From 444151a0c0c31eac99663c19dda87a7c543bee84 Mon Sep 17 00:00:00 2001
From: Mikko Rasa <tdb@tdb.fi>
Date: Mon, 28 Nov 2016 23:31:19 +0200
Subject: [PATCH] Improve the color curve postprocessor

---
 shaderlib/colorcurve.glsl | 27 +++++++++++----------------
 source/colorcurve.cpp     | 26 +++++++++++++++++---------
 source/colorcurve.h       | 17 ++++++++++-------
 3 files changed, 38 insertions(+), 32 deletions(-)

diff --git a/shaderlib/colorcurve.glsl b/shaderlib/colorcurve.glsl
index fa3ba510..63f63138 100644
--- a/shaderlib/colorcurve.glsl
+++ b/shaderlib/colorcurve.glsl
@@ -3,27 +3,22 @@ import postprocess;
 uniform sampler1D curve;
 uniform ToneMapping
 {
-	float peak;
-	float brightness;
+	float exposure;
+	vec3 brightness_response;
 };
 
 ////// fragment
 void main()
 {
-	vec4 sample = texture(source, texcoord);
-	float maxc = max(sample.r, max(sample.g, sample.b));
-	if(maxc>1.0-peak)
+	vec4 incoming = texture(source, texcoord);
+	float maxc = max(incoming.r, max(incoming.g, incoming.b));
+	if(maxc>0.0)
 	{
-		vec3 saturated = sample.rgb/maxc;
-		if(maxc>1.0+peak)
-		{
-			sample.rgb = mix(vec3(1.0), saturated, 1.0/pow(brightness, maxc-1.0-peak));
-		}
-		else
-		{
-			float x = (1.0+peak-maxc)/(2.0*peak);
-			sample.rgb = saturated.rgb*(1.0-peak+(1.0-x*x)*peak);
-		}
+		vec3 saturated = incoming.rgb/maxc;
+		maxc = pow(maxc*exposure+brightness_response.y, brightness_response.x)-brightness_response.z;
+		float c = min(maxc, 1.0);
+		float minc = min(saturated.r, min(saturated.g, saturated.b));
+		incoming.rgb = mix(saturated, vec3(1.0), min((maxc-c)/(1.0-minc), 1.0))*c;
 	}
-	frag_color = vec4(texture(curve, sample.r).r, texture(curve, sample.g).r, texture(curve, sample.b).r, sample.a);
+	frag_color = vec4(texture(curve, incoming.r).r, texture(curve, incoming.g).r, texture(curve, incoming.b).r, incoming.a);
 }
diff --git a/source/colorcurve.cpp b/source/colorcurve.cpp
index 81b5a43e..f1c09030 100644
--- a/source/colorcurve.cpp
+++ b/source/colorcurve.cpp
@@ -23,23 +23,31 @@ ColorCurve::ColorCurve():
 	curve.set_wrap(CLAMP_TO_EDGE);
 	texturing.attach(1, curve);
 
-	set_peak(0.2);
-	set_brightness(1.5);
+	set_exposure_adjust(0.0f);
+	set_brightness_response(0.4f);
 	set_linear();
 }
 
-void ColorCurve::set_peak(float p)
+void ColorCurve::set_exposure_adjust(float e)
+{
+	shdata.uniform("exposure", pow(2.0f, e));
+}
+
+void ColorCurve::set_brightness_response(float b)
+{
+	if(b<=0 || b>1)
+		throw invalid_argument("ColorCurve::set_brightness_response");
+	float t = (b<1 ? pow(b, 1/(1-b)) : 0.0f);
+	shdata.uniform("brightness_response", b, t, pow(t, b));
+}
+
+void ColorCurve::set_peak(float)
 {
-	if(p<0 || p>1)
-		throw invalid_argument("ColorCurve::set_peak");
-	shdata.uniform("peak", p);
 }
 
 void ColorCurve::set_brightness(float b)
 {
-	if(b<1)
-		throw invalid_argument("ColorCurve::set_brightness");
-	shdata.uniform("brightness", b);
+	set_brightness_response(1/b);
 }
 
 void ColorCurve::set_gamma(float g)
diff --git a/source/colorcurve.h b/source/colorcurve.h
index 2b854bec..608af448 100644
--- a/source/colorcurve.h
+++ b/source/colorcurve.h
@@ -12,8 +12,8 @@ namespace GL {
 
 /**
 Processes oversaturated colors to preserve hues.  When one color component
-exceeds 1.0, the others are scaled towards white.  A transition curve is also
-applied near 1.0 to prevent the abrupt change in the gradient.
+exceeds 1.0, the overflow is distributed to the other components, scaling the
+color towards white.
 
 Gamma or sRGB correction can also be applied to the output.  It can be used to
 improve color reproduction by performing lighting calculations in linear color
@@ -31,12 +31,15 @@ private:
 public:
 	ColorCurve();
 
-	/// Sets the size of the peak zone.  Must be between 0 and 1, inclusive.
-	void set_peak(float);
+	/** Set exposure adjustment in EV units.  Positive values brighten the
+	image, negative values darken it.  Zero is neutral. */
+	void set_exposure_adjust(float);
+
+	/** Sets the exponent of the */
+	void set_brightness_response(float);
 
-	/** Sets brightness for oversaturated colors.  Must be >= 1.  Suggested
-	values are between 1.5 and 2.0; a value of 1.0 will clamp colors to the
-	saturated value. */
+	// Deprecated functions
+	void set_peak(float);
 	void set_brightness(float);
 
 	/** Sets the gamma value used for mapping output colors.  Allowed range is
-- 
2.43.0