TUI/Art further alpha improvements

Blended alpha against black should make uniform on images with saved
alpha colors.

Improved clip detection

src/tui/widgets/art.rs

@@ -14,6 +14,17 @@ pub struct Art {
     pub area: Rect,
 }
 
+// blend alpha onto black canvas
+fn alpha([r, g, b, a]: [u8; 4]) -> [u8; 3] {
+    if a == u8::MAX {
+        return [r, g, b];
+    } else if a == 0 {
+        return [0, 0, 0];
+    };
+    let a = a as f32 / u8::MAX as f32;
+    [r, g, b].map(|c| (c as f32 * a).round() as u8)
+}
+
 impl ContainedWidget for Art {
     fn draw(&mut self, frame: &mut ratatui::Frame, stylesheet: super::StyleSheet) {
         if self.area.width == 0 || self.area.height == 0 {
@@ -23,6 +34,8 @@ impl ContainedWidget for Art {
         let (w, h) = (self.area.width as usize, self.area.height as usize * 2);
         if let Some(thumbnail) = library.thumbnail(w, h) {
             let fill = stylesheet.base.bg.unwrap_or(Color::Black);
+            // clip at 5% or less
+            const CLIP: u8 = u8::MAX / 20;
             let lines: Vec<Line> = thumbnail
                 .chunks(2)
                 .map(|rows| {
@@ -36,26 +49,27 @@ impl ContainedWidget for Art {
 
                             // no alpha blending because the 16 terminal colors aren't readable
                             let content = if let Some(bg) = bgiter.next() {
-                                if bg == fg {
-                                    // All alpha only draw space to support
-                                    // terminal emulator transparency
-                                    if fg[3] == 0 {
-                                        style = style.bg(fill);
-                                        " "
-                                    // Uniform solid only draw block to avoid
-                                    // terminal emulator transparency
-                                    } else {
-                                        style = style.fg(Color::Rgb(fg[0], fg[1], fg[2]));
-                                        "█"
-                                    }
+                                // All alpha only draw space to support
+                                // terminal emulator transparency
+                                if bg[3] <= CLIP && fg[3] <= CLIP {
+                                    style = style.bg(fill);
+                                    " "
+                                // Uniform solid only draw block to avoid
+                                // terminal emulator transparency
+                                } else if bg == fg {
+                                    let fg = alpha(*fg);
+                                    style = style.fg(Color::Rgb(fg[0], fg[1], fg[2]));
+                                    "█"
                                 } else {
+                                    let (fg, bg) = (alpha(*fg), alpha(*bg));
                                     style = style.fg(Color::Rgb(fg[0], fg[1], fg[2])).bg(Color::Rgb(bg[0], bg[1], bg[2]));
                                     "▀"
                                 }
-                            } else if fg[3] == 0 {
+                            } else if fg[3] <= CLIP {
                                 style = style.bg(fill);
                                 " "
                             } else {
+                                let fg = alpha(*fg);
                                 style = style.fg(Color::Rgb(fg[0], fg[1], fg[2]));
                                 "▀"
                             };