Use stronger compression in eval (#509)

src/lib.rs

@@ -646,8 +646,18 @@ fn optimize_raw<T: Deflater>(
     max_size: Option<usize>,
     deflater: &T,
 ) -> Option<PngData> {
-    // Must use normal (lazy) compression, as faster ones (greedy) are not representative
-    let eval_compression = 5;
+    // Libdeflate has four algorithms: 1-4 = 'greedy', 5-7 = 'lazy', 8-9 = 'lazy2', 10-12 = 'near-optimal'
+    // 5 is the minimumm required for a decent evaluation result
+    // 7 is not noticeably slower than 5 and improves evaluation of filters in 'fast' mode (o2 and lower)
+    // 8 is a little slower but not noticeably when used only for reductions (o3 and higher)
+    // 9 is not appreciably better than 8
+    // 10 and higher are quite slow - good for filters but only good for reductions if matching the main zc level
+    let eval_compression = match opts.deflate {
+        Deflaters::Libdeflater { compression } => {
+            if opts.fast_evaluation { 7 } else { 8 }.min(compression)
+        }
+        _ => 8,
+    };
     // None and Bigrams work well together, especially for alpha reductions
     let eval_filters = indexset! {RowFilter::None, RowFilter::Bigrams};
     // This will collect all versions of images and pick one that compresses best

src/main.rs

@@ -180,7 +180,7 @@ fn main() {
         )
         .arg(
             Arg::new("fast")
-                .help("Use fast filter evaluation")
+                .help("Use fast filter evaluation (helpful when you have more filters enabled than CPU cores)")
                 .long("fast")
                 .action(ArgAction::SetTrue),
         )
@@ -242,7 +242,7 @@ fn main() {
         )
         .arg(
             Arg::new("zopfli")
-                .help("Use the slower but better compressing Zopfli algorithm")
+                .help("Use the slow but stronger Zopfli compressor (recommended use is with all filters and `--fast` enabled)")
                 .short('Z')
                 .long("zopfli")
                 .action(ArgAction::SetTrue),

tests/flags.rs

@@ -11,7 +11,6 @@ use std::ops::Deref;
 use std::path::Path;
 use std::path::PathBuf;
 
-const GRAYSCALE: u8 = 0;
 const RGB: u8 = 2;
 const INDEXED: u8 = 3;
 const RGBA: u8 = 6;
@@ -597,7 +596,7 @@ fn fix_errors() {
         }
     };
 
-    assert_eq!(png.raw.ihdr.color_type.png_header_code(), GRAYSCALE);
+    assert_eq!(png.raw.ihdr.color_type.png_header_code(), INDEXED);
     assert_eq!(png.raw.ihdr.bit_depth, BitDepth::Eight);
 
     // Cannot check if pixels are equal because image crate cannot read corrupt (input) PNGs

tests/regression.rs

@@ -177,8 +177,8 @@ fn issue_52_05() {
         None,
         RGBA,
         BitDepth::Eight,
-        INDEXED,
-        BitDepth::One,
+        RGB,
+        BitDepth::Eight,
     );
 }