improve: lightweight preview to reduce network traffic (#733)

* To reduce bandwidth traffic in a remote environment, a lossy compression-based preview mode is provided for displaying simple visualizations in node-based widgets.

* Added 'preview=[image format]' option to the '/view' API.
* Updated node to use preview for displaying images as widgets.
* Excluded preview usage in the open image, save image, mask editor where the original data is required.

* Made preview_format parameterizable for extensibility.

* default preview format changed: jpeg -> webp

* Support advanced preview_format option.
- grayscale option for visual debugging
- quality option for aggressive reducing

L?;format;quality?

ex)
jpeg => rgb, jpeg, quality 90
L;webp;80 => grayscale, webp, quality 80
L;png => grayscale, png, quality 90
webp;50 => rgb, webp, quality 50

* move comment

* * add settings for preview_format
* default value is ''(= don't reencode)

---------

Co-authored-by: Lt.Dr.Data <lt.dr.data@gmail.com>

server.py

@@ -217,6 +217,28 @@ class PromptServer():
                 file = os.path.join(output_dir, filename)
 
                 if os.path.isfile(file):
+                    if 'preview' in request.rel_url.query:
+                        with Image.open(file) as img:
+                            preview_info = request.rel_url.query['preview'].split(';')
+
+                            if preview_info[0] == "L" or preview_info[0] == "l":
+                                img = img.convert("L")
+                                image_format = preview_info[1]
+                            else:
+                                img = img.convert("RGB")  # jpeg doesn't support RGBA
+                                image_format = preview_info[0]
+
+                            quality = 90
+                            if preview_info[-1].isdigit():
+                                quality = int(preview_info[-1])
+
+                            buffer = BytesIO()
+                            img.save(buffer, format=image_format, optimize=True, quality=quality)
+                            buffer.seek(0)
+
+                            return web.Response(body=buffer.read(), content_type=f'image/{image_format}',
+                                                headers={"Content-Disposition": f"filename=\"{filename}\""})
+
                     if 'channel' not in request.rel_url.query:
                         channel = 'rgba'
                     else: