Improve indexed to grayscale conversion (#674)

If no palette reduction occurred (or palette reductions were off), a
fully transparent palette entry may not have been zeroed to black. By
ensuring this gets cleaned in the `indexed_to_channels` transformation,
we may able to achieve a grayscale conversion that would otherwise have
been RGB.

This is the final piece of the puzzle in #649 to achieve better file
sizes on the first run and avoid further changes on a second run.

Results from the images in #649 (combining this PR and #673):
9.1.3 1st run: 2,191,773
9.1.3 2nd run: 2,191,629
PR 1st run: 2,191,380
PR 2nd run: 2,191,380

benches/reductions.rs

@@ -257,7 +257,7 @@ fn reductions_palette_8_to_grayscale_8(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, &Options::default()).unwrap();
 
-    b.iter(|| color::indexed_to_channels(&png.raw, true));
+    b.iter(|| color::indexed_to_channels(&png.raw, true, false));
 }
 
 #[bench]

src/reduction/color.rs

@@ -138,15 +138,30 @@ pub fn reduced_rgb_to_grayscale(png: &PngImage) -> Option<PngImage> {
 
 /// Attempt to convert indexed to a different color type, returning the resulting image if successful
 #[must_use]
-pub fn indexed_to_channels(png: &PngImage, allow_grayscale: bool) -> Option<PngImage> {
+pub fn indexed_to_channels(
+    png: &PngImage,
+    allow_grayscale: bool,
+    optimize_alpha: bool,
+) -> Option<PngImage> {
     if png.ihdr.bit_depth != BitDepth::Eight {
         return None;
     }
-    let palette = match &png.ihdr.color_type {
-        ColorType::Indexed { palette } => palette,
+    let mut palette = match &png.ihdr.color_type {
+        ColorType::Indexed { palette } => palette.clone(),
         _ => return None,
     };
 
+    // Ensure fully transparent colors are black, which can help with grayscale conversion
+    if optimize_alpha {
+        for color in &mut palette {
+            if color.a == 0 {
+                color.r = 0;
+                color.g = 0;
+                color.b = 0;
+            }
+        }
+    }
+
     // Determine which channels are required
     let is_gray = if allow_grayscale {
         palette.iter().all(|c| c.r == c.g && c.g == c.b)

src/reduction/mod.rs

@@ -106,7 +106,9 @@ pub(crate) fn perform_reductions(
     // Attempt to convert from indexed to channels
     // This may give a better result due to dropping the PLTE chunk
     if !cheap && opts.color_type_reduction && !deadline.passed() {
-        if let Some(reduced) = indexed_to_channels(&png, opts.grayscale_reduction) {
+        if let Some(reduced) =
+            indexed_to_channels(&png, opts.grayscale_reduction, opts.optimize_alpha)
+        {
             // This result should not be passed on to subsequent reductions
             eval.try_image(Arc::new(reduced));
             evaluation_added = true;

tests/reduction.rs

@@ -578,6 +578,18 @@ fn palette_8_should_be_grayscale_8() {
     );
 }
 
+#[test]
+fn palette_2_should_be_grayscale_alpha_8() {
+    test_it_converts(
+        "tests/files/palette_2_should_be_grayscale_alpha_8.png",
+        true,
+        INDEXED,
+        BitDepth::Two,
+        GRAYSCALE_ALPHA,
+        BitDepth::Eight,
+    );
+}
+
 #[test]
 fn palette_8_should_be_rgb() {
     test_it_converts(