Enable NUM_THREADS option for reproject().

Closes #60.

Usage:

  from rasterio.warp import reproject()
  reproject(..., num_threads=2)

In practice, I'm not seeing a speed up for modestly sized
images. I think you'd have to get into some very large rasters
to see a difference.

docs/datasets.rst

@@ -38,25 +38,16 @@ Attributes
 
 In addition to the file-like attributes shown above, a dataset has a number
 of other read-only attributes that help explain its role in spatial information
-systems.
+systems. The ``driver``
 
 .. code-block:: pycon
 
     >>> dataset.driver
     u'GTiff'
-    >>> dataset.shape
-    (718, 791)
     >>> dataset.height, dataset.width
     (718, 791)
     >>> dataset.shape
     (718, 791)
-    >>> dataset.transform
-    [101985.0, 300.0379266750948, 0.0, 2826915.0, 0.0, -300.041782729805]
-    >>> dataset.crs
-    {u'units': u'm', u'no_defs': True, u'ellps': u'WGS84', u'proj': u'utm', u'zone': 18}
-
-.. code-block:: pycon
-
     >>> dataset.count
     3
     >>> dataset.dtypes
@@ -64,6 +55,20 @@ systems.
     >>> dataset.nodatavals
     [0.0, 0.0, 0.0]
 
+What makes geospatial raster datasets different from other raster files is
+that their pixels map to regions of the Earth. A dataset has a coordinate
+reference system and an affine transformation matrix that maps pixel
+coordinates to coordinates in that reference system.
+
+.. code-block:: pycon
+
+    >>> dataset.crs
+    {u'units': u'm', u'no_defs': True, u'ellps': u'WGS84', u'proj': u'utm', u'zone': 18}
+    >>> dataset.transform
+    [101985.0, 300.0379266750948, 0.0, 2826915.0, 0.0, -300.041782729805]
+
+
+
 Reading data
 ------------
 

rasterio/_gdal.pxd

@@ -5,11 +5,13 @@ cdef extern from "cpl_conv.h":
     void *  CPLMalloc (size_t)
     void    CPLFree (void *ptr)
     void    CPLSetThreadLocalConfigOption (char *key, char *val)
+    const char *CPLGetConfigOption (char *, char *)
 
 cdef extern from "cpl_string.h":
     int CSLCount (char **papszStrList)
     char ** CSLAddNameValue (char **papszStrList, const char *pszName, const char *pszValue)
     int CSLFindName (char **papszStrList, const char *pszName)
+    const char * CSLFetchNameValue (char **papszStrList, const char *pszName)
     char ** CSLSetNameValue (char **list, char *name, char *val)
     void    CSLDestroy (char **list)
 

rasterio/_warp.pyx

@@ -86,6 +86,10 @@ def reproject(
     cdef char *dstwkt= NULL
     cdef const char *proj_c
     cdef void *osr
+    cdef char **warp_extras
+    cdef char *key_c
+    cdef char *val_c
+    cdef const char* pszWarpThreads
 
     # If the source is an ndarray, we copy to a MEM dataset.
     # We need a src_transform and src_dst in this case. These will
@@ -223,8 +227,23 @@ def reproject(
         log.debug("Created transformer")
 
         psWOptions = _gdal.GDALCreateWarpOptions()
-        # TODO: get options from kwargs
-        # psWOptions = GDALCloneWarpOptions( psOptions );
+        
+        warp_extras = psWOptions.papszWarpOptions
+        for k, v in kwargs.items():
+            k, v = k.upper(), str(v).upper()
+            key_b = k.encode('utf-8')
+            val_b = v.encode('utf-8')
+            key_c = key_b
+            val_c = val_b
+            warp_extras = _gdal.CSLSetNameValue(warp_extras, key_c, val_c)
+        
+        pszWarpThreads = _gdal.CSLFetchNameValue(warp_extras, "NUM_THREADS")
+        if pszWarpThreads == NULL:
+            pszWarpThreads = _gdal.CPLGetConfigOption(
+                                "GDAL_NUM_THREADS", "1")
+            warp_extras = _gdal.CSLSetNameValue(
+                warp_extras, "NUM_THREADS", pszWarpThreads)
+
         log.debug("Created warp options")
     
         psWOptions.eResampleAlg = <_gdal.GDALResampleAlg>resampling
@@ -261,7 +280,7 @@ def reproject(
             log.debug(
                 "Chunk and warp window: %d, %d, %d, %d",
                 0, 0, cols, rows)
-            eErr = oWarper.ChunkAndWarpImage(0, 0, cols, rows)
+            eErr = oWarper.ChunkAndWarpMulti(0, 0, cols, rows)
             log.debug("Chunked and warped: %d", retval)
     
     except Exception, e:

rasterio/tests/test_warp.py

@@ -106,3 +106,33 @@ def test_warp_from_to_file(tmpdir):
                 reproject(rasterio.band(src, i), rasterio.band(dst, i))
     # subprocess.call(['open', tiffname])
 
+def test_warp_from_to_file_multi(tmpdir):
+    """File to file"""
+    tiffname = str(tmpdir.join('foo.tif'))
+    with rasterio.open('rasterio/tests/data/RGB.byte.tif') as src:
+        dst_transform = [-8789636.708, 300.0, 0.0, 2943560.235, 0.0, -300.0]
+        dst_crs = dict(
+                    proj='merc',
+                    a=6378137,
+                    b=6378137,
+                    lat_ts=0.0,
+                    lon_0=0.0,
+                    x_0=0.0,
+                    y_0=0,
+                    k=1.0,
+                    units='m',
+                    nadgrids='@null',
+                    wktext=True,
+                    no_defs=True)
+        kwargs = src.meta.copy()
+        kwargs.update(
+            transform=dst_transform,
+            crs=dst_crs)
+        with rasterio.open(tiffname, 'w', **kwargs) as dst:
+            for i in (1, 2, 3):
+                reproject(
+                    rasterio.band(src, i), 
+                    rasterio.band(dst, i),
+                    num_threads=2)
+    # subprocess.call(['open', tiffname])
+