ASS subtitle - Different alignment at block and inner level

Question

I am trying to achieve an ASS subtitle that aligns the rectangular bounding block at the horizontal center of the window, while maintaining left-justification inside that block for multiline subtitle entries. Below is an ASCII art illustration,

+---------------------------------+
|           This is               |
|           what I want           |
+---------------------------------+

However, either specify Alignment in Style definition or using \a switch inside a piece of subtitle results in either entirely left-justified or centered line in the container.

+---------------------------------+
|Entirely left-justified          |
|        Entirely centered        |
+---------------------------------+

My apologies if I missed something obvious in the specification. I am also open to other text-based subtitle format that is capable of this and supported by ffmpeg.

Answer 1

If you're familiar with an ass file, you can position your text by using \pos(150,250).

Example:

Dialogue: 0,0:01:00.00,0:01:15.00,Default,,0,0,0,,{\fad(500,500)\pos(150,250)}{\i1}{\b1}Your Text

The first number is from the left, and the second one is from the top of the video.

An easier solution, rather than write each line yourself, is to download Aegisub to make the ass files for you. Using Aegisub, you can drag the text lines to the position you need.