Allow grayscale reduction from 16 to 4 or less

benches/reductions.rs

@@ -12,7 +12,7 @@ fn reductions_16_to_8_bits(b: &mut Bencher) {
     let input = test::black_box(PathBuf::from("tests/files/rgb_16_should_be_rgb_8.png"));
     let png = PngData::new(&input, false).unwrap();
 
-    b.iter(|| bit_depth::reduce_bit_depth(&png.raw, 1));
+    b.iter(|| bit_depth::reduced_bit_depth_16_to_8(&png.raw));
 }
 
 #[bench]
@@ -22,7 +22,7 @@ fn reductions_8_to_4_bits(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, false).unwrap();
 
-    b.iter(|| bit_depth::reduce_bit_depth(&png.raw, 1));
+    b.iter(|| bit_depth::reduced_bit_depth_8_or_less(&png.raw, 1));
 }
 
 #[bench]
@@ -32,7 +32,7 @@ fn reductions_8_to_2_bits(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, false).unwrap();
 
-    b.iter(|| bit_depth::reduce_bit_depth(&png.raw, 1));
+    b.iter(|| bit_depth::reduced_bit_depth_8_or_less(&png.raw, 1));
 }
 
 #[bench]
@@ -42,7 +42,7 @@ fn reductions_8_to_1_bits(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, false).unwrap();
 
-    b.iter(|| bit_depth::reduce_bit_depth(&png.raw, 1));
+    b.iter(|| bit_depth::reduced_bit_depth_8_or_less(&png.raw, 1));
 }
 
 #[bench]
@@ -52,7 +52,7 @@ fn reductions_4_to_2_bits(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, false).unwrap();
 
-    b.iter(|| bit_depth::reduce_bit_depth(&png.raw, 1));
+    b.iter(|| bit_depth::reduced_bit_depth_8_or_less(&png.raw, 1));
 }
 
 #[bench]
@@ -62,7 +62,7 @@ fn reductions_4_to_1_bits(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, false).unwrap();
 
-    b.iter(|| bit_depth::reduce_bit_depth(&png.raw, 1));
+    b.iter(|| bit_depth::reduced_bit_depth_8_or_less(&png.raw, 1));
 }
 
 #[bench]
@@ -72,7 +72,7 @@ fn reductions_2_to_1_bits(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, false).unwrap();
 
-    b.iter(|| bit_depth::reduce_bit_depth(&png.raw, 1));
+    b.iter(|| bit_depth::reduced_bit_depth_8_or_less(&png.raw, 1));
 }
 
 #[bench]
@@ -82,7 +82,7 @@ fn reductions_grayscale_8_to_4_bits(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, false).unwrap();
 
-    b.iter(|| bit_depth::reduce_bit_depth(&png.raw, 1));
+    b.iter(|| bit_depth::reduced_bit_depth_8_or_less(&png.raw, 1));
 }
 
 #[bench]
@@ -92,7 +92,7 @@ fn reductions_grayscale_8_to_2_bits(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, false).unwrap();
 
-    b.iter(|| bit_depth::reduce_bit_depth(&png.raw, 1));
+    b.iter(|| bit_depth::reduced_bit_depth_8_or_less(&png.raw, 1));
 }
 
 #[bench]
@@ -102,7 +102,7 @@ fn reductions_grayscale_8_to_1_bits(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, false).unwrap();
 
-    b.iter(|| bit_depth::reduce_bit_depth(&png.raw, 1));
+    b.iter(|| bit_depth::reduced_bit_depth_8_or_less(&png.raw, 1));
 }
 
 #[bench]
@@ -112,7 +112,7 @@ fn reductions_grayscale_4_to_2_bits(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, false).unwrap();
 
-    b.iter(|| bit_depth::reduce_bit_depth(&png.raw, 1));
+    b.iter(|| bit_depth::reduced_bit_depth_8_or_less(&png.raw, 1));
 }
 
 #[bench]
@@ -122,7 +122,7 @@ fn reductions_grayscale_4_to_1_bits(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, false).unwrap();
 
-    b.iter(|| bit_depth::reduce_bit_depth(&png.raw, 1));
+    b.iter(|| bit_depth::reduced_bit_depth_8_or_less(&png.raw, 1));
 }
 
 #[bench]
@@ -132,7 +132,7 @@ fn reductions_grayscale_2_to_1_bits(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, false).unwrap();
 
-    b.iter(|| bit_depth::reduce_bit_depth(&png.raw, 1));
+    b.iter(|| bit_depth::reduced_bit_depth_8_or_less(&png.raw, 1));
 }
 
 #[bench]

src/lib.rs

@@ -686,19 +686,9 @@ fn perform_reductions(
     }
 
     if opts.bit_depth_reduction {
-        if let Some(reduced) = reduce_bit_depth(&png, 1) {
+        if let Some(reduced) = reduced_bit_depth_16_to_8(&png) {
-            let previous = png.clone();
-            let bits = reduced.ihdr.bit_depth;
             png = Arc::new(reduced);
             eval.try_image(png.clone());
-            if (bits == BitDepth::One || bits == BitDepth::Two)
-                && previous.ihdr.bit_depth != BitDepth::Four
-            {
-                // Also try 16-color mode for all lower bits images, since that may compress better
-                if let Some(reduced) = reduce_bit_depth(&previous, 4) {
-                    eval.try_image(Arc::new(reduced));
-                }
-            }
             reduction_occurred = true;
         }
         if deadline.passed() {
@@ -719,6 +709,24 @@ fn perform_reductions(
         }
     }
 
