masking blend modes

stepmania/src/RageDisplay_OGL.cpp

@@ -1637,6 +1637,14 @@ void RageDisplay_OGL::SetBlendMode( BlendMode mode )
 		iSourceRGB = GL_ONE; iDestRGB = GL_ZERO;
 		iSourceAlpha = GL_ONE; iDestAlpha = GL_ZERO;
 		break;
+	case BLEND_ALPHA_MASK:
+		iSourceRGB = GL_ZERO; iDestRGB = GL_ONE;
+		iSourceAlpha = GL_ZERO; iDestAlpha = GL_SRC_ALPHA;
+		break;
+	case BLEND_ALPHA_KNOCK_OUT:
+		iSourceRGB = GL_ZERO; iDestRGB = GL_ONE;
+		iSourceAlpha = GL_ZERO; iDestAlpha = GL_ONE_MINUS_SRC_ALPHA;
+		break;
 	case BLEND_WEIGHTED_MULTIPLY:
 		/* output = 2*(dst*src).  0.5,0.5,0.5 is identity; darker colors darken the image,
 		 * and brighter colors lighten the image. */

stepmania/src/RageTypes.cpp

@@ -74,7 +74,33 @@ static const char *BlendModeNames[] =
 {
 	"Normal",
 	"Add",
+
+	/*
+	 * Copy the source directly to the destination.  Alpha is not premultiplied.
+	 *
+	 * Co = Cs
+	 * Ao = As
+	 */
 	"CopySrc",
+
+	/*
+	 * Leave the color alone, and apply the source alpha on top of the existing alpha.
+	 * Transparent areas in the source become transparent in the destination.
+	 *
+	 * Be sure to disable alpha test with this blend mode.
+	 *
+	 * Co = Cd
+	 * Ao = Ad*As
+	 */
+	"AlphaMask",
+
+	/*
+	 * Leave the color alone, and apply the source alpha on top of the existing alpha.
+	 * Transparent areas in the source become transparent in the destination.
+	 * Co = Cd
+	 * Ao = Ad*(1-As)
+	 */
+	"AlphaKnockOut",
 	"WeightedMultiply",
 	"InvertDest",
 	"NoEffect"

stepmania/src/RageTypes.h

@@ -10,6 +10,8 @@ enum BlendMode
 	BLEND_NORMAL,
 	BLEND_ADD,
 	BLEND_COPY_SRC,
+	BLEND_ALPHA_MASK,
+	BLEND_ALPHA_KNOCK_OUT,
 	BLEND_WEIGHTED_MULTIPLY,
 	BLEND_INVERT_DEST,
 	BLEND_NO_EFFECT,