feat(core,player,app): subtitle size, background and colour settings

Plain-text subtitles (SRT) were drawn in mpv's bare default, thin white
text that bright scenes swallow. Settings → Playback now picks a size,
an outline or a dark box, and white or yellow text. Styled (ASS) tracks
keep their own look and take only the size.

The box options are mpv 0.38's, so they are set best effort: an older
libmpv logs and plays on. The style syncs with the other playback
settings, and the Android bridge keeps the stored style instead of
resetting it on save.

crates/pstr-player/src/lib.rs

@@ -49,6 +49,7 @@ pub use chapters::{Chapter, ChapterRole, chapter_at, chapter_end, credits_start,
 pub use error::{Error, Result};
 pub use player::{
     EndReason, MAX_VOLUME, Player, PlayerConfig, PlayerEvent, READAHEAD_BLOCKS, VideoOutput,
+    subtitle_options,
 };
 pub use registry::{PROTOCOL, StreamHandle, StreamRegistry};
 pub use render::VideoRenderer;

crates/pstr-player/src/player.rs

@@ -12,6 +12,7 @@ use std::sync::Arc;
 
 use libmpv2::events::{Event, PropertyData};
 use libmpv2::{Format, Mpv};
+use pstr_core::prefs::{SubtitleBackdrop, SubtitleColour, SubtitleStyle};
 use pstr_stream::VideoStream;
 use tokio::runtime::Runtime;
 
@@ -181,6 +182,8 @@ pub struct PlayerConfig {
     /// Whether subtitles are shown at all. Off is a choice a viewer makes once
     /// and expects to keep, and no language preference can express it.
     pub subtitles: bool,
+    /// How plain subtitles look. See [`subtitle_options`].
+    pub subtitle_style: SubtitleStyle,
     /// Raw mpv options, applied last, so anything here wins.
     pub options: Vec<(String, String)>,
 }
@@ -206,6 +209,7 @@ impl Default for PlayerConfig {
             audio_language: None,
             subtitle_language: None,
             subtitles: true,
+            subtitle_style: SubtitleStyle::default(),
             options: Vec::new(),
         }
     }
@@ -293,6 +297,17 @@ impl Player {
             if !config.subtitles {
                 init.set_property("sid", "no")?;
             }
+            // Best effort: `sub-border-style` is mpv 0.38's, and an older
+            // libmpv refusing it must cost the box, not the whole player. The
+            // first refusal ends the run, because what follows it only makes
+            // sense on top of it: a box's colour on a plain outline is a fat
+            // grey smear.
+            for (name, value) in subtitle_options(&config.subtitle_style) {
+                if let Err(error) = init.set_property(name, value.as_str()) {
+                    tracing::warn!("subtitle style {name}={value}: {error}");
+                    break;
+                }
+            }
             for (name, value) in &config.options {
                 init.set_property(name.as_str(), value.as_str())?;
             }
@@ -593,6 +608,32 @@ impl Player {
     }
 }
 
+/// The mpv options that draw `style`.
+///
+/// Leaves `sub-ass-override` at mpv's default, `scale`: a styled track takes
+/// the size and keeps its own fonts, colours and placement. See
+/// [`SubtitleStyle`] for why.
+pub fn subtitle_options(style: &SubtitleStyle) -> Vec<(&'static str, String)> {
+    let mut options = vec![("sub-scale", format!("{:.2}", style.size.scale()))];
+    match style.colour {
+        // mpv's own default; stated so a live change back from yellow lands.
+        SubtitleColour::White => options.push(("sub-color", "#FFFFFFFF".into())),
+        SubtitleColour::Yellow => options.push(("sub-color", "#FFFFE600".into())),
+    }
+    if style.backdrop == SubtitleBackdrop::Box {
+        // `opaque-box` draws the outline as a box hugging each line, so the
+        // outline's colour and size become the box's colour and margin.
+        // Ordered so the style comes first: see the caller.
+        options.extend([
+            ("sub-border-style", "opaque-box".into()),
+            // ARGB: about seventy percent black.
+            ("sub-outline-color", "#B0000000".into()),
+            ("sub-outline-size", "6".into()),
+        ]);
+    }
+    options
+}
+
 /// mpv's borrowed event to ours.
 fn translate(event: Event<'_>) -> PlayerEvent {
     match event {
@@ -620,6 +661,47 @@ fn translate(event: Event<'_>) -> PlayerEvent {
 mod tests {
     use super::*;
 
+    #[test]
+    fn the_default_subtitle_style_changes_nothing_mpv_would_not_do_anyway() {
+        let options = subtitle_options(&SubtitleStyle::default());
+        assert_eq!(
+            options,
+            vec![
+                ("sub-scale", "1.00".to_string()),
+                ("sub-color", "#FFFFFFFF".to_string()),
+            ]
+        );
+    }
+
+    #[test]
+    fn a_boxed_subtitle_style_turns_the_outline_into_a_box() {
+        let style = SubtitleStyle {
+            size: pstr_core::prefs::SubtitleSize::Larger,
+            backdrop: SubtitleBackdrop::Box,
+            colour: SubtitleColour::Yellow,
+        };
+        let options = subtitle_options(&style);
+        let value = |name: &str| {
+            options
+                .iter()
+                .find(|(option, _)| *option == name)
+                .map(|(_, value)| value.as_str())
+        };
+        assert_eq!(value("sub-scale"), Some("1.50"));
+        assert_eq!(value("sub-color"), Some("#FFFFE600"));
+        assert_eq!(value("sub-border-style"), Some("opaque-box"));
+        assert_eq!(value("sub-outline-color"), Some("#B0000000"));
+        assert_eq!(
+            options[2].0, "sub-border-style",
+            "the style before what depends on it"
+        );
+        assert_eq!(
+            value("sub-ass-override"),
+            None,
+            "a styled track keeps its own look"
+        );
+    }
+
     #[test]
     fn every_end_reason_maps_to_something_the_ui_can_act_on() {
         use libmpv2::mpv_end_file_reason as raw;