+    if opts.bit_depth_reduction {
+        if let Some(reduced) = reduced_bit_depth_8_or_less(&png, 1) {
+            let previous = png;
+            png = Arc::new(reduced);
+            eval.try_image(png.clone());
+            if png.ihdr.bit_depth < BitDepth::Four && previous.ihdr.bit_depth == BitDepth::Eight {
+                // Also try 16-color mode for all lower bits images, since that may compress better
+                if let Some(reduced) = reduced_bit_depth_8_or_less(&previous, 4) {
+                    eval.try_image(Arc::new(reduced));
+                }
+            }
+            reduction_occurred = true;
+        }
+        if deadline.passed() {
+            return;
+        }
+    }
+
     if reduction_occurred {
         eval.set_baseline(baseline);
     }

src/reduction/bit_depth.rs

@@ -2,13 +2,10 @@ use crate::colors::{BitDepth, ColorType};
 use crate::headers::IhdrData;
 use crate::png::PngImage;
 
-/// Attempt to reduce the bit depth of the image, returning the reduced image if successful
+/// Attempt to reduce a 16-bit image to 8-bit, returning the reduced image if successful
 #[must_use]
-pub fn reduce_bit_depth(png: &PngImage, minimum_bits: usize) -> Option<PngImage> {
+pub fn reduced_bit_depth_16_to_8(png: &PngImage) -> Option<PngImage> {
     if png.ihdr.bit_depth != BitDepth::Sixteen {
-        if png.channels_per_pixel() == 1 {
-            return reduce_bit_depth_8_or_less(png, minimum_bits);
-        }
         return None;
     }
 
@@ -29,11 +26,12 @@ pub fn reduce_bit_depth(png: &PngImage, minimum_bits: usize) -> Option<PngImage>
     })
 }
 
+/// Attempt to reduce an 8/4/2-bit image to a lower bit depth, returning the reduced image if successful
 #[must_use]
-pub fn reduce_bit_depth_8_or_less(png: &PngImage, mut minimum_bits: usize) -> Option<PngImage> {
+pub fn reduced_bit_depth_8_or_less(png: &PngImage, mut minimum_bits: usize) -> Option<PngImage> {
     assert!((1..8).contains(&minimum_bits));
     let bit_depth = png.ihdr.bit_depth as usize;
-    if minimum_bits >= bit_depth || bit_depth > 8 {
+    if minimum_bits >= bit_depth || bit_depth > 8 || png.channels_per_pixel() != 1 {
         return None;
     }
     // Calculate the current number of pixels per byte

src/reduction/mod.rs

@@ -8,12 +8,12 @@ use std::borrow::Cow;
 pub mod alpha;
 use crate::alpha::reduced_alpha_channel;
 pub mod bit_depth;
-use crate::bit_depth::reduce_bit_depth_8_or_less;
 pub mod color;
 use crate::color::*;
 
 pub(crate) use crate::alpha::cleaned_alpha_channel;
-pub(crate) use crate::bit_depth::reduce_bit_depth;
+pub(crate) use crate::bit_depth::reduced_bit_depth_16_to_8;
+pub(crate) use crate::bit_depth::reduced_bit_depth_8_or_less;
 
 /// Attempt to reduce the number of colors in the palette
 /// Returns `None` if palette hasn't changed
@@ -196,7 +196,6 @@ pub fn reduce_color_type(
     grayscale_reduction: bool,
     optimize_alpha: bool,
 ) -> Option<PngImage> {
-    let was_single_channel = png.channels_per_pixel() == 1;
     let mut reduced = Cow::Borrowed(png);
 
     // Go down one step at a time - maybe not the most efficient, but it's safe
@@ -233,13 +232,6 @@ pub fn reduce_color_type(
         }
     }
 
-    // Some conversions will allow us to perform bit depth reduction that wasn't possible before
-    if !was_single_channel && reduced.channels_per_pixel() == 1 {
-        if let Some(r) = reduce_bit_depth_8_or_less(&reduced, 1) {
-            reduced = Cow::Owned(r);
-        }
-    }
-
     match reduced {
         Cow::Owned(r) => Some(r),
         _ => None,

tests/reduction.rs

@@ -712,6 +712,18 @@ fn grayscale_16_should_be_grayscale_8() {
     );
 }
 
+#[test]
+fn grayscale_16_should_be_grayscale_1() {
+    test_it_converts(
+        "tests/files/grayscale_16_should_be_grayscale_1.png",
+        false,
+        GRAYSCALE,
+        BitDepth::Sixteen,
+        GRAYSCALE,
+        BitDepth::One,
+    );
+}
+
 #[test]
 fn grayscale_8_should_be_grayscale_8() {
     test_it_converts(