ENH Partial support for reading gray+alpha PNGs

Currently, reading gray+alpha PNGs is only possible by throwing away the alpha channel.
luispedro · Sep 13, 2016 · 8d000d0 · 8d000d0
1 parent 2476c5b
commit 8d000d0
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Showing 4 changed files with 33 additions and 7 deletions.
diff --git a/ChangeLog b/ChangeLog
@@ -2,6 +2,7 @@ Version 0.5.1+
 	* Add `supports_format` function
 	* Make png compression level tunable when calling imsave
 	* Add imsave_multi
+	* Add partial support for reading PNG files in Gray+alpha format
 
 Version 0.5.1 2014-11-06 by luispedro
 	* Improve tests to work after installation

diff --git a/imread/_imread.cpp b/imread/_imread.cpp
@@ -169,11 +169,12 @@ PyObject* py_supports_format(PyObject* self, PyObject* args) {
 PyObject* py_imread_may_multi(PyObject* self, PyObject* args, bool is_multi, bool is_blob) {
     PyObject* filename_or_blob_object;
     const char* formatstr;
-    if (!PyArg_ParseTuple(args, "Os", &filename_or_blob_object, &formatstr)) {
+    PyObject* optsDict;
+    if (!PyArg_ParseTuple(args, "OsO", &filename_or_blob_object, &formatstr, &optsDict)) {
         PyErr_SetString(PyExc_RuntimeError,TypeErrorMsg);
         return NULL;
     }
-    options_map opts;
+    options_map opts = parse_options(optsDict);
 
     try {
         std::auto_ptr<ImageFormat> format(get_format(formatstr));

diff --git a/imread/imread.py b/imread/imread.py
@@ -26,7 +26,7 @@ def _as_grey(im, as_grey):
     transform = np.array([ 0.30,  0.59,  0.11])
     return np.dot(im, transform)
 
-def imread(filename, as_grey=False, formatstr=None, return_metadata=False):
+def imread(filename, as_grey=False, formatstr=None, return_metadata=False, opts=None):
     '''
     im = imread(filename, as_grey=False, formatstr={filename extension}, return_metadata=False)
     im,meta = imread(filename, as_grey=False, formatstr={filename extension}, return_metadata=True)
@@ -45,6 +45,11 @@ def imread(filename, as_grey=False, formatstr=None, return_metadata=False):
         does not correspond to the format, you can pass it explicitly.
     return_metadata : bool, optional
         Whether to return metadata (default: False)
+    opts : dict, optional
+        Other options. Support will depend on the backend. Currently used
+
+            strip_alpha: bool
+                Whether to strip the alpha channel.
 
     Returns
     -------
@@ -60,9 +65,11 @@ def imread(filename, as_grey=False, formatstr=None, return_metadata=False):
     imread_from_blob : function
         Read from a in-memory string
     '''
+    if opts is None:
+        opts = {}
     formatstr = _parse_formatstr(filename, formatstr, 'imread')
     reader = special.get(formatstr, _imread.imread)
-    imdata,meta = reader(filename, formatstr)
+    imdata,meta = reader(filename, formatstr, opts)
     imdata = _as_grey(imdata, as_grey)
     if return_metadata:
         return imdata, meta
@@ -71,7 +78,7 @@ def imread(filename, as_grey=False, formatstr=None, return_metadata=False):
 imload = imread
 
 
-def imread_from_blob(blob, formatstr=None, as_grey=False, return_metadata=False):
+def imread_from_blob(blob, formatstr=None, as_grey=False, return_metadata=False, opts=None):
     '''
     imdata = imread_from_blob(blob, formatstr=None, as_grey=False, return_metadata={True})
     imdata,metadata = imread_from_blob(blob, formatstr=None, as_grey={False}, return_metadata=True)
@@ -93,6 +100,11 @@ def imread_from_blob(blob, formatstr=None, as_grey=False, return_metadata=False)
         Whether to convert to grey scale image (default: no)
     return_metadata : bool, optional
         Whether to return metadata (default: False)
+    opts : dict, optional
+        Other options. Support will depend on the backend. Currently used
+
+            strip_alpha: bool
+                Whether to strip the alpha channel.
 
     Returns
     -------
@@ -108,10 +120,12 @@ def imread_from_blob(blob, formatstr=None, as_grey=False, return_metadata=False)
     imread : function
         Read from a file on disk
     '''
+    if opts is None:
+        opts = {}
     reader = _imread.imread_from_blob
     if formatstr is None:
         formatstr = detect_format(blob, is_blob=True)
-    imdata,meta = reader(blob, formatstr)
+    imdata,meta = reader(blob, formatstr, opts)
     imdata = _as_grey(imdata, as_grey)
     if return_metadata:
         return imdata,meta

diff --git a/imread/lib/_png.cpp b/imread/lib/_png.cpp
@@ -1,4 +1,4 @@
-// Copyright 2012-2014 Luis Pedro Coelho <[email protected]>
+// Copyright 2012-2016 Luis Pedro Coelho <[email protected]>
 // License: MIT (see COPYING.MIT file)
 
 #include "base.h"
@@ -109,6 +109,10 @@ std::auto_ptr<Image> PNGFormat::read(byte_source* src, ImageFactory* factory, co
     // PNGs are in "network" order (ie., big-endian)
     if (bit_depth == 16 && !is_big_endian()) png_set_swap(p.png_ptr);
 
+    const bool strip_alpha = get_optional_bool(opts, "strip_alpha", false);
+    if (strip_alpha) {
+        png_set_strip_alpha(p.png_ptr);
+    }
     int d = -1;
     switch (png_get_color_type(p.png_ptr, p.png_info)) {
         case PNG_COLOR_TYPE_PALETTE:
@@ -122,6 +126,12 @@ std::auto_ptr<Image> PNGFormat::read(byte_source* src, ImageFactory* factory, co
         case PNG_COLOR_TYPE_GRAY:
             d = -1;
             break;
+        case PNG_COLOR_TYPE_GRAY_ALPHA:
+            if (!strip_alpha) {
+                throw CannotReadError("imread.png: Color type (4: grayscale with alpha channel) can  only be read when strip_alpha is set to true.");
+            }
+            d = -1;
+            break;
         default: {
             std::ostringstream out;
             out << "imread.png: Color type ("