Subtitle Edit - ssa dark font hevc files

[maverickluke]

Does anyone know why when an ssa is added to an mkv video which is HEVC the font is unreadable because the font color is black? I am encoding mkv video files to mp4 using vidcoder so I can watch on my Apple TV. I use subtitle edit to take an srt or mks saving it as an ssa. I then use mkvtoolnix to create a new mkv video with the ssa added with the video and audio. When I then play the mkv video on my computer the subtitles are unreadable because the font is a black font. The converted mp4 video file has the same black font also. This only happens with HEVC mkv files and not AVC files.

What do I need to do to fix this?

[ProWo]

The mp4 container accepts some video and audio codecs, but only vobsub and mov text as subtitles.
Maybe you don't need to convert your mkv at all, just remux it to mp4. It depends on the codecs of the mkv.
You can try clever FFmpeg-GUI.
Load your mkv, click main, click multiplex, the streams of the mkv are already loaded, click add and add your srt subtitle.
Choose mp4 as container and click multiplex.
If the codecs are compatible, you will get your desired mp4 video.

[maverickluke]

I am looking to permanently add subtitles to an mp4 file. These are usually for non-english foreign parts. I have been successfully doing this for years with AVC codec files. (See my original post for details. The only issue I am having is trying to do the same thing with video files encoded with the HEVC codec.

[maverickluke]

I also used clever FFmpeg-GUI to remux the mp4 adding an ssa subtitle. When I played the remuxed mp4 the subtitles didn't even appear on the screen. I used MediaInfo to confirm the subtitle was added.

[ProWo]

You must select the subtitle in the player.
It's not selected automatically.