From cdccc51b2e71492ec716898681b82e645e9175eb Mon Sep 17 00:00:00 2001
From: Casey Bianco-Davis <caseycasey739@gmail.com>
Date: Fri, 21 Mar 2025 12:15:09 +0100
Subject: [PATCH] Grease Pencil: Draw Tool: Improve random color value and
 saturation setting results

This makes the random vertex color brightness work on a relative basis.

Because humans see relative differences in brightness more then absolute
brightness, simple adding randomness to `Value` will cause dark colors to be
effected more heavily then light colors.

This also fixes a problem desaturated colors with becoming red after
saturation randomization.

Pull Request: https://projects.blender.org/blender/blender/pulls/107315
---
 .../intern/grease_pencil_randomize.cc         | 38 +++++++++++++------
 1 file changed, 27 insertions(+), 11 deletions(-)

diff --git a/source/blender/editors/grease_pencil/intern/grease_pencil_randomize.cc b/source/blender/editors/grease_pencil/intern/grease_pencil_randomize.cc
index 16133f9a26a..fb723c25b7e 100644
--- a/source/blender/editors/grease_pencil/intern/grease_pencil_randomize.cc
+++ b/source/blender/editors/grease_pencil/intern/grease_pencil_randomize.cc
@@ -180,25 +180,41 @@ ColorGeometry4f randomize_color(const BrushGpencilSettings &settings,
   }
 
   float3 hsv;
-  rgb_to_hsv_v(color, hsv);
+  linearrgb_to_srgb_v3_v3(hsv, color);
+  rgb_to_hsv_v(hsv, hsv);
 
-  hsv += float3(random_hue * settings.random_hue,
-                random_saturation * settings.random_saturation,
-                random_value * settings.random_value);
+  hsv[0] += random_hue * settings.random_hue;
 
-  /* Wrap hue. */
-  if (hsv[0] > 1.0f) {
-    hsv[0] -= 1.0f;
-  }
-  else if (hsv[0] < 0.0f) {
-    hsv[0] += 1.0f;
-  }
+  const float absolute_brightness = hsv[2] + random_value * settings.random_value;
 
+  /*
+   * To match relative brightness we want the ratio of the original to modified Value to not
+   * depend on the brightness of the input Value, Exp is used because we need a function that
+   * is positive for all 'x' and has the property that 'f(-x) = 1/f(x)' this is so that
+   * we make the Value on average growth and shrink by the same amount. To detriment the rate
+   * of the Exponential we set slope to match addition for small random values at an arbitrary
+   * Base Value.
+   */
+  constexpr float base_value = 0.5f;
+  const float relative_brightness = hsv[2] *
+                                    math::exp(random_value * settings.random_value / base_value);
+
+  /* Use the fourth power. */
+  const float blend_factor = math::pow(settings.random_value, 4.0f);
+  /* Use relative brightness at low randomness, and switch to absolute at high. */
+  hsv[2] = math::interpolate(relative_brightness, absolute_brightness, blend_factor);
+
+  /* Multiply by the current saturation to prevent grays from becoming red. */
+  hsv[1] += random_saturation * settings.random_saturation * 2.0f * hsv[1];
+
+  /* Wrap hue, clamp saturation and value. */
+  hsv[0] = math::fract(hsv[0]);
   hsv[1] = math::clamp(hsv[1], 0.0f, 1.0f);
   hsv[2] = math::clamp(hsv[2], 0.0f, 1.0f);
 
   ColorGeometry4f random_color;
   hsv_to_rgb_v(hsv, random_color);
+  srgb_to_linearrgb_v3_v3(random_color, random_color);
   random_color.a = color.a;
   return random_color;
 }