thumbnails: Take crop/scaling info from a json file next to the image file

common/common/images.py

@@ -1,27 +1,52 @@
 
+import json
 import os
 from io import BytesIO
 
 from PIL import Image
 
-# The region of the frame image to place on the template.
-# Format is (top_left_x, top_left_y, bottom_right_x, bottom_right_y).
-# The frame is scaled to the size of the template before this is done.
-FRAME_CROP = None # no crop
+"""
+A template is two files:
+	NAME.png
+	NAME.json
+The image is the template image itself.
+The JSON file contains the following:
+	{
+		crop: BOX,
+		location: BOX,
+	}
+where BOX is a 4-tuple [left x, top y, right x, bottom y] describing a rectangle in image coordinates.
 
-# The location in the template to place the frame image after cropping.
-# Format is (x, y) of top-left corner.
-FRAME_LOCATION = (0, 90)
+To create a thumbnail, the input frame is first cropped to the bounds of the "crop" box,
+then resized to the size of the "location" box, then pasted underneath the template image at that
+location within the template image.
 
+For example, a JSON file of:
+	{
+		"crop": [50, 100, 1870, 980],
+		"location": [320, 180, 1600, 900]
+	}
+would crop the input frame from (50, 100) to (1870, 980), resize it to 720x1280,
+and place it at (320, 180).
+
+If the original frame and the template differ in size, the frame is first resized to the template.
+This allows you to work with a consistent coordinate system regardless of the input frame size.
+"""
 
 def compose_thumbnail_template(base_dir, template_name, frame_data):
 	template_path = os.path.join(base_dir, "thumbnail_templates", f"{template_name}.png")
+	info_path = os.path.join(base_dir, "thumbnail_templates", f"{template_name}.json")
+
 	template = Image.open(template_path)
 	# PIL can't load an image from a byte string directly, we have to pretend to be a file
 	frame = Image.open(BytesIO(frame_data))
 
-	# The parameters of how we overlay the template are hard-coded for now.
-	# We can make this configurable later if needed.
+	with open(info_path) as f:
+		info = json.load(f)
+	crop = info['crop']
+	loc_left, loc_top, loc_right, loc_bottom = info['location']
+	location = loc_left, loc_top
+	location_size = loc_right - loc_left, loc_bottom - loc_top
 
 	# Create a new blank image of the same size as the template
 	result = Image.new('RGBA', template.size)
@@ -30,10 +55,11 @@ def compose_thumbnail_template(base_dir, template_name, frame_data):
 	# and we don't really care about performance.
 	if frame.size != template.size:
 		frame = frame.resize(template.size, Image.LANCZOS)
-	# Insert the frame at the desired location and cropping
-	if FRAME_CROP is not None:
-		frame = frame.crop(FRAME_CROP)
-	result.paste(frame, FRAME_LOCATION)
+	# Insert the frame at the desired location, cropping and scaling.
+	# Technically we might end up resizing twice here which is bad for quality,
+	# but the case of frame size != template size should be rare enough that it doesn't matter.
+	frame = frame.crop(crop).resize(location_size, Image.LANCZOS)
+	result.paste(frame, location)
 	# Place the template "on top", letting the frame be seen only where the template's alpha
 	# lets it through.
 	result.alpha_composite(template)