3 files changed, 59 insertions, 3 deletions

diff --git a/MCServer/Plugins/APIDump/APIDesc.lua b/MCServer/Plugins/APIDump/APIDesc.lua
index 28e7744ed..9bcd6edde 100644
--- a/MCServer/Plugins/APIDump/APIDesc.lua
+++ b/MCServer/Plugins/APIDump/APIDesc.lua
@@ -200,6 +200,8 @@ g_APIDesc =
 				msFillAir = { Notes = "Dst is overwritten by Src only where Src has air blocks" },
 				msImprint = { Notes = "Src overwrites Dst anywhere where Dst has non-air blocks" },
 				msLake = { Notes = "Special mode for merging lake images" },
+				msSpongePrint = { Notes = "Similar to msImprint, sponge block doesn't overwrite anything, all other blocks overwrite everything"},
+				msMask = { Notes = "The blocks that are exactly the same are kept in Dst, all differing blocks are replaced by air"},
 			},
 			ConstantGroups =
 			{
@@ -293,7 +295,6 @@ g_APIDesc =
 						</tr>
 						</tbody></table>
 
-
 						<p>
 						<strong>msSpongePrint</strong> - used for most prefab-generators to merge the prefabs. Similar to
 						msImprint, but uses the sponge block as the NOP block instead, so that the prefabs may carve out air
@@ -306,10 +307,26 @@ g_APIDesc =
 						</tr><tr>
 						<td> A </td><td> sponge </td><td> A </td><td> Sponge is the NOP block </td>
 						</tr><tr>
-						<td> *        </td><td> B    </td><td> B    </td><td> Everything else overwrites anything </td>
+						<td> * </td><td> B </td><td> B </td><td> Everything else overwrites anything </td>
 						</tr>
 						</tbody></table>
-					]],
+
+						<p>
+						<strong>msMask</strong> - the blocks that are the same in the other area are kept, all the
+						differing blocks are replaced with air. Meta is used in the comparison, too, two blocks of the
+						same type but different meta are considered different and thus replaced with air.
+						</p>
+						<table><tbody><tr>
+						<th colspan="2"> area block </th><th> </th><th> Notes </th>
+						</tr><tr>
+						<th> this </th><th> Src </th><th> result </th><th> </th>
+						</tr><tr>
+						<td> A </td><td> A </td><td> A </td><td> Same blocks are kept </td>
+						</tr><tr>
+						<td> A </td><td> non-A </td><td> air </td><td> Differing blocks are replaced with air </td>
+						</tr>
+						</tbody></table>
+]],
 				},  -- Merge strategies
 			},  -- AdditionalInfo
 		},  -- cBlockArea
diff --git a/src/BlockArea.cpp b/src/BlockArea.cpp
index 2b950378a..543dbe04d 100644
--- a/src/BlockArea.cpp
+++ b/src/BlockArea.cpp
@@ -173,6 +173,21 @@ static inline void MergeCombinatorSpongePrint(BLOCKTYPE & a_DstType, BLOCKTYPE a
 
 
 
+/** Combinator used for cBlockArea::msMask merging */
+static inline void MergeCombinatorMask(BLOCKTYPE & a_DstType, BLOCKTYPE a_SrcType, NIBBLETYPE & a_DstMeta, NIBBLETYPE a_SrcMeta)
+{
+	// If the blocks are the same, keep the dest; otherwise replace with air
+	if ((a_SrcType != a_DstType) || (a_SrcMeta != a_DstMeta))
+	{
+		a_DstType = E_BLOCK_AIR;
+		a_DstMeta = 0;
+	}
+}
+
+
+
+
+
 ///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
 // cBlockArea:
 
@@ -710,6 +725,21 @@ void cBlockArea::Merge(const cBlockArea & a_Src, int a_RelX, int a_RelY, int a_R
 			break;
 		}  // case msSpongePrint
 		
+		case msMask:
+		{
+			InternalMergeBlocks(
+				m_BlockTypes, a_Src.GetBlockTypes(),
+				DstMetas, SrcMetas,
+				SizeX, SizeY, SizeZ,
+				SrcOffX, SrcOffY, SrcOffZ,
+				DstOffX, DstOffY, DstOffZ,
+				a_Src.GetSizeX(), a_Src.GetSizeY(), a_Src.GetSizeZ(),
+				m_Size.x, m_Size.y, m_Size.z,
+				MergeCombinatorMask
+			);
+			break;
+		}  // case msMask
+		
 		default:
 		{
 			LOGWARNING("Unknown block area merge strategy: %d", a_Strategy);
diff --git a/src/BlockArea.h b/src/BlockArea.h
index d37f0d182..34a0ef926 100644
--- a/src/BlockArea.h
+++ b/src/BlockArea.h
@@ -52,6 +52,7 @@ public:
 		msImprint,
 		msLake,
 		msSpongePrint,
+		msMask,
 	} ;
 	
 	cBlockArea(void);
@@ -152,6 +153,14 @@ public:
 	+----------+--------+--------+
 	| A        | sponge | A      |  Sponge is the NOP block
 	| *        | B      | B      |  Everything else overwrites anything
+	
+	msMask:
+	Combines two areas, the blocks that are the same are kept, differing ones are reset to air
+	|  area block  |        |
+	| this | Src   | result |
+	+------+-------+--------+
+	| A    | A     | A      |  Same blocks are kept
+	| A    | non-A | air    |  Everything else is replaced with air
 
 	*/
 	void Merge(const cBlockArea & a_Src, int a_RelX, int a_RelY, int a_RelZ, eMergeStrategy a_Strategy);