Extended to dither each RGB channel separately.

git-svn-id: file:///srv/svn/repos/haiku/trunk/current@6407 a95241bf-73f2-0310-859d-f6bbb57e9c96

headers/private/print/libprint/Halftone.h

@@ -24,6 +24,12 @@ public:
 		TYPE2,
 		TYPE3
 	};
+	enum GrayFunction {
+		kMixToGray,
+		kRedChannel,
+		kGreenChannel,
+		kBlueChannel
+	};
 	Halftone(color_space cs, double gamma = 1.4, DITHERTYPE dither_type = TYPE3);
 	~Halftone();
 	int dither(uchar *dst, const uchar *src, int x, int y, int width);
@@ -33,6 +39,7 @@ public:
 	void setPattern(const uchar *pattern);
 	PFN_gray getGrayFunction() const;
 	void setGrayFunction(PFN_gray gray);
+	void setGrayFunction(GrayFunction grayFunction);
 
 protected:
 	void createGammaTable(double gamma);

src/add-ons/print/drivers/shared/libprint/Halftone.cpp

@@ -18,7 +18,7 @@ using namespace std;
 
 #include "Pattern.h"
 
-uint gray(rgb_color c)
+static uint gray(rgb_color c)
 {
 	if (c.red == c.green && c.red == c.blue) {
 		return 255 - c.red;
@@ -27,6 +27,21 @@ uint gray(rgb_color c)
 	}
 }
 
+static uint channel_red(rgb_color c)
+{
+	return c.red;	
+}
+
+static uint channel_green(rgb_color c)
+{
+	return c.green;	
+}
+
+static uint channel_blue(rgb_color c)
+{
+	return c.blue;	
+}
+
 Halftone::Halftone(color_space cs, double gamma, DITHERTYPE dither_type)
 {
 	__pixel_depth = color_space2pixel_depth(cs);
@@ -108,6 +123,23 @@ int Halftone::dither(
 	return (this->*__dither)(dst, src, x, y, width);
 }
 
+void Halftone::setGrayFunction(GrayFunction grayFunction)
+{
+	PFN_gray function = NULL;
+	switch (grayFunction) {
+		case kMixToGray: function = gray;
+			break;
+		case kRedChannel: function = channel_red;
+			break;
+		case kGreenChannel: function = channel_green;
+			break;
+		case kBlueChannel: function = channel_blue;
+			break;
+	};
+	setGrayFunction(function);
+}
+
+
 int Halftone::ditherGRAY1(
 	uchar *dst,
 	const uchar *src,