Fix solid background color opacity (#847)

Since landing the patch adding transparency support to Alacritty,
there's been an issue where othewise solid background cells were also
being rendered partially transparent. Now, all filled background cells
are rendered fully opaque.

Some logic was added to support discarding filled backgrounds which had
the same color as the default background. This means that, if the
default background is #000 and a cell has that background, it will never
be rendered opaque. This may not be correct.

Note that many truecolor vim color schemes print spaces for default
colored background cells. Performance can be dramatically improved by
using ctermbg=NONE guibg=NONE to skip rendering those cells.

1 files changed, 32 insertions, 7 deletions

diff --git a/src/term/mod.rs b/src/term/mod.rs
index a16971b5..97460421 100644
--- a/src/term/mod.rs
+++ b/src/term/mod.rs
@@ -299,6 +299,14 @@ impl<'a> RenderableCellsIter<'a> {
         }
     }
 
+    #[inline]
+    fn compute_bg_alpha(&self, bg: &Color) -> f32 {
+        match *bg {
+            Color::Named(NamedColor::Background) => 0.0,
+            _ => 1.0
+        }
+    }
+
     fn compute_bg_rgb(&self, bg: &Color) -> Rgb {
         match *bg {
             Color::Spec(rgb) => rgb,
@@ -314,6 +322,7 @@ pub struct RenderableCell {
     pub c: char,
     pub fg: Rgb,
     pub bg: Rgb,
+    pub bg_alpha: f32,
     pub flags: cell::Flags,
 }
 
@@ -363,19 +372,35 @@ impl<'a> Iterator for RenderableCellsIter<'a> {
                 };
 
                 // Apply inversion and lookup RGB values
-                let (fg, bg) = if selected || cell.inverse() {
-                    (self.compute_bg_rgb(&cell.bg), self.compute_fg_rgb(&cell.fg, &cell))
+                let mut bg_alpha = 1.0;
+                let fg_rgb;
+                let bg_rgb;
+
+                let invert = selected ^ cell.inverse();
+
+                if invert {
+                    if cell.fg == cell.bg {
+                        bg_rgb = self.colors[NamedColor::Foreground];
+                        fg_rgb = self.colors[NamedColor::Background];
+                        bg_alpha = 1.0
+                    } else {
+                        bg_rgb = self.compute_fg_rgb(&cell.fg, &cell);
+                        fg_rgb = self.compute_bg_rgb(&cell.bg);
+                    }
                 } else {
-                    (self.compute_fg_rgb(&cell.fg, &cell), self.compute_bg_rgb(&cell.bg))
-                };
+                    fg_rgb = self.compute_fg_rgb(&cell.fg, &cell);
+                    bg_rgb = self.compute_bg_rgb(&cell.bg);
+                    bg_alpha = self.compute_bg_alpha(&cell.bg);
+                }
 
                 return Some(RenderableCell {
                     line: line,
                     column: column,
                     flags: cell.flags,
                     c: cell.c,
-                    fg: fg,
-                    bg: bg,
+                    fg: fg_rgb,
+                    bg: bg_rgb,
+                    bg_alpha: bg_alpha,
                 })
             }
 
@@ -976,7 +1001,7 @@ impl Term {
             self.mode,
             config,
             selection,
-            self.cursor_style
+            self.cursor_style,
         )
     }