0 means fit the other argument only.

Toolbox/resize.py

@@ -18,8 +18,33 @@ else:
     except ValueError:
         print('you did it wrong')
         quit()
+
+def fit_into_bounds(image_width, image_height, frame_width, frame_height):
+    '''
+    Given the w+h of the image and the w+h of the frame,
+    return new w+h that fits the image into the frame
+    while maintaining the aspect ratio.
+
+    (1920, 1080, 400, 400) -> (400, 225)
+    '''
+    width_ratio = frame_width / image_width
+    height_ratio = frame_height / image_height
+    ratio = min(width_ratio, height_ratio)
+
+    new_width = int(image_width * ratio)
+    new_height = int(image_height * ratio)
+
+    return (new_width, new_height)
+
+(image_width, image_height) = i.size
+
+if new_x == 0:
+    (new_x, new_y) = fit_into_bounds(image_width, image_height, 10000000, new_y)
+if new_y == 0:
+    (new_x, new_y) = fit_into_bounds(image_width, image_height, new_x, 10000000)
+
 print(i.size, new_x, new_y)
-i = i.resize( (new_x, new_y) )
+i = i.resize( (new_x, new_y), Image.ANTIALIAS)
 if newname is None:
     if '.' in filename:
         suffix = '_{width}x{height}'.format(width=new_x, height=new_y)