Storage size must be the actual encoded (i.e. stored) resolution of the video stream, without any anamorphic de-squeeze applied. Other values will lead to distorted rendering.

This will fix the the rendering for files with PlayRes different from the video's storage resolution if the video is not anamorphic. Anamorphic files are currently mistreated even beyond subtitle rendering (at least with how ass_renderer is used by bubblesub) and will require more effort to fix. Nonetheless, this already constitutes a clear improvement.

Setting pixel aspect to 1.0 is also incorrect for anamorppic content. If rendering at video resolution as ass_renderer does, libass can calculate the pixel aspect itself if storage_size and frame_size are both set correctly.

Reported in: https://github.com/bubblesub/bubblesub/issues/85

ASS has standard default values for those headers but ass_renderer assigns different ones. If those fields are linked with the actual C object from libass, i.e. the values are automatically updated when libass changes them, simply removing the custom defaults is the correct thing to do as libass will take care of it. If not, and those fields are exposed/used somewhere (after #1 removes the usage as a bogus storage size) a patch similar to the following will emulate the correct fallbacks:

From 9c921570adae3bcdbdffc7b782be1ec5accf2edf Mon Sep 17 00:00:00 2001
From: Oneric <[email protected]>
Date: Wed, 16 Feb 2022 19:38:24 +0100
Subject: [PATCH] Fix default values for ASS headers

 - ScaledBorderAndShadow defaults to no
 - WrapStyle defaults to 0 ("smart" mode)
 - PlayRes{X,Y} has obscure fallback values
---
 ass_renderer/libass.py | 24 +++++++++++++++++-------
 1 file changed, 17 insertions(+), 7 deletions(-)

diff --git a/ass_renderer/libass.py b/ass_renderer/libass.py
index 0cad674..b714057 100644
--- a/ass_renderer/libass.py
+++ b/ass_renderer/libass.py
@@ -369,16 +369,26 @@ class AssTrack(ctypes.Structure):
         text = write_ass(ass_file).encode("utf-8")
         _libass.ass_process_data(ctypes.byref(self), text, len(text))
 
-        self.play_res_x = int(
-            ass_file.script_info.get("PlayResX") or video_resolution[0]
+        self.wrap_style = int(ass_file.script_info.get("WrapStyle") or 0)
+        self.scaled_border_and_shadow = (
+            ass_file.script_info.get("ScaledBorderAndShadow", "no") == "yes"
         )
-        self.play_res_y = int(
-            ass_file.script_info.get("PlayResY") or video_resolution[1]
+        res_x = int(
+            ass_file.script_info.get("PlayResX") or 0
         )
-        self.wrap_style = int(ass_file.script_info.get("WrapStyle") or 1)
-        self.scaled_border_and_shadow = (
-            ass_file.script_info.get("ScaledBorderAndShadow", "yes") == "yes"
+        res_y = int(
+            ass_file.script_info.get("PlayResY") or 0
         )
+        if res_x <= 0 and res_y <= 0:
+            res_x = 384
+            res_y = 288
+        elif res_x <= 0:
+            res_x = 1280 if res_y == 1024 else max(1, res_y * 4 // 3)
+        elif res_y <= 0:
+            res_y = 1024 if res_x == 1280 else max(1, res_x * 3 // 4)
+        self.play_res_x = res_x
+        self.play_res_y = res_y
+
 
 
 _libc.free.argtypes = [ctypes.c_void_p]
-- 
2.30.2