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.

2 files changed, 8 insertions, 6 deletions

diff --git a/res/text.f.glsl b/res/text.f.glsl
index dd60333c..5915960f 100644
--- a/res/text.f.glsl
+++ b/res/text.f.glsl
@@ -14,7 +14,7 @@
 #version 330 core
 in vec2 TexCoords;
 in vec3 fg;
-in vec3 bg;
+in vec4 bg;
 flat in float vb;
 flat in int background;
 
@@ -27,8 +27,11 @@ uniform sampler2D mask;
 void main()
 {
     if (background != 0) {
+        if (bg.a == 0.0)
+            discard;
+
         alphaMask = vec4(1.0);
-        color = vec4(bg + vb, 1.0) * bgOpacity;
+        color = vec4(bg.rgb + vb, 1.0);
     } else {
         vec3 textColor = texture(mask, TexCoords).rgb;
         alphaMask = vec4(textColor, textColor.r);
diff --git a/res/text.v.glsl b/res/text.v.glsl
index 081ca6b9..9580c71f 100644
--- a/res/text.v.glsl
+++ b/res/text.v.glsl
@@ -26,11 +26,11 @@ layout (location = 3) in vec4 uv;
 // text fg color
 layout (location = 4) in vec3 textColor;
 // Background color
-layout (location = 5) in vec3 backgroundColor;
+layout (location = 5) in vec4 backgroundColor;
 
 out vec2 TexCoords;
 out vec3 fg;
-out vec3 bg;
+out vec4 bg;
 
 // Terminal properties
 uniform vec2 termDim;
@@ -63,7 +63,6 @@ void main()
         gl_Position = projection * vec4(finalPosition.xy, 0.0, 1.0);
         TexCoords = vec2(0, 0);
     } else {
-
         // Glyphs are offset within their cell; account for y-flip
         vec2 cellOffset = vec2(glyphOffset.x, glyphOffset.y - glyphSize.y);
 
@@ -76,6 +75,6 @@ void main()
 
     vb = visualBell;
     background = backgroundPass;
-    bg = backgroundColor / vec3(255.0, 255.0, 255.0);
+    bg = vec4(backgroundColor.rgb / 255.0, backgroundColor.a);
     fg = textColor / vec3(255.0, 255.0, 255.0);
 }