add a new BlendMode::Equation : ReverseSubtract

include/SFML/Graphics/BlendMode.hpp

@@ -69,7 +69,8 @@ struct SFML_GRAPHICS_API BlendMode
     enum Equation
     {
         Add,            ///< Pixel = Src * SrcFactor + Dst * DstFactor
-        Subtract ///< Pixel = Src * SrcFactor - Dst * DstFactor
+        Subtract,       ///< Pixel = Src * SrcFactor - Dst * DstFactor
+        ReverseSubtract ///< Pixel = Dst * DstFactor - Src * SrcFactor
     };
 
     ////////////////////////////////////////////////////////////

src/SFML/Graphics/GLExtensions.hpp

@@ -64,6 +64,7 @@
     #define GLEXT_glBlendEquation                     glBlendEquationOES
     #define GLEXT_GL_FUNC_ADD                         GL_FUNC_ADD_OES
     #define GLEXT_GL_FUNC_SUBTRACT                    GL_FUNC_SUBTRACT_OES
+    #define GLEXT_GL_FUNC_REVERSE_SUBTRACT            GL_FUNC_REVERSE_SUBTRACT_OES
 
     // The following extensions are optional.
 
@@ -142,6 +143,7 @@
     // Core since 1.2 - EXT_blend_subtract
     #define GLEXT_blend_subtract                      sfogl_ext_EXT_blend_subtract
     #define GLEXT_GL_FUNC_SUBTRACT                    GL_FUNC_SUBTRACT_EXT
+    #define GLEXT_GL_FUNC_REVERSE_SUBTRACT            GL_FUNC_REVERSE_SUBTRACT_EXT
 
     // Core since 1.3 - ARB_multitexture
     #define GLEXT_multitexture                        sfogl_ext_ARB_multitexture

src/SFML/Graphics/RenderTarget.cpp

@@ -67,6 +67,7 @@ namespace
         {
             case sf::BlendMode::Add:             return GLEXT_GL_FUNC_ADD;
             case sf::BlendMode::Subtract:        return GLEXT_GL_FUNC_SUBTRACT;
+            case sf::BlendMode::ReverseSubtract: return GLEXT_GL_FUNC_REVERSE_SUBTRACT;
         }
 
         sf::err() << "Invalid value for sf::BlendMode::Equation! Fallback to sf::BlendMode::Add." << std::endl;