Implement Shading objects

@@ -46,6 +46,7 @@ part 'src/font.dart';
 part 'src/font_descriptor.dart';
 part 'src/font_metrics.dart';
 part 'src/formxobject.dart';
+part 'src/function.dart';
 part 'src/graphic_state.dart';
 part 'src/graphics.dart';
 part 'src/image.dart';
@@ -60,6 +61,7 @@ part 'src/page_format.dart';
 part 'src/page_list.dart';
 part 'src/point.dart';
 part 'src/rect.dart';
+part 'src/shading.dart';
 part 'src/signature.dart';
 part 'src/stream.dart';
 part 'src/ttf_parser.dart';

+/*
+ * Copyright (C) 2017, David PHAM-VAN <dev.nfet.net@gmail.com>
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// ignore_for_file: omit_local_variable_types
+
+part of pdf;
+
+class PdfFunction extends PdfObjectStream {
+  PdfFunction(
+    PdfDocument pdfDocument, {
+    this.colors,
+  }) : super(pdfDocument);
+
+  final List<PdfColor> colors;
+
+  @override
+  void _prepare() {
+    for (final PdfColor color in colors) {
+      buf.putBytes(<int>[
+        (color.red * 255.0).round() & 0xff,
+        (color.green * 255.0).round() & 0xff,
+        (color.blue * 255.0).round() & 0xff,
+      ]);
+    }
+
+    super._prepare();
+
+    params['/FunctionType'] = const PdfNum(0);
+    params['/BitsPerSample'] = const PdfNum(8);
+    params['/Order'] = const PdfNum(3);
+    params['/Domain'] = PdfArray.fromNum(const <num>[0, 1]);
+    params['/Range'] = PdfArray.fromNum(const <num>[0, 1, 0, 1, 0, 1]);
+    params['/Size'] = PdfNum(colors.length);
+  }
+}

@@ -94,6 +94,13 @@ class PdfGraphics {
     buf.putString('W n\n');
   }
 
+  /// Apply a shader
+  void applyShader(PdfShading shader) {
+    // The shader needs to be registered in the page resources
+    page.shading[shader.name] = shader;
+    buf.putString('${shader.name} sh\n');
+  }
+
   /// This releases any resources used by this Graphics object. You must use
   /// this method once finished with it.
   ///

@@ -199,5 +199,5 @@ class PdfImage extends PdfXObject {
   final PdfImageOrientation orientation;
 
   /// Name of the image
-  String get name => '/Image$objser';
+  String get name => '/I$objser';
 }

@@ -59,6 +59,9 @@ class PdfPage extends PdfObject {
   /// The fonts associated with this page
   final Map<String, PdfFont> fonts = <String, PdfFont>{};
 
+  /// The fonts associated with this page
+  final Map<String, PdfShading> shading = <String, PdfShading>{};
+
   /// The xobjects or other images in the pdf
   final Map<String, PdfXObject> xObjects = <String, PdfXObject>{};
 
@@ -117,11 +120,23 @@ class PdfPage extends PdfObject {
     /// This holds any resources for this page
     final PdfDict resources = PdfDict();
 
+    resources['/ProcSet'] = PdfArray(const <PdfName>[
+      PdfName('/PDF'),
+      PdfName('/Text'),
+      PdfName('/ImageB'),
+      PdfName('/ImageC'),
+    ]);
+
     // fonts
     if (fonts.isNotEmpty) {
       resources['/Font'] = PdfDict.fromObjectMap(fonts);
     }
 
+    // shading
+    if (shading.isNotEmpty) {
+      resources['/Shading'] = PdfDict.fromObjectMap(shading);
+    }
+
     // Now the XObjects
     if (xObjects.isNotEmpty) {
       resources['/XObject'] = PdfDict.fromObjectMap(xObjects);

+/*
+ * Copyright (C) 2017, David PHAM-VAN <dev.nfet.net@gmail.com>
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// ignore_for_file: omit_local_variable_types
+
+part of pdf;
+
+enum PdfShadingType { function, axial, radial }
+
+class PdfShading extends PdfObject {
+  PdfShading(
+    PdfDocument pdfDocument, {
+    @required this.shadingType,
+    @required this.function,
+    @required this.start,
+    @required this.end,
+  })  : assert(shadingType != null),
+        assert(function != null),
+        assert(start != null),
+        assert(end != null),
+        super(pdfDocument);
+
+  /// Name of the Shading object
+  String get name => '/S$objser';
+
+  final PdfShadingType shadingType;
+
+  final PdfFunction function;
+
+  final PdfPoint start;
+
+  final PdfPoint end;
+
+  @override
+  void _prepare() {
+    super._prepare();
+
+    params['/ShadingType'] = PdfNum(shadingType.index + 1);
+    params['/AntiAlias'] = const PdfBool(true);
+    params['/ColorSpace'] = const PdfName('/DeviceRGB');
+    params['/Coords'] =
+        PdfArray.fromNum(<double>[start.x, start.y, end.x, end.y]);
+    params['/Domain'] = PdfArray.fromNum(<num>[0, 1]);
+    params['/Extend'] = PdfArray(const <PdfBool>[PdfBool(true), PdfBool(true)]);
+    params['/Function'] = function.ref();
+  }
+}