Image alpha channel is now an f32 (#65)

In the next release of Peniko, our alpha channels will all be `f32` as
we move to the new color code.

Also, new version of `cargo-nextest` fails when there are no tests and
this crate has no tests, so don't fail.

.github/workflows/ci.yml

@@ -174,7 +174,7 @@ jobs:
           save-if: ${{ github.event_name != 'merge_group' }}
 
       - name: cargo nextest
-        run: cargo nextest run --workspace --locked --all-features --no-fail-fast
+        run: cargo nextest run --workspace --locked --all-features --no-fail-fast --no-tests=pass
 
       - name: cargo test --doc
         run: cargo test --doc --workspace --locked --all-features --no-fail-fast

CHANGELOG.md

@@ -15,6 +15,10 @@ You can find its changes [documented below](#020-2024-09-19).
 
 This release has an [MSRV] of 1.70.
 
+### Changed
+
+- `Image` now stores the alpha as an `f32` ([#65][] by [@waywardmonkeys][])
+
 ## [0.2.0][] (2024-09-19)
 
 This release has an [MSRV] of 1.70.
@@ -50,6 +54,7 @@ This release has an [MSRV] of 1.70.
 [#46]: https://github.com/linebender/peniko/pull/46
 [#47]: https://github.com/linebender/peniko/pull/47
 [#52]: https://github.com/linebender/peniko/pull/52
+[#65]: https://github.com/linebender/peniko/pull/65
 
 [@DJMcNab]: https://github.com/DJMcNab
 [@ratmice]: https://github.com/ratmice

src/image.rs

@@ -1,10 +1,6 @@
 // Copyright 2022 the Peniko Authors
 // SPDX-License-Identifier: Apache-2.0 OR MIT
 
-#[cfg(all(not(feature = "std"), feature = "libm"))]
-#[allow(unused_imports)]
-use kurbo::common::FloatFuncs as _;
-
 use super::{Blob, Extend};
 
 /// Defines the pixel format of an [image](Image).
@@ -46,7 +42,7 @@ pub struct Image {
     /// Extend mode.
     pub extend: Extend,
     /// An additional alpha multiplier to use with the image.
-    pub alpha: u8,
+    pub alpha: f32,
 }
 
 impl Image {
@@ -60,7 +56,7 @@ impl Image {
             height,
             extend: Extend::Pad,
             // Opaque
-            alpha: u8::MAX,
+            alpha: 1.,
         }
     }
 
@@ -73,16 +69,14 @@ impl Image {
 
     /// Returns the image with the alpha multiplier multiplied again by `alpha`.
     /// The behaviour of this transformation is undefined if `alpha` is negative.
-    ///
-    /// If any resulting alphas would overflow, these currently saturate (to opaque).
     #[must_use]
     #[track_caller]
     pub fn multiply_alpha(mut self, alpha: f32) -> Self {
         debug_assert!(
             alpha.is_finite() && alpha >= 0.0,
             "A non-finite or negative alpha ({alpha}) is meaningless."
         );
-        self.alpha = ((self.alpha as f32) * alpha).round() as u8;
+        self.alpha *= alpha;
         self
     }
 }