Implement ~BLEND(r,g,b,o) image path function (bug #11590)

It takes a specified RGB color and blends it with the target image by
certain opacity, specified either as a factor (e.g. 0.25) or percentage
(25%).

I chose BLEND over TINT for consistency with AnimationWML. Feel free to
discuss.

changelog

@@ -44,6 +44,9 @@ Version 1.11.0-svn:
    * Added [modify_side] color= attribute, which changes a side's team color
      range (feature/bug #18772)
    * Removed support for the deprecated [removeitem]
+   * Implemented ~BLEND(r,g,b,o) image path function, which blends the image
+     with a specified RGB color according to certain opacity (factor or
+     percentage) (feature/bug #11590)
  * Unit changes and balancing:
    * New extra_define ENABLE_WOLF_ADVANCEMENT to optionally enable Wolves
      advancement to Great Wolves and Direwolves

data/scenario-test.cfg

@@ -59,6 +59,14 @@ Xu          , Xu          , Qxu         , Qxu         , Ql          , Ql
             max_hitpoints=90
             experience=99
             alignment=liminal
+            [modifications]
+                [object]
+                    [effect]
+                        apply_to=image_mod
+                        add="BLEND(255,0,255,25%)"
+                    [/effect]
+                [/object]
+            [/modifications]
         [/leader]
         side=1
         controller=human

src/image_modifications.cpp

@@ -15,6 +15,7 @@
 
 #include "color_range.hpp"
 #include "config.hpp"
+#include "display.hpp"
 #include "foreach.hpp"
 #include "game_config.hpp"
 #include "image.hpp"
@@ -339,6 +340,32 @@ int cs_modification::get_b() const
 	return b_;
 }
 
+surface blend_modification::operator()(const surface& src) const
+{
+	return blend_surface(src, a_, display::rgb(r_, g_, b_));
+
+}
+
+int blend_modification::get_r() const
+{
+	return r_;
+}
+
+int blend_modification::get_g() const
+{
+	return g_;
+}
+
+int blend_modification::get_b() const
+{
+	return b_;
+}
+
+float blend_modification::get_a() const
+{
+	return a_;
+}
+
 surface bl_modification::operator()(const surface& src) const
 {
 	return blur_alpha_surface(src, depth_);
@@ -586,6 +613,36 @@ REGISTER_MOD_PARSER(CS, args)
 	return new cs_modification(r, g, b);
 }
 
+// Color blending
+REGISTER_MOD_PARSER(BLEND, args)
+{
+	const std::vector<std::string>& params = utils::split(args, ',');
+
+	if(params.size() != 4) {
+		ERR_DP << "~BLEND() requires exactly 4 arguments\n";
+		return NULL;
+	}
+
+	float opacity = 0.0f;
+	const std::string& opacity_str = params[3];
+	const std::string::size_type p100_pos = opacity_str.find('%');
+
+	if(p100_pos == std::string::npos)
+		opacity = lexical_cast_default<float>(opacity_str);
+	else {
+		// make multiplier
+		const std::string& parsed_field = opacity_str.substr(0, p100_pos);
+		opacity = lexical_cast_default<float>(parsed_field);
+		opacity /= 100.0f;
+	}
+
+	return new blend_modification(
+		lexical_cast_default<int>(params[0]),
+		lexical_cast_default<int>(params[1]),
+		lexical_cast_default<int>(params[2]),
+		opacity);
+}
+
 // Crop/slice
 REGISTER_MOD_PARSER(CROP, args)
 {

src/image_modifications.hpp

@@ -298,6 +298,26 @@ private:
 	int r_, g_, b_;
 };
 
+/**
+ * Color blending (BLEND) modification
+ */
+class blend_modification : public modification
+{
+public:
+	blend_modification(int r, int g, int b, float a)
+		: r_(r), g_(g), b_(b), a_(a)
+	{}
+	virtual surface operator()(const surface& src) const;
+	int get_r() const;
+	int get_g() const;
+	int get_b() const;
+	float get_a() const;
+
+private:
+	int r_, g_, b_;
+	float a_;
+};
+
 /**
  * Gaussian-like blur (BL) modification.
  */