Add extension module to unpremultiply and swizzle pixels

Avoid the multiple passes over the data necessary when doing the
conversion through PIL.

Author: bgilbert

openslide/_convert.c

@@ -0,0 +1,118 @@
+/*
+ * openslide-python - Python bindings for the OpenSlide library
+ *
+ * Copyright (c) 2015 Carnegie Mellon University
+ *
+ * This library is free software; you can redistribute it and/or modify it
+ * under the terms of version 2.1 of the GNU Lesser General Public License
+ * as published by the Free Software Foundation.
+ *
+ * This library is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+ * or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
+ * License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this library; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include <Python.h>
+
+typedef unsigned char u8;
+
+#ifdef WORDS_BIGENDIAN
+#define CA 0
+#define CR 1
+#define CG 2
+#define CB 3
+#else
+#define CB 0
+#define CG 1
+#define CR 2
+#define CA 3
+#endif
+
+static void
+argb2rgba(u8 *buf, Py_ssize_t len)
+{
+    Py_ssize_t cur;
+
+    for (cur = 0; cur < len; cur += 4) {
+        u8 a = buf[cur + CA];
+        u8 r = buf[cur + CR];
+        u8 g = buf[cur + CG];
+        u8 b = buf[cur + CB];
+        if (a != 0 && a != 255) {
+            r = r * 255 / a;
+            g = g * 255 / a;
+            b = b * 255 / a;
+        }
+        buf[cur + 0] = r;
+        buf[cur + 1] = g;
+        buf[cur + 2] = b;
+        buf[cur + 3] = a;
+    }
+}
+
+// Takes one argument: a contiguous buffer object.  Modifies it in place.
+static PyObject *
+_convert_argb2rgba(PyObject *self, PyObject *args)
+{
+    PyObject *ret = NULL;
+    Py_buffer view;
+
+    if (!PyArg_ParseTuple(args, "s*", &view))
+        return NULL;
+    if (!PyBuffer_IsContiguous(&view, 'A')) {
+        PyErr_SetString(PyExc_ValueError, "Argument is not contiguous");
+        goto DONE;
+    }
+    if (view.readonly) {
+        PyErr_SetString(PyExc_ValueError, "Argument is not writable");
+        goto DONE;
+    }
+    if (view.len % 4) {
+        PyErr_SetString(PyExc_ValueError, "Argument has invalid size");
+        goto DONE;
+    }
+
+    Py_BEGIN_ALLOW_THREADS
+    argb2rgba(view.buf, view.len);
+    Py_END_ALLOW_THREADS
+
+    Py_INCREF(Py_None);
+    ret = Py_None;
+
+DONE:
+    PyBuffer_Release(&view);
+    return ret;
+}
+
+static PyMethodDef ConvertMethods[] = {
+    {"argb2rgba", _convert_argb2rgba, METH_VARARGS,
+        "Convert aRGB to RGBA in place."},
+    {NULL, NULL, 0, NULL}
+};
+
+#if PY_MAJOR_VERSION >= 3
+static struct PyModuleDef convertmodule = {
+    PyModuleDef_HEAD_INIT,
+    "_convert",
+    NULL,
+    0,
+    ConvertMethods
+};
+
+PyMODINIT_FUNC
+PyInit__convert(void)
+{
+    return PyModule_Create(&convertmodule);
+}
+#else
+PyMODINIT_FUNC
+init_convert(void)
+{
+    Py_InitModule("_convert", ConvertMethods);
+}
+#endif

openslide/lowlevel.py

@@ -54,6 +54,24 @@
 else:
     _lib = cdll.LoadLibrary('libopenslide.so.0')
 
+try:
+    from . import _convert
+    def _load_image(buf, size):
+        '''buf must be a mutable buffer.'''
+        _convert.argb2rgba(buf)
+        return PIL.Image.frombuffer('RGBA', size, buf, 'raw', 'RGBA', 0, 1)
+except ImportError:
+    def _load_image(buf, size):
+        '''buf can be a string, but should be a ctypes buffer to avoid an
+        extra copy in the caller.'''
+        # First reorder the bytes in a pixel from native-endian aRGB to
+        # big-endian RGBa to work around limitations in RGBa loader
+        rawmode = (sys.byteorder == 'little') and 'BGRA' or 'ARGB'
+        buf = PIL.Image.frombuffer('RGBA', size, buf, 'raw', rawmode, 0,
+                1).tostring()
+        # Now load the image as RGBA, undoing premultiplication
+        return PIL.Image.frombuffer('RGBA', size, buf, 'raw', 'RGBa', 0, 1)
+
 class OpenSlideError(Exception):
     """An error produced by the OpenSlide library.
 
@@ -161,17 +179,6 @@ def _func(name, restype, argtypes, errcheck=_check_error):
         func.errcheck = errcheck
     return func
 
-def _load_image(buf, size):
-    '''buf can be a string, but should be a ctypes buffer to avoid an extra
-    copy in the caller.'''
-    # First reorder the bytes in a pixel from native-endian aRGB to
-    # big-endian RGBa to work around limitations in RGBa loader
-    rawmode = (sys.byteorder == 'little') and 'BGRA' or 'ARGB'
-    buf = PIL.Image.frombuffer('RGBA', size, buf, 'raw', rawmode, 0,
-            1).tostring()
-    # Now load the image as RGBA, undoing premultiplication
-    return PIL.Image.frombuffer('RGBA', size, buf, 'raw', 'RGBa', 0, 1)
-
 try:
     detect_vendor = _func('openslide_detect_vendor', c_char_p, [_utf8_p],
             _check_string)

setup.py

@@ -1,5 +1,5 @@
 import os
-from setuptools import setup
+from setuptools import setup, Extension
 
 # Load version string
 _verfile = os.path.join(os.path.dirname(__file__), 'openslide', '_version.py')
@@ -12,6 +12,9 @@
     packages=[
         'openslide',
     ],
+    ext_modules=[
+        Extension('openslide._convert', ['openslide/_convert.c']),
+    ],
     maintainer='OpenSlide project',
     maintainer_email='[email protected]',
     description='Python interface to OpenSlide',