Prevent conversion from grayscale when disabled

benches/reductions.rs

@@ -229,7 +229,7 @@ fn reductions_rgba_to_palette_8(b: &mut Bencher) {
     let input = test::black_box(PathBuf::from("tests/files/rgba_8_should_be_palette_8.png"));
     let png = PngData::new(&input, &Options::default()).unwrap();
 
-    b.iter(|| color::reduced_to_indexed(&png.raw));
+    b.iter(|| color::reduced_to_indexed(&png.raw, true));
 }
 
 #[bench]
@@ -237,7 +237,7 @@ fn reductions_rgb_to_palette_8(b: &mut Bencher) {
     let input = test::black_box(PathBuf::from("tests/files/rgb_8_should_be_palette_8.png"));
     let png = PngData::new(&input, &Options::default()).unwrap();
 
-    b.iter(|| color::reduced_to_indexed(&png.raw));
+    b.iter(|| color::reduced_to_indexed(&png.raw, true));
 }
 
 #[bench]
@@ -247,7 +247,7 @@ fn reductions_grayscale_8_to_palette_8(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, &Options::default()).unwrap();
 
-    b.iter(|| color::reduced_to_indexed(&png.raw));
+    b.iter(|| color::reduced_to_indexed(&png.raw, true));
 }
 
 #[bench]

src/colors.rs

@@ -70,6 +70,14 @@ impl ColorType {
         matches!(self, ColorType::RGB { .. } | ColorType::RGBA)
     }
 
+    #[inline]
+    pub(crate) fn is_gray(&self) -> bool {
+        matches!(
+            self,
+            ColorType::Grayscale { .. } | ColorType::GrayscaleAlpha
+        )
+    }
+
     #[inline]
     pub(crate) fn has_alpha(&self) -> bool {
         matches!(self, ColorType::GrayscaleAlpha | ColorType::RGBA)

src/reduction/color.rs

@@ -32,13 +32,16 @@ where
 }
 
 #[must_use]
-pub fn reduced_to_indexed(png: &PngImage) -> Option<PngImage> {
+pub fn reduced_to_indexed(png: &PngImage, allow_grayscale: bool) -> Option<PngImage> {
     if png.ihdr.bit_depth != BitDepth::Eight {
         return None;
     }
     if matches!(png.ihdr.color_type, ColorType::Indexed { .. }) {
         return None;
     }
+    if !allow_grayscale && png.ihdr.color_type.is_gray() {
+        return None;
+    }
 
     let mut raw_data = Vec::with_capacity(png.data.len() / png.channels_per_pixel());
     let palette: Vec<_> = match png.ihdr.color_type {

src/reduction/mod.rs

@@ -108,7 +108,7 @@ pub(crate) fn perform_reductions(
     // Attempt to reduce to indexed
     let mut indexed = None;
     if opts.color_type_reduction && !deadline.passed() {
-        if let Some(reduced) = reduced_to_indexed(&png) {
+        if let Some(reduced) = reduced_to_indexed(&png, opts.grayscale_reduction) {
             // Make sure the palette gets sorted (but don't bother evaluating both results)
             let new = Arc::new(sorted_palette(&reduced).unwrap_or(reduced));
             // For relatively small differences, enter this into the evaluator