io: qt4 plugin: allow multiple images to be displayed. Keep track of Windows so that they don't get deallocated before they are closed.

scikits/image/io/qt_plugin.py

@@ -1,31 +1,55 @@
 import plugin
 import numpy as np
+import sys
+
+app = [None]
+windows = []
 
 try:
-    from PyQt4.QtGui import QApplication, QImage, QPixmap, QLabel
+    from PyQt4.QtGui import (QApplication, QMainWindow, QImage, QPixmap,
+                             QLabel)
 except ImportError:
-    have_qt = False
+    pass
 else:
-    have_qt = True
 
-def show(arr):
-    arr = np.ascontiguousarray(arr.astype(np.uint8))
-    if arr.ndim != 3:
-        raise ValueError("Qt only displays colour images.")
+    class ImageWindow(QMainWindow):
+        def __init__(self, arr):
+            QMainWindow.__init__(self)
 
-    if arr.shape[-1] == 4:
-        raise ValueError("Alpha channels not yet supported.")
+            img = QImage(arr.data, arr.shape[1], arr.shape[0],
+                         QImage.Format_RGB888)
+            pm = QPixmap.fromImage(img)
 
-    img = QImage(arr.data, arr.shape[1], arr.shape[0], QImage.Format_RGB888)
-    pm = QPixmap.fromImage(img)
+            label = QLabel()
+            label.setPixmap(pm)
+            label.show()
 
-    app = QApplication([])
+            self.label = label
+            self.setCentralWidget(self.label)
 
-    label = QLabel()
-    label.setPixmap(pm)
-    label.show()
+        def closeEvent(self, event):
+            # Allow window to be destroyed by removing any
+            # references to it
+            windows.remove(self)
 
-    app.exec_()
+    def show(arr, block=True):
+        arr = np.ascontiguousarray(arr.astype(np.uint8))
+        if arr.ndim != 3:
+            raise ValueError("Qt only displays colour images.")
+
+        if arr.shape[-1] == 4:
+            raise ValueError("Alpha channels not yet supported.")
+
+        if not '-qt4thread' in sys.argv and app[0] is None:
+            app[0] = QApplication([])
+
+        iw = ImageWindow(arr)
+        iw.show()
+
+        # Keep track of window so that it doesn't get destroyed
+        windows.append(iw)
+
+        if app[0] and block:
+            app[0].exec_()
 
-if have_qt:
     plugin.register('qt', show=show)