Add cairo example code, and use example-generated image in readme

examples/cairo_example.py

@@ -0,0 +1,68 @@
+#! /usr/bin/env python
+# -*- coding: utf-8 -*-
+
+# Copyright 2015 Hamilton Kibbe <ham@hamiltonkib.be>
+
+# 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.
+
+"""
+This example demonstrates the use of pcb-tools with cairo to render a composite
+image from a set of gerber files. Each layer is loaded and drawn using a
+GerberCairoContext. The color and opacity of each layer can be set individually.
+Once all thedesired layers are drawn on the context, the context is written to
+a .png file.
+"""
+
+import os
+from gerber import read
+from gerber.render import GerberCairoContext
+
+GERBER_FOLDER = os.path.abspath(os.path.join(os.path.dirname(__file__), 'gerbers'))
+
+
+
+
+
+# Open the gerber files
+copper = read(os.path.join(GERBER_FOLDER, 'copper.GTL'))
+mask = read(os.path.join(GERBER_FOLDER, 'soldermask.GTS'))
+silk = read(os.path.join(GERBER_FOLDER, 'silkscreen.GTO'))
+drill = read(os.path.join(GERBER_FOLDER, 'ncdrill.DRD'))
+
+
+# Create a new drawing context
+ctx = GerberCairoContext()
+
+# Draw the copper layer
+copper.render(ctx)
+
+# Set opacity and color for soldermask layer
+ctx.alpha = 0.65
+ctx.color = (0.2, 0.2, 0.75)
+
+# Draw the soldermask layer
+mask.render(ctx)
+
+# Set opacity and color for silkscreen layer
+ctx.alpha = 0.9
+ctx.color = (1, 1, 1)
+
+# Draw the silkscreen layer
+silk.render(ctx)
+
+# Set opacity for drill layer
+ctx.alpha = 1.
+drill.render(ctx)
+
+# Write output to png file
+ctx.dump(os.path.join(os.path.dirname(__file__), 'cairo_example.png'))