From 271f5fdf3c4d04ca502b20768ac3a4b342b121fe Mon Sep 17 00:00:00 2001
From: William Gallafent <william@gallaf.net>
Date: Wed, 21 May 2014 21:40:15 +0100
Subject: [PATCH] Initial implementation of premultiplier, which is used when
 saving TIFF (since by convention this uses premultiplied alpha)

---
 .../gil/extension/io/formats/tiff/write.hpp   | 36 ++++++---
 include/boost/gil/premultiply.hpp             | 77 +++++++++++++++++++
 2 files changed, 101 insertions(+), 12 deletions(-)
 create mode 100644 include/boost/gil/premultiply.hpp

diff --git a/include/boost/gil/extension/io/formats/tiff/write.hpp b/include/boost/gil/extension/io/formats/tiff/write.hpp
index 21b8fb7d0..1a821bee6 100644
--- a/include/boost/gil/extension/io/formats/tiff/write.hpp
+++ b/include/boost/gil/extension/io/formats/tiff/write.hpp
@@ -30,6 +30,8 @@ extern "C" {
 #include <boost/static_assert.hpp>
 #include <boost/shared_ptr.hpp>
 
+#include <boost/gil/premultiply.hpp>
+
 #include <boost/gil/extension/io/tiff_tags.hpp>
 
 #include <boost/gil/extension/io/detail/base.hpp>
@@ -177,12 +179,17 @@ private:
         typedef typename View::x_iterator x_it_t;
         x_it_t row_it = x_it_t( &(*row.begin()));
 
-        for( typename View::y_coord_t y = 0; y < view.height(); ++y )
+				// @todo: is there an overhead to doing this when there's no
+				// alpha to premultiply by? I'd hope it's optimised out.
+				auto pm_view = premultiply_view <typename View:: value_type> (view);
+
+        for( typename View::y_coord_t y = 0; y < pm_view.height(); ++y )
         {
-            std::copy( view.row_begin( y )
-                     , view.row_end( y )
-                     , row_it
-                     );
+					std::copy( pm_view.row_begin( y )
+										 , pm_view.row_end( y )
+										 , row_it
+						);
+
 
             this->_io_dev.write_scaline( row
                                        , (uint32) y
@@ -192,8 +199,8 @@ private:
             // @todo: do optional bit swapping here if you need to...
         }
     }
-
-    template< typename View >
+	
+    template< typename View>
     void write_tiled_data( const View&            view
                          , tiff_tile_width::type  tw
                          , tiff_tile_length::type th
@@ -221,12 +228,16 @@ private:
 
         byte_t* row_addr = reinterpret_cast< byte_t* >( &row.front() );
 
-        for( typename View::y_coord_t y = 0; y < view.height(); ++y )
+				// @todo: is there an overhead to doing this when there's no
+				// alpha to premultiply by? I'd hope it's optimised out.
+				auto pm_view = premultiply_view <typename View:: value_type> (view);
+
+        for( typename View::y_coord_t y = 0; y < pm_view.height(); ++y )
         {
-            std::copy( view.row_begin( y )
-                     , view.row_end  ( y )
-                     , row.begin()
-                     );
+					std::copy( pm_view.row_begin( y )
+										 , pm_view.row_end( y )
+										 , row.begin()
+						);
 
             this->_io_dev.write_scaline( row_addr
                                        , (uint32) y
@@ -253,6 +264,7 @@ private:
         internal_write_tiled_data(view, tw, th, row, row_it);
     }
 
+	// @todo: premultiply
     template< typename View,
               typename IteratorType
             >
diff --git a/include/boost/gil/premultiply.hpp b/include/boost/gil/premultiply.hpp
new file mode 100644
index 000000000..c6eb157fe
--- /dev/null
+++ b/include/boost/gil/premultiply.hpp
@@ -0,0 +1,77 @@
+#ifndef PREMULTIPLY_HPP
+#define PREMULTIPLY_HPP
+
+#include <iostream>
+
+#include <boost/gil/rgba.hpp>
+#include <boost/mpl/remove.hpp>
+
+namespace boost { namespace gil {
+
+template <typename SrcP, typename DstP>
+struct channel_premultiply {
+	channel_premultiply (SrcP const & src, DstP & dst)
+		: src_ (src), dst_ (dst)
+	{}
+	
+	template <typename Channel>
+	void operator () (Channel c) const {
+		// @todo: need to do a “channel_convert” too, in case the channel types aren't the same?
+		get_color (dst_, Channel()) = channel_multiply(get_color(src_,Channel()), alpha_or_max(src_));
+	}
+	SrcP const & src_;
+	DstP & dst_;
+};
+
+struct premultiply {
+	template <typename SrcP, typename DstP>
+	void operator()(const SrcP& src, DstP& dst) const {
+		typedef typename color_space_type<SrcP>::type src_colour_space_t;
+		typedef typename color_space_type<DstP>::type dst_colour_space_t;
+		typedef typename mpl:: remove <src_colour_space_t, alpha_t>:: type src_colour_channels;
+		mpl:: for_each <src_colour_channels> ( channel_premultiply <SrcP, DstP> (src, dst) );
+		if (mpl:: contains <dst_colour_space_t, alpha_t>:: value)
+			get_color (dst,alpha_t()) = alpha_or_max (src);
+	}
+};
+
+template <typename SrcConstRefP,  // const reference to the source pixel
+          typename DstP>          // Destination pixel value (models PixelValueConcept)
+class premultiply_deref_fn {
+public:
+    typedef premultiply_deref_fn const_t;
+    typedef DstP                value_type;
+    typedef value_type          reference;      // read-only dereferencing
+    typedef const value_type&   const_reference;
+    typedef SrcConstRefP        argument_type;
+    typedef reference           result_type;
+    BOOST_STATIC_CONSTANT(bool, is_mutable=false);
+
+    result_type operator()(argument_type srcP) const {
+        result_type dstP;
+        premultiply () (srcP,dstP);
+        return dstP;
+    }
+};
+
+template <typename SrcView, typename DstP>
+struct premultiplied_view_type {
+private:
+    typedef typename SrcView::const_t::reference src_pix_ref;  // const reference to pixel in SrcView
+    typedef premultiply_deref_fn<src_pix_ref, DstP> deref_t; // the dereference adaptor that performs color conversion
+    typedef typename SrcView::template add_deref<deref_t> add_ref_t;
+public:
+    typedef typename add_ref_t::type type; // the color converted view type
+    static type make(const SrcView& sv) { return add_ref_t::make(sv, deref_t()); }
+};
+
+template <typename DstP, typename View> inline
+typename premultiplied_view_type<View,DstP>::type premultiply_view(const View& src) {
+    return premultiplied_view_type<View,DstP>::make(src);
+}
+
+	}
+}
+
+
+#endif