Support construction from rgb8 values (#136)

This is a common pattern in code in Vello examples and other places
where they're just constructing an opaque color from the r,g,b values
and a solid alpha.

CHANGELOG.md

@@ -20,6 +20,7 @@ This release has an [MSRV][] of 1.82.
 * Support for the ACES2065-1 color space. ([#124][] by [@tomcur][])
 * A documentation example implementing `ColorSpace`. ([#130][] by [@tomcur][])
 * Conversions of `[u8; 4]` and packed `u32` into `Rgba8` and `PremulRgba8` are now provided. ([#135][] by [@tomcur][])
+* Support construction of `AlphaColor<Srgb>`, `OpaqueColor<Srgb>` and `PremulColor<Srgb>` from rgb8 values. ([#136][] by [@waywardmonkeys][])
 
 ### Fixed
 
@@ -121,6 +122,7 @@ This is the initial release.
 [#129]: https://github.com/linebender/color/pull/129
 [#130]: https://github.com/linebender/color/pull/130
 [#135]: https://github.com/linebender/color/pull/135
+[#136]: https://github.com/linebender/color/pull/136
 
 [Unreleased]: https://github.com/linebender/color/compare/v0.2.2...HEAD
 [0.2.2]: https://github.com/linebender/color/releases/tag/v0.2.2

color/src/lib.rs

@@ -138,6 +138,22 @@ impl AlphaColor<Srgb> {
         let components = [u8_to_f32(r), u8_to_f32(g), u8_to_f32(b), u8_to_f32(a)];
         Self::new(components)
     }
+
+    /// Create a color from 8-bit rgb values with an opaque alpha.
+    ///
+    /// Note: for conversion from the [`Rgba8`] type, just use the `From` trait.
+    pub const fn from_rgb8(r: u8, g: u8, b: u8) -> Self {
+        let components = [u8_to_f32(r), u8_to_f32(g), u8_to_f32(b), 1.];
+        Self::new(components)
+    }
+}
+
+impl OpaqueColor<Srgb> {
+    /// Create a color from 8-bit rgb values.
+    pub const fn from_rgb8(r: u8, g: u8, b: u8) -> Self {
+        let components = [u8_to_f32(r), u8_to_f32(g), u8_to_f32(b)];
+        Self::new(components)
+    }
 }
 
 impl PremulColor<Srgb> {
@@ -148,6 +164,14 @@ impl PremulColor<Srgb> {
         let components = [u8_to_f32(r), u8_to_f32(g), u8_to_f32(b), u8_to_f32(a)];
         Self::new(components)
     }
+
+    /// Create a color from 8-bit rgb values with an opaque alpha.
+    ///
+    /// Note: for conversion from the [`Rgba8`] type, just use the `From` trait.
+    pub const fn from_rgb8(r: u8, g: u8, b: u8) -> Self {
+        let components = [u8_to_f32(r), u8_to_f32(g), u8_to_f32(b), 1.];
+        Self::new(components)
+    }
 }
 
 // Keep clippy from complaining about unused libm in nostd test case.