Try both gray and indexed low depths

shssoichiro · Feb 18, 2024 · 0c33cb3 · 0c33cb3
1 parent 4cb8296
commit 0c33cb3
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Showing 3 changed files with 32 additions and 19 deletions.
diff --git a/benches/reductions.rs b/benches/reductions.rs
@@ -31,7 +31,7 @@ fn reductions_8_to_4_bits(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, &Options::default()).unwrap();
 
-    b.iter(|| bit_depth::reduced_bit_depth_8_or_less(&png.raw));
+    b.iter(|| bit_depth::reduced_bit_depth(&png.raw, 8));
 }
 
 #[bench]
@@ -41,7 +41,7 @@ fn reductions_8_to_2_bits(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, &Options::default()).unwrap();
 
-    b.iter(|| bit_depth::reduced_bit_depth_8_or_less(&png.raw));
+    b.iter(|| bit_depth::reduced_bit_depth(&png.raw, 8));
 }
 
 #[bench]
@@ -51,7 +51,7 @@ fn reductions_8_to_1_bits(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, &Options::default()).unwrap();
 
-    b.iter(|| bit_depth::reduced_bit_depth_8_or_less(&png.raw));
+    b.iter(|| bit_depth::reduced_bit_depth(&png.raw, 8));
 }
 
 #[bench]
@@ -61,7 +61,7 @@ fn reductions_4_to_2_bits(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, &Options::default()).unwrap();
 
-    b.iter(|| bit_depth::reduced_bit_depth_8_or_less(&png.raw));
+    b.iter(|| bit_depth::reduced_bit_depth(&png.raw, 8));
 }
 
 #[bench]
@@ -71,7 +71,7 @@ fn reductions_4_to_1_bits(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, &Options::default()).unwrap();
 
-    b.iter(|| bit_depth::reduced_bit_depth_8_or_less(&png.raw));
+    b.iter(|| bit_depth::reduced_bit_depth(&png.raw, 8));
 }
 
 #[bench]
@@ -81,7 +81,7 @@ fn reductions_2_to_1_bits(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, &Options::default()).unwrap();
 
-    b.iter(|| bit_depth::reduced_bit_depth_8_or_less(&png.raw));
+    b.iter(|| bit_depth::reduced_bit_depth(&png.raw, 8));
 }
 
 #[bench]
@@ -91,7 +91,7 @@ fn reductions_grayscale_8_to_4_bits(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, &Options::default()).unwrap();
 
-    b.iter(|| bit_depth::reduced_bit_depth_8_or_less(&png.raw));
+    b.iter(|| bit_depth::reduced_bit_depth(&png.raw, 8));
 }
 
 #[bench]
@@ -101,7 +101,7 @@ fn reductions_grayscale_8_to_2_bits(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, &Options::default()).unwrap();
 
-    b.iter(|| bit_depth::reduced_bit_depth_8_or_less(&png.raw));
+    b.iter(|| bit_depth::reduced_bit_depth(&png.raw, 8));
 }
 
 #[bench]
@@ -111,7 +111,7 @@ fn reductions_grayscale_8_to_1_bits(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, &Options::default()).unwrap();
 
-    b.iter(|| bit_depth::reduced_bit_depth_8_or_less(&png.raw));
+    b.iter(|| bit_depth::reduced_bit_depth(&png.raw, 8));
 }
 
 #[bench]
@@ -121,7 +121,7 @@ fn reductions_grayscale_4_to_2_bits(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, &Options::default()).unwrap();
 
-    b.iter(|| bit_depth::reduced_bit_depth_8_or_less(&png.raw));
+    b.iter(|| bit_depth::reduced_bit_depth(&png.raw, 8));
 }
 
 #[bench]
@@ -131,7 +131,7 @@ fn reductions_grayscale_4_to_1_bits(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, &Options::default()).unwrap();
 
-    b.iter(|| bit_depth::reduced_bit_depth_8_or_less(&png.raw));
+    b.iter(|| bit_depth::reduced_bit_depth(&png.raw, 8));
 }
 
 #[bench]
@@ -141,7 +141,7 @@ fn reductions_grayscale_2_to_1_bits(b: &mut Bencher) {
     ));
     let png = PngData::new(&input, &Options::default()).unwrap();
 
-    b.iter(|| bit_depth::reduced_bit_depth_8_or_less(&png.raw));
+    b.iter(|| bit_depth::reduced_bit_depth(&png.raw, 8));
 }
 
 #[bench]

diff --git a/src/reduction/bit_depth.rs b/src/reduction/bit_depth.rs
@@ -63,8 +63,9 @@ pub fn scaled_bit_depth_16_to_8(png: &PngImage) -> Option<PngImage> {
 
 /// Attempt to reduce an 8-bit image to a lower bit depth, returning the reduced image if successful
 #[must_use]
-pub fn reduced_bit_depth_8_or_less(png: &PngImage) -> Option<PngImage> {
-    if png.ihdr.bit_depth != BitDepth::Eight || png.channels_per_pixel() != 1 {
+pub fn reduced_bit_depth(png: &PngImage, below_depth: usize) -> Option<PngImage> {
+    assert!(below_depth <= 8);
+    if below_depth <= 1 || png.ihdr.bit_depth != BitDepth::Eight || png.channels_per_pixel() != 1 {
         return None;
     }
 
@@ -78,6 +79,9 @@ pub fn reduced_bit_depth_8_or_less(png: &PngImage) -> Option<PngImage> {
             5..=16 => 4,
             _ => return None,
         };
+        if minimum_bits >= below_depth {
+            return None;
+        }
     } else {
         // Finding minimum depth for grayscale is much more complicated
         let mut mask = 1;
@@ -97,7 +101,7 @@ pub fn reduced_bit_depth_8_or_less(png: &PngImage) -> Option<PngImage> {
                     if byte & mask != compare {
                         // This depth is not possible, try the next one up
                         minimum_bits <<= 1;
-                        if minimum_bits == 8 {
+                        if minimum_bits >= below_depth {
                             return None;
                         }
                         mask = (1 << minimum_bits) - 1;

diff --git a/src/reduction/mod.rs b/src/reduction/mod.rs
@@ -144,14 +144,23 @@ pub(crate) fn perform_reductions(
 
     // Attempt to reduce to a lower bit depth
     if opts.bit_depth_reduction && !deadline.passed() {
-        // Try reducing the previous png, falling back to the indexed one if it exists
-        // This allows a grayscale depth reduction to be preferred over an indexed depth reduction
-        let reduced = reduced_bit_depth_8_or_less(&png)
-            .or_else(|| indexed.and_then(|png| reduced_bit_depth_8_or_less(&png)));
+        // First try reducing the `png` var (could be grayscale or indexed)
+        let reduced = reduced_bit_depth(&png, 8);
+        let mut indexed_depth_limit = 8;
         if let Some(reduced) = reduced {
+            // For cheap evaluations, skip the `indexed` attempt (if it exists then this one is grayscale which is preferred)
+            // Otherwise require the indexed to reach a lower depth than what we've achieved here
+            indexed_depth_limit = if cheap { 0 } else { reduced.ihdr.bit_depth as usize };
             eval.try_image(Arc::new(reduced));
             evaluation_added = true;
         }
+        // Now see if we can reduce the `indexed` var
+        if !deadline.passed() {
+            if let Some(reduced) = indexed.and_then(|png| reduced_bit_depth(&png, indexed_depth_limit)) {
+                eval.try_image(Arc::new(reduced));
+                evaluation_added = true;
+            }
+        }
     }
 
     if evaluation_added {