A quick "refresh"... user bsawlor (thank you!) pointed out a float crash. Sliders return floats now and thus needs casting in this demo.

DemoPrograms/Demo_PNG_Thumbnail_Viewer.py

@@ -1,11 +1,23 @@
 #!/usr/bin/env python
+import PIL
 from PIL import Image
 from sys import exit
 import PySimpleGUI as sg
 import os
 import io
-import numpy as np
+import base64
 
+"""
+    Demo PNG Thumbnail Viewer
+
+    Displays PNG files from a folder.
+    
+    OK, so... this isn't the best Demo Program, that's for sure.  It's one of the older
+    demos in the repo.  There are likely better ones to use.  The convert_to_bytes function is
+    the best thing in this demo.
+
+    Copyright 2021 PySimpleGUI.org
+"""
 
 
 thumbnails = {}
@@ -13,28 +25,62 @@ ROWS = 8
 COLUMNS = 8
 sg.set_options(border_width=0)
 # Get the folder containing the images from the user
-# folder = 'A:/TEMP/pdfs'
 folder = sg.popup_get_folder('Image folder to open')
 if folder is None:
     sg.popup_cancel('Cancelling')
     exit(0)
 
 
-def image_file_to_bytes(filename, size):
+
+
+def convert_to_bytes(file_or_bytes, resize=None):
+    """
+    Will convert into bytes and optionally resize an image that is a file or a base64 bytes object.
+    Turns into  PNG format in the process so that can be displayed by tkinter
+    :param file_or_bytes: either a string filename or a bytes base64 image object
+    :type file_or_bytes:  (Union[str, bytes])
+    :param resize:  optional new size
+    :type resize: (Tuple[int, int] or None)
+    :param fill: If True then the image is filled/padded so that the image is not distorted
+    :type fill: (bool)
+    :return: (bytes) a byte-string object
+    :rtype: (bytes)
+    """
+    if isinstance(file_or_bytes, str):
+        img = PIL.Image.open(file_or_bytes)
+    else:
         try:
-        image = Image.open(filename)
-        image.thumbnail(size, Image.ANTIALIAS)
-        bio = io.BytesIO()  # a binary memory resident stream
-        image.save(bio, format='PNG')  # save image as png to it
-        imgbytes = bio.getvalue()
-    except:
-        imgbytes = None
-    return imgbytes
+            img = PIL.Image.open(io.BytesIO(base64.b64decode(file_or_bytes)))
+        except Exception as e:
+            dataBytesIO = io.BytesIO(file_or_bytes)
+            img = PIL.Image.open(dataBytesIO)
+
+    cur_width, cur_height = img.size
+    if resize:
+        new_width, new_height = resize
+        scale = min(new_height / cur_height, new_width / cur_width)
+        img = img.resize((int(cur_width * scale), int(cur_height * scale)), PIL.Image.ANTIALIAS)
+    with io.BytesIO() as bio:
+        img.save(bio, format="PNG")
+        del img
+        return bio.getvalue()
+#
+# old, original PIL code.
+# def image_file_to_bytes(filename, size):
+#     try:
+#         image = Image.open(filename)
+#         image.thumbnail(size, Image.ANTIALIAS)
+#         bio = io.BytesIO()  # a binary memory resident stream
+#         image.save(bio, format='PNG')  # save image as png to it
+#         imgbytes = bio.getvalue()
+#     except:
+#         imgbytes = None
+#     return imgbytes
 
 
 def set_image_to_blank(key):
-    img = Image.new('RGB', (100, 100), (255, 255, 255))
-    img.thumbnail((1, 1), Image.ANTIALIAS)
+    img = PIL.Image.new('RGB', (100, 100), (255, 255, 255))
+    img.thumbnail((1, 1), PIL.Image.ANTIALIAS)
     bio = io.BytesIO()
     img.save(bio, format='PNG')
     imgbytes = bio.getvalue()
@@ -43,8 +89,8 @@ def set_image_to_blank(key):
 
 # get list of PNG files in folder
 png_files = [os.path.join(folder, f)
-             for f in os.listdir(folder) if '.png' in f]
-filenames_only = [f for f in os.listdir(folder) if '.png' in f]
+             for f in os.listdir(folder) if f.endswith('.png')]
+filenames_only = [f for f in os.listdir(folder) if f.endswith('.png')]
 
 if len(png_files) == 0:
     sg.popup('No PNG images in folder')
@@ -65,7 +111,7 @@ col_buttons = [[]]
 
 # define layout, show and read the window
 col = [[sg.Text(png_files[0], size=(80, 3), key='filename')],
-       [sg.Image(data=image_file_to_bytes(png_files[0], (500, 500)), key='image')], ]
+       [sg.Image(data=convert_to_bytes(png_files[0], (500, 500)), key='image')], ]
 
 layout = [
     [sg.Menu(menu)],
@@ -82,11 +128,11 @@ while True:
     
     for x in range(ROWS):               # update thumbnails
         for y in range(COLUMNS):
-            cur_index = display_index + (x * 4) + y
+            cur_index = display_index + (x * COLUMNS) + y
             if cur_index < len(png_files):
                 filename = png_files[cur_index]
                 if filename not in thumbnails:
-                    imgbytes = image_file_to_bytes(filename, (100, 100))
+                    imgbytes = convert_to_bytes(filename, (100, 100))
                     thumbnails[filename] = imgbytes
                 else:
                     imgbytes = thumbnails[filename]
@@ -96,7 +142,7 @@ while True:
                 set_image_to_blank((x, y))
 
     event, values = window.read()
-    display_index = values['-slider-']
+    display_index = int(values['-slider-'])
     # --------------------- Button & Keyboard ---------------------
     if event in (sg.WIN_CLOSED, 'Exit'):
         break
@@ -128,10 +174,10 @@ while True:
         sg.popup('Demo PNG Viewer Program', 'Please give PySimpleGUI a try!')
     elif type(event) is tuple:
         x, y = event
-        image_index = display_index + (x * 4) + y
+        image_index = display_index + (x * COLUMNS) + y
         if image_index < len(png_files):
             filename = png_files[image_index]
-            imgbytes = image_file_to_bytes(filename, (500, 500))
+            imgbytes = convert_to_bytes(filename, (500, 500))
             window['image'].update(data=imgbytes)
             window['filename'].update(filename)