Add --oxipng and --zopfli flags to change optimization level

README.md

@@ -112,15 +112,17 @@ Arguments:
   <OUTPUT>  Name of the file in which to save the spritesheet
 
 Options:
-  -r, --ratio <RATIO>      Set the output pixel ratio [default: 1]
-      --retina             Set the pixel ratio to 2 (equivalent to `--ratio=2`)
-      --unique             Store only unique images in the spritesheet, and map them to multiple names
-      --recursive          Include images in sub-directories
-      --spacing <SPACING>  Add pixel spacing between sprites [default: 0]
-  -m, --minify-index-file  Remove whitespace from the JSON index file
-      --sdf                Output a spritesheet using a signed distance field for each sprite
-  -h, --help               Print help
-  -V, --version            Print version
+  -r, --ratio <RATIO>        Set the output pixel ratio [default: 1]
+      --retina               Set the pixel ratio to 2 (equivalent to `--ratio=2`)
+      --unique               Store only unique images in the spritesheet, and map them to multiple names
+      --recursive            Include images in sub-directories
+      --spacing <SPACING>    Add pixel spacing between sprites [default: 0]
+      --oxipng <LEVEL>       Specify the PNG optimization level (0–6, default: 2)
+      --zopfli <ITERATIONS>  Optimize the output PNG with zopfli (1–255, very slow)
+  -m, --minify-index-file    Remove whitespace from the JSON index file
+      --sdf                  Output a spritesheet using a signed distance field for each sprite
+  -h, --help                 Print help
+  -V, --version              Print version
 ```
 
 ## Using Spreet as a Rust library

src/bin/spreet/cli.rs

@@ -28,6 +28,12 @@ pub struct Cli {
     /// Add pixel spacing between sprites
     #[arg(long, default_value_t = 0, value_parser = is_non_negative)]
     pub spacing: u8,
+    /// Specify the PNG optimization level (0–6, default: 2)
+    #[arg(long, group = "optlevel", value_name = "LEVEL", value_parser = is_max_6)]
+    pub oxipng: Option<u8>,
+    /// Optimize the output PNG with zopfli (1–255, very slow)
+    #[arg(long, group = "optlevel", value_name = "ITERATIONS", value_parser = is_positive)]
+    pub zopfli: Option<u8>,
     /// Remove whitespace from the JSON index file
     #[arg(short, long)]
     pub minify_index_file: bool,
@@ -64,3 +70,13 @@ fn is_non_negative(s: &str) -> Result<u8, String> {
             Ok(result)
         })
 }
+
+/// Clap validator to ensure that an unsigned integer parsed from a string is no more than 6.
+fn is_max_6(s: &str) -> Result<u8, String> {
+    u8::from_str(s)
+        .map_err(|e| e.to_string())
+        .and_then(|result| match result {
+            i if i <= 6 => Ok(result),
+            _ => Err(String::from("must be a number no more than 6")),
+        })
+}

src/bin/spreet/main.rs

@@ -1,7 +1,8 @@
 use std::collections::BTreeMap;
+use std::num::NonZero;
 
 use clap::Parser;
-use spreet::{get_svg_input_paths, load_svg, sprite_name, Sprite, Spritesheet};
+use spreet::{get_svg_input_paths, load_svg, sprite_name, Optlevel, Sprite, Spritesheet};
 
 mod cli;
 
@@ -64,10 +65,19 @@ fn main() {
         std::process::exit(exitcode::DATAERR);
     };
 
+    let optlevel = match (args.oxipng, args.zopfli) {
+        (None, None) => Optlevel::default(),
+        (Some(level), None) => Optlevel::Oxipng { level },
+        (None, Some(iterations)) => Optlevel::Zopfli {
+            iterations: NonZero::new(iterations).unwrap(),
+        },
+        (Some(_), Some(_)) => unreachable!(),
+    };
+
     // Save the bitmapped spritesheet to a local PNG.
     let file_prefix = args.output;
     let spritesheet_path = format!("{file_prefix}.png");
-    if let Err(e) = spritesheet.save_spritesheet(&spritesheet_path) {
+    if let Err(e) = spritesheet.save_spritesheet_at(&spritesheet_path, optlevel) {
         eprintln!("Error: could not save spritesheet to {spritesheet_path} ({e})");
         std::process::exit(exitcode::IOERR);
     };

src/sprite/mod.rs

@@ -2,6 +2,7 @@ use std::collections::btree_map::Entry;
 use std::collections::BTreeMap;
 use std::fs::File;
 use std::io::Write;
+use std::num::NonZero;
 use std::path::Path;
 
 use crunch::{Item, PackedItem, PackedItems, Rotation};
@@ -399,6 +400,12 @@ struct PixmapItem {
     sprite: Sprite,
 }
 
+/// Optimization level for PNG image output.
+pub enum Optlevel {
+    Oxipng { level: u8 },
+    Zopfli { iterations: NonZero<u8> },
+}
+
 impl Spritesheet {
     pub fn new(
         sprites: BTreeMap<String, Sprite>,
@@ -500,23 +507,44 @@ impl Spritesheