Add comp-op support.

lib/carto/tree/reference.json

@@ -55,6 +55,41 @@
                 "default-meaning": "no filters",
                 "type": "imagefilters",
                 "doc": "A list of image filters. Currently supported are agg-stack-blur, emboss, blur, gray, sobel, edge-detect, x-gradient, y-gradient, sharpen"
+            },
+            "comp-op": {
+                "css": "composite-operation",
+                "default-value": "src-over",
+                "default-meaning": "add the current layer on top of other layers",
+                "doc": "Composite operation. This defines how this layer should behave relative to layers atop or below it.",
+                "type": ["clear",
+                    "src",
+                    "dst",
+                    "src-over",
+                    "dst-over",
+                    "src-in",
+                    "dst-in",
+                    "src-out",
+                    "dst-out",
+                    "src-atop",
+                    "dst-atop",
+                    "xor",
+                    "plus",
+                    "minus",
+                    "multiply",
+                    "screen",
+                    "overlay",
+                    "darken",
+                    "lighten",
+                    "color-dodge",
+                    "color-burn",
+                    "hard-light",
+                    "soft-light",
+                    "difference",
+                    "exclusion",
+                    "contrast",
+                    "invert",
+                    "invert-rgb"
+                ]
             }
         },
         "map": {

lib/carto/tree/style.js

@@ -16,19 +16,30 @@ tree.Style.prototype.toXML = function(env) {
         });
     }));
 
+    var comp_op = _.flatten(this.definitions.map(function(definition) {
+        return definition.rules.filter(function(rule) {
+            return (rule.name === 'composite-operation');
+        });
+    }));
+
     var rules = this.definitions.map(function(definition) {
         return definition.toXML(env, existing);
     });
 
-    var image_filters_xml = '';
+    var image_filters_xml = '',
+        comp_op_xml = '';
 
     if (image_filters.length) {
         image_filters_xml = 'image-filters="' + image_filters.map(function(f) {
             return f.value.eval(env).toString(env, null, ';');
         }).join(',') + '"';
     }
+    
+    if (comp_op.length) {
+        comp_op_xml = 'comp-op="' + comp_op[0].value.eval(env).toString() + '"';
+    }
 
-    return '<Style name="' + this.name + '" filter-mode="first" ' + image_filters_xml + '>\n' + rules.join('') + '</Style>';
+    return '<Style name="' + this.name + '" filter-mode="first" ' + image_filters_xml + ' ' + comp_op_xml + '>\n' + rules.join('') + '</Style>';
 };
 
 })(require('../tree'));

test/rendering/image_filters.mss

@@ -3,5 +3,6 @@
   line-color:#F00;
   line-width: 0.5;
   image-filters: blur(), sharpen();
+  composite-operation: src-in;
   line-dasharray: 2, 3;
 }