add test for alpha blending during bilinear scaling - refs #997

tests/data/good_maps/raster-alpha.xml

@@ -0,0 +1,19 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!DOCTYPE Map[]>
+<Map srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over" background-color="red">
+
+<Style name="white" filter-mode="first">
+  <Rule>
+    <RasterSymbolizer opacity="1" scaling="bilinear" />
+  </Rule>
+</Style>
+<Layer name="white"
+  srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over">
+    <StyleName>white</StyleName>
+    <Datasource>
+       <Parameter name="file">../raster/white-alpha.tiff</Parameter>
+       <Parameter name="type">gdal</Parameter>
+    </Datasource>
+  </Layer>
+
+</Map>

tests/python_tests/raster_alpha_test.py

@@ -0,0 +1,29 @@
+#!/usr/bin/env python
+
+from nose.tools import *
+from utilities import execution_path
+from copy import deepcopy
+
+import os, mapnik
+
+def setup():
+    # All of the paths used are relative, if we run the tests
+    # from another directory we need to chdir()
+    os.chdir(execution_path('.'))
+
+def test_map_alpha_compare():
+    m = mapnik.Map(600,400)
+    mapnik.load_map(m,'../data/good_maps/raster-alpha.xml')
+    m.zoom_all()
+    actual = '/tmp/mapnik-raster-alpha.png'
+    expected = 'images/support/raster-alpha.png'
+    im = mapnik.Image(m.width,m.height)
+    mapnik.render(m,im)
+    im.save(actual)
+    expected_im = mapnik.Image.open(expected)
+    eq_(im.tostring(),expected_im.tostring(), 'failed comparing actual (%s) and expected(%s)' % (actual,'tests/python_tests/'+ expected))
+
+
+if __name__ == "__main__":
+    setup()
+    [eval(run)() for run in dir() if 'test_' in run]