Viewer info line: compute HSV from linear RGB

Fixes #286

CHANGELOG.md

@@ -8,6 +8,7 @@
 ## Version 2.3.13
 
 - Fix default value for file premult in ReadSVG (should be premultiplied).
+- HSV values in the viewer info lines are now computed from linear RGB #286.
 
 
 ## Version 2.3.12

Engine/Lut.cpp

@@ -1612,9 +1612,13 @@ LutManager::VLogLut()
     return LutManager::m_instance.getLut("V-Log", from_func_VLog, to_func_VLog);
 }
 
-// r,g,b values are from 0 to 1
+// r,g,b values are linear values from 0 to 1
 // h = [0,OFXS_HUE_CIRCLE], s = [0,1], v = [0,1]
 //		if s == 0, then h = 0 (undefined)
+// Reference:
+// "Color gamut transform pairs", Alvy Ray Smith, Proceeding SIGGRAPH '78
+// https://doi.org/10.1145/800248.807361
+// http://www.icst.pku.edu.cn/F/course/ImageProcessing/2018/resource/Color78.pdf
 void
 rgb_to_hsv( float r,
             float g,
@@ -1623,8 +1627,8 @@ rgb_to_hsv( float r,
             float *s,
             float *v )
 {
-    float min = std::min(std::min(r, g), b);
-    float max = std::max(std::max(r, g), b);
+    float min = (std::min)((std::min)(r, g), b);
+    float max = (std::max)((std::max)(r, g), b);
 
     *v = max;                               // v
 
@@ -1655,6 +1659,13 @@ rgb_to_hsv( float r,
     }
 }
 
+// r,g,b values are linear values from 0 to 1
+// h = [0,OFXS_HUE_CIRCLE], s = [0,1], v = [0,1]
+//		if s == 0, then h = 0 (undefined)
+// Reference:
+// "Color gamut transform pairs", Alvy Ray Smith, Proceeding SIGGRAPH '78
+// https://doi.org/10.1145/800248.807361
+// http://www.icst.pku.edu.cn/F/course/ImageProcessing/2018/resource/Color78.pdf
 void
 hsv_to_rgb(float h,
            float s,

Gui/InfoViewerWidget.cpp

@@ -326,8 +326,9 @@ InfoViewerWidget::setColor(float r,
     rgbaValues->setText(values);
     rgbaValues->update();
     float h, s, v, l;
-    // Nuke's HSV display is based on sRGB, an L is Rec.709.
-    // see http://forums.thefoundry.co.uk/phpBB2/viewtopic.php?t=2283
+    // Nuke's HSV display is based on sRGB until Nuke 8, an L is Rec.709.
+    // see https://community.foundry.com/discuss/topic/100271
+    // This was changed in Nuke 9 to use linear values.
     double srgb_r = Color::to_func_srgb(r);
     double srgb_g = Color::to_func_srgb(g);
     double srgb_b = Color::to_func_srgb(b);
@@ -339,7 +340,18 @@ InfoViewerWidget::setColor(float r,
     color->update();
 
     const QFont &hsvFont = hvl_lastOption->font();
-    Color::rgb_to_hsv(srgb_r, srgb_g, srgb_b, &h, &s, &v);
+    // Nuke 5-8 version used sRGB colors to compute HSV
+    //Color::rgb_to_hsv(srgb_r, srgb_g, srgb_b, &h, &s, &v);
+
+    // However, as Alvin Ray Smith said in his paper,
+    // "We shall assume that an RGB monitor is a linear device"
+    // We thus use linear values (same as Nuke 9 and later).
+    // Fixes https://github.com/NatronGitHub/Natron/issues/286
+    // Reference:
+    // "Color gamut transform pairs", Alvy Ray Smith, Proceeding SIGGRAPH '78
+    // https://doi.org/10.1145/800248.807361
+    // http://www.icst.pku.edu.cn/F/course/ImageProcessing/2018/resource/Color78.pdf
+    Color::rgb_to_hsv(r, g, b, &h, &s, &v);
     l = 0.2125 * r + 0.7154 * g + 0.0721 * b; // L according to Rec.709
 
     QString lS = _colorValid ? QString::number(l, 'f', 5) : QString::fromUtf8("?");