = ramp_lookup(ramp, c[0], 0);
+	ColorOut[1] = ramp_lookup(ramp, c[1], 1);
+	ColorOut[2] = ramp_lookup(ramp, c[2], 2);
 
 	ColorOut = mix(ColorIn, ColorOut, Fac);
 }