[gegl] examples: Add a better XML+Python example

[Daniel Sabo <daniels src gnome org>]

commit dc4fb9fd048d9d0c32da26006a6ca390489412ee
Author: Daniel Sabo <DanielSabo gmail com>
Date:   Mon Oct 21 19:39:42 2013 -0700

    examples: Add a better XML+Python example

 examples/xml-graph.py |   88 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 88 insertions(+), 0 deletions(-)
---
diff --git a/examples/xml-graph.py b/examples/xml-graph.py
new file mode 100644
index 0000000..a2e97a3
--- /dev/null
+++ b/examples/xml-graph.py
@@ -0,0 +1,88 @@
+#!/usr/bin/env python
+from __future__ import print_function
+
+from gi.repository import Gegl
+import sys
+
+def check_operations(required_ops):
+  known_ops = Gegl.list_operations()
+
+  missing_ops = []
+
+  for op in required_ops:
+    if not op in known_ops:
+      print("Could not find required operation:", op)
+      missing_ops.append(op)
+
+  return not missing_ops
+
+out_file_name = "xml-graph-output.png"
+
+text_xml = """
+<?xml version='1.0' encoding='UTF-8'?>
+<gegl>
+  <node operation='gegl:text'>
+    <params>
+      <param name='string'>Hello World</param>
+      <param name='color'>rgb(1.0, 1.0, 1.0)</param>
+      <param name='size'>24</param>
+    </params>
+  </node>
+</gegl>
+"""
+
+background_xml = """
+<?xml version='1.0' encoding='UTF-8'?>
+<gegl>
+  <node operation='gegl:plasma'>
+    <params>
+      <param name='width'>256</param>
+      <param name='height'>128</param>
+    </params>
+  </node>
+</gegl>
+"""
+
+if __name__ == '__main__':
+  Gegl.init(sys.argv)
+
+  if not check_operations(["gegl:png-save", "gegl:text", "gegl:plasma", "gegl:translate"]):
+    sys.exit(1)
+
+  graph = Gegl.Node()
+
+  # Load the graphs from xml, if they were in external files
+  # we would use Gegl.Node.new_from_file() instead.
+  lower_graph = Gegl.Node.new_from_xml(background_xml, "/")
+  upper_graph = Gegl.Node.new_from_xml(text_xml, "/")
+
+  # Add a reference from our main node to the xml graphs so they
+  # can't go out of scope as long as "graph" is alive.
+  graph.add_child(lower_graph)
+  graph.add_child(upper_graph)
+
+  # Center the upper graph on the lower graph
+  text_bbox = upper_graph.get_bounding_box()
+  background_bbox = lower_graph.get_bounding_box()
+
+  x_offset = max((background_bbox.width - text_bbox.width) / 2, 0)
+  y_offset = max((background_bbox.height - text_bbox.height) / 2, 0)
+
+  translate = graph.create_child("gegl:translate")
+  translate.set_property("x", x_offset)
+  translate.set_property("y", y_offset)
+
+  upper_graph.connect_to("output", translate, "input")
+
+  # Use the "gegl:over" to combine the two graphs
+  over = graph.create_child("gegl:over")
+  lower_graph.connect_to("output", over, "input")
+  translate.connect_to("output", over, "aux")
+
+  # Save the result to a png file
+  save_node = graph.create_child ("gegl:png-save")
+  save_node.set_property("path", out_file_name)
+  over.connect_to("output", save_node, "input")
+  save_node.process()
+
+  print("Save to", out_file_name)