Add Color::plus/minus utils

Signed-off-by: falkTX <falktx@falktx.com>

dgl/Color.hpp

@@ -1,6 +1,6 @@
 /*
  * DISTRHO Plugin Framework (DPF)
- * Copyright (C) 2012-2021 Filipe Coelho <falktx@falktx.com>
+ * Copyright (C) 2012-2022 Filipe Coelho <falktx@falktx.com>
  *
  * Permission to use, copy, modify, and/or distribute this software for any purpose with
  * or without fee is hereby granted, provided that the above copyright notice and this
@@ -70,6 +70,30 @@ struct Color {
     */
     Color withAlpha(float alpha) const noexcept;
 
+   /**
+      Create a new color based on this one but with subtracted numeric value on all elements.
+      Value must be in [0..255] range.
+    */
+    Color minus(int value) const noexcept;
+
+   /**
+      Create a new color based on this one but with subtracted floating-point value on all elements.
+      Value must be in [0..1] range.
+    */
+    Color minus(float value) const noexcept;
+
+   /**
+      Create a new color based on this one but with added numeric value on all elements.
+      Value must be in [0..255] range.
+    */
+    Color plus(int value) const noexcept;
+
+   /**
+      Create a new color based on this one but with added floating-point value on all elements.
+      Value must be in [0..1] range.
+    */
+    Color plus(float value) const noexcept;
+
    /**
       Create a color specified by hue, saturation and lightness.
       Values must in [0..1] range.

dgl/src/Color.cpp

@@ -121,6 +121,48 @@ Color Color::withAlpha(const float alpha2) const noexcept
     return color;
 }
 
+Color Color::minus(const int value) const noexcept
+{
+    const float fvalue = static_cast<float>(value)/255.f;
+    Color color(*this);
+    color.red -= fvalue;
+    color.green -= fvalue;
+    color.blue -= fvalue;
+    color.fixBounds();
+    return color;
+}
+
+Color Color::minus(const float value) const noexcept
+{
+    Color color(*this);
+    color.red -= value;
+    color.green -= value;
+    color.blue -= value;
+    color.fixBounds();
+    return color;
+}
+
+Color Color::plus(const int value) const noexcept
+{
+    const float fvalue = static_cast<float>(value)/255.f;
+    Color color(*this);
+    color.red += fvalue;
+    color.green += fvalue;
+    color.blue += fvalue;
+    color.fixBounds();
+    return color;
+}
+
+Color Color::plus(const float value) const noexcept
+{
+    Color color(*this);
+    color.red += value;
+    color.green += value;
+    color.blue += value;
+    color.fixBounds();
+    return color;
+}
+
 Color Color::fromHSL(float hue, float saturation, float lightness, float alpha)
 {
     float m1, m2;