Use the palette crate for blending

I wasn't pleased with how text was rendering on a colored background,
and after some research I discovered that I was neglecting to convert
from sRGB to a linear RGB colorspace, and that this was causing the
blending to have inaccurate results.

This diff pulls in the palette crate to handle the heavy lifting.
It doesn't appear to have any especially fast optimizations for this
though, which is a shame because the blending code I was using
previously was largely integer math with few branches.  The colorspace
conversion has several floating point operations and branches that
are unavoidable :-/

We run a bit hotter on the CPU but the results are much more pleasing
to the eye.

Cargo.toml

@@ -10,6 +10,7 @@ freetype = "0.3.0"
 harfbuzz-sys = "0.1.15"
 libc = "0.2.36"
 mio = "0.6.12"
+palette = "0.2.1"
 resize = "0.3.0"
 serde = "1.0.27"
 serde_derive = "1.0.27"

src/font/mod.rs

@@ -421,9 +421,8 @@ impl Font {
     ) -> Result<&ftwrap::FT_GlyphSlotRec_, Error> {
         let info = &mut self.fonts[font_idx];
 
-        let render_mode =
-            //ftwrap::FT_Render_Mode::FT_RENDER_MODE_NORMAL;
-            ftwrap::FT_Render_Mode::FT_RENDER_MODE_LCD;
+        let render_mode = ftwrap::FT_Render_Mode::FT_RENDER_MODE_NORMAL;
+        //ftwrap::FT_Render_Mode::FT_RENDER_MODE_LCD;
 
         // when changing the load flags, we also need
         // to change them for harfbuzz otherwise it won't

src/main.rs

@@ -12,6 +12,7 @@ extern crate mio;
 extern crate serde;
 #[macro_use]
 extern crate serde_derive;
+extern crate palette;
 extern crate term;
 extern crate toml;
 #[macro_use]

src/xgfx.rs

@@ -15,6 +15,8 @@ pub type Result<T> = result::Result<T, Error>;
 pub use xkeysyms::*;
 
 use super::term::color::RgbColor;
+use palette::Blend;
+use palette::pixel::Srgb;
 
 pub struct Connection {
     conn: xcb::Connection,
@@ -278,6 +280,21 @@ impl<'a> Drop for Context<'a> {
 #[derive(Copy, Clone, Debug)]
 pub struct Color(u32);
 
+impl From<Srgb> for Color {
+    #[inline]
+    fn from(s: Srgb) -> Color {
+        let b: [u8; 4] = s.to_pixel();
+        Color::rgba(b[0], b[1], b[2], b[3])
+    }
+}
+
+impl From<Color> for Srgb {
+    #[inline]
+    fn from(c: Color) -> Srgb {
+        Srgb::from_pixel(&c.as_rgba())
+    }
+}
+
 impl Color {
     #[inline]
     pub fn rgb(red: u8, green: u8, blue: u8) -> Color {
@@ -307,43 +324,41 @@ impl Color {
     pub fn composite(&self, dest: Color, operator: &Operator) -> Color {
         match operator {
             &Operator::Over => {
-                let (src_r, src_g, src_b, src_a) = self.as_rgba();
-                let (dst_r, dst_g, dst_b, _dst_a) = dest.as_rgba();
-
-                // Alpha blending per https://stackoverflow.com/a/12016968/149111
-                let inv_alpha = 256u16 - src_a as u16;
-                let alpha = src_a as u16 + 1;
-
-                Color::rgb(
-                    ((alpha * src_r as u16 + inv_alpha * dst_r as u16) >> 8) as u8,
-                    ((alpha * src_g as u16 + inv_alpha * dst_g as u16) >> 8) as u8,
-                    ((alpha * src_b as u16 + inv_alpha * dst_b as u16) >> 8) as u8,
-                )
+                let src: Srgb = (*self).into();
+                let dest: Srgb = dest.into();
+                Srgb::from_linear(src.to_linear().over(dest.to_linear())).into()
             }
 
             &Operator::Source => *self,
 
             &Operator::Multiply => {
-                let (src_r, src_g, src_b, src_a) = self.as_rgba();
-                let (dst_r, dst_g, dst_b, _dst_a) = dest.as_rgba();
-                let r = ((src_r as u16 * dst_r as u16) >> 8) as u8;
-                let g = ((src_g as u16 * dst_g as u16) >> 8) as u8;
-                let b = ((src_b as u16 * dst_b as u16) >> 8) as u8;
-
-                Color::rgba(r, g, b, src_a)
+                let src: Srgb = (*self).into();
+                let dest: Srgb = dest.into();
+                let result: Color = Srgb::from_linear(src.to_linear().multiply(dest.to_linear()))
+                    .into();
+                result.into()
             }
 
             &Operator::MultiplyThenOver(ref tint) => {
-                self.composite(*tint, &Operator::Multiply).composite(
-                    dest,
-                    &Operator::Over,
-                )
+                // First multiply by the tint color.  This colorizes the glyph.
+                let src: Srgb = (*self).into();
+                let tint: Srgb = (*tint).into();
+                let mut tinted = src.to_linear().multiply(tint.to_linear());
+                // We take the alpha from the source.  This is important because
+                // we're using Multiply to tint the glyph and if we don't reset the
+                // alpha we tend to end up with a background square of the tint color.
+                tinted.alpha = src.alpha;
+
+                // Then blend the tinted glyph over the destination background
+                let dest: Srgb = dest.into();
+                Srgb::from_linear(tinted.over(dest.to_linear())).into()
             }
         }
     }
 }
 
 impl From<RgbColor> for Color {
+    #[inline]
     fn from(color: RgbColor) -> Self {
         Color::rgb(color.red, color.green, color.blue)
     }