Raise an exception for ENVI images with frame offsets.

ENVI image files with frame offsets are not supported. If an image with frame
offsets were opened, image data would be read incorrectly so an exception is
now thrown instead (this addresses issue #35).

- io/envi.py:
  - (EnviFeatureNotSupported): New class
  - (_has_frame_offset): New function to determine if image has frame offsets.
  - (check_compatibility): New function to raise exception when an unsupported
    feature is indicated in an ENVI header.
  - (open): Check compatibility before opening image.
- spectral.py (open): Check compatibility before opening image.
- tests/envi.py: Added tests for ENVI feature compatibilty

Author: tboggs

spectral/io/envi.py

@@ -78,6 +78,10 @@ def __init__(self, dtype):
           'data type names.'.format(np.dtype(dtype).name)
         super(EnviDataTypeError, self).__init__(msg)
 
+class EnviFeatureNotSupported(NotImplementedError):
+    '''A specified ENVI capability is not supported by the spectral module.'''
+    pass
+
 def _validate_dtype(dtype):
     '''Raises EnviDataTypeError if dtype can not be written to ENVI file.'''
     typename = np.dtype(dtype).name
@@ -170,6 +174,44 @@ class Params:
     p.filename = None
     return p
 
+def _has_frame_offset(params):
+    '''
+    Returns True if header params indicate non-zero frame offsets.
+
+    Arguments:
+
+        `params` (dict):
+
+            Dictionary of header parameters assocaited with hdr file.
+
+    Returns:
+
+        bool
+
+    This function returns True when either "major frame offsets" or
+    "minor frame offsets" is specified and contains a non-zero value.
+    '''
+    for param in ['major frame offsets', 'minor frame offsets']:
+        if param in params:
+            val = params[param]
+            if np.iterable(val):
+                offsets = [int(x) for x in val]
+            else:
+                offsets = [int(val)] * 2
+            if not np.all(np.equal(offsets, 0)):
+                return True
+    return False
+
+def check_compatibility(header):
+    '''
+    Verifies that all features of an ENVI header are supported.
+    '''
+    from .spyfile import find_file_path
+    if type(header) in [str, unicode]:
+        header = read_envi_header(find_file_path(header))
+    if _has_frame_offset(header):
+        raise EnviFeatureNotSupported(
+            'ENVI image frame offsets are not supported.')
 
 def open(file, image=None):
     '''
@@ -210,7 +252,7 @@ def open(file, image=None):
 
     headerPath = find_file_path(file)
     h = read_envi_header(headerPath)
-
+    check_compatibility(h)
     p = gen_params(h)
 
     inter = h["interleave"]
@@ -594,6 +636,7 @@ def _write_image(hdr_file, data, header, **kwargs):
     '''
     Write `data` as an ENVI file using the metadata in `header`.
     '''
+    check_compatibility(header)
     force = kwargs.get('force', False)
     img_ext = kwargs.get('ext', '.img')
 

spectral/spectral.py

@@ -471,6 +471,8 @@ def open_image(file):
     # Try to open it as an ENVI header file.
     try:
         return envi.open(pathname)
+    except envi.EnviFeatureNotSupported:
+        raise
     except:
         pass
 

spectral/tests/envi.py

@@ -175,6 +175,60 @@ def test_save_load_classes(self):
         gt2 = spy.open_image(fname).read_band(0)
         assert(np.all(gt == gt2))
 
+    def test_open_nonzero_frame_offset_fails(self):
+        '''Opening files with nonzero frame offsets should fail.'''
+        import os
+        import spectral as spy
+        img = spy.open_image('92AV3C.lan')
+        fname = os.path.join(testdir, 'test_open_nonzero_frame_offset_fails.hdr')
+        spy.envi.save_image(fname, img)
+        fout = open(fname, 'a')
+        fout.write('major frame offsets = 128\n')
+        fout.close()
+        try:
+            img2 = spy.envi.open(fname)
+        except spy.envi.EnviFeatureNotSupported:
+            pass
+        else:
+            raise Exception('File erroneously opened.')
+
+    def test_open_zero_frame_offset_passes(self):
+        '''Files with frame offsets set to zero should open.'''
+        import os
+        import spectral as spy
+        img = spy.open_image('92AV3C.lan')
+        fname = os.path.join(testdir, 'test_open_zero_frame_offset_passes.hdr')
+        spy.envi.save_image(fname, img)
+        fout = open(fname, 'a')
+        fout.write('major frame offsets = 0\n')
+        fout.write('minor frame offsets = {0, 0}\n')
+        fout.close()
+        img2 = spy.envi.open(fname)
+
+    def test_save_nonzero_frame_offset_fails(self):
+        '''Opening files with nonzero frame offsets should fail.'''
+        import os
+        import spectral as spy
+        img = spy.open_image('92AV3C.lan')
+        fname = os.path.join(testdir, 'test_save_nonzero_frame_offset_fails.hdr')
+        meta = {'major frame offsets' : [128, 0]}
+        try:
+            spy.envi.save_image(fname, img, metadata=meta)
+        except spy.envi.EnviFeatureNotSupported:
+            pass
+        else:
+            raise Exception('File erroneously saved.')
+
+    def test_save_zero_frame_offset_passes(self):
+        '''Opening files with nonzero frame offsets should fail.'''
+        import os
+        import spectral as spy
+        img = spy.open_image('92AV3C.lan')
+        fname = os.path.join(testdir, 'test_save_zero_frame_offset_passes.hdr')
+        meta = {'major frame offsets' : 0}
+        spy.envi.save_image(fname, img, metadata=meta)
+
+
 def run():
     print('\n' + '-' * 72)
     print('Running ENVI tests.')