Round palette sizes in --extra

Cargo.toml

@@ -24,9 +24,9 @@ required-features = ["binary"]
 clap = { version = "4.3.24", features = ["cargo"], optional = true }
 gif = { version = "0.13.0-beta.1", default-features = false, features = ["std", "raii_no_panic"] }
 gif-dispose = "5.0.0-beta.1"
-imagequant = "4.2.2"
 imgref = "1.10.0"
-lodepng = { version = "3.9.3", optional = true }
+imagequant = "4.3.0"
+lodepng = { version = "3.10.1", optional = true }
 pbr = { version = "1.1.1", optional = true }
 resize = { version = "0.8.3", features = ["rayon"] }
 rgb = "0.8.37"

src/lib.rs

@@ -445,6 +445,18 @@ impl Writer {
         }
 
         let mut res = liq.quantize(&mut img)?;
+
+        // GIF only stores power-of-two palette sizes
+        if self.settings.extra_effort {
+            let len = res.palette_len();
+            // it has little impact on compression (128c -> 64c is only 7% smaller)
+            if (len < 128 || len > 220) && len != len.next_power_of_two() {
+                liq.set_max_colors(len.next_power_of_two() as _)?;
+                liq.set_quality(0, 100)?;
+                res = liq.quantize(&mut img)?;
+            }
+        }
+
         res.set_dithering_level((f32::from(self.settings.s.quality) / 50.0 - 1.).max(0.2))?;
 
         let mut out = Vec::new();