Added subtractive and multiplicative blend modes (thanks vrld)

src/modules/graphics/Graphics.cpp

@@ -109,6 +109,8 @@ namespace graphics
 	{
 		{ "alpha", Graphics::BLEND_ALPHA },
 		{ "additive", Graphics::BLEND_ADDITIVE },
+		{ "subtractive", Graphics::BLEND_SUBTRACTIVE },
+		{ "multiplicative", Graphics::BLEND_MULTIPLICATIVE },
 	};
 
 	StringMap<Graphics::BlendMode, Graphics::BLEND_MAX_ENUM> Graphics::blendModes(Graphics::blendModeEntries, sizeof(Graphics::blendModeEntries));

src/modules/graphics/Graphics.h

@@ -52,6 +52,8 @@ namespace graphics
 		{
 			BLEND_ALPHA = 1,
 			BLEND_ADDITIVE,
+			BLEND_SUBTRACTIVE,
+			BLEND_MULTIPLICATIVE,
 			BLEND_MAX_ENUM
 		};
 

src/modules/graphics/opengl/Graphics.cpp

@@ -533,10 +533,19 @@ namespace opengl
 
 	void Graphics::setBlendMode( Graphics::BlendMode mode )
 	{
-		if(mode == BLEND_ADDITIVE)
+		glAlphaFunc(GL_GEQUAL, 0);
-			glBlendFunc(GL_SRC_ALPHA, GL_ONE);
+
-		else // mode == BLEND_ALPHA
+		if (mode == BLEND_SUBTRACTIVE)
+			glBlendEquation(GL_FUNC_REVERSE_SUBTRACT);
+		else
+			glBlendEquation(GL_FUNC_ADD);
+
+		if (mode == BLEND_ALPHA)
 			glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
+		else if (mode == BLEND_MULTIPLICATIVE)
+			glBlendFunc(GL_DST_COLOR, GL_ONE_MINUS_SRC_ALPHA);
+		else
+			glBlendFunc(GL_SRC_ALPHA, GL_ONE);
 	}
 
 	void Graphics::setColorMode ( Graphics::ColorMode mode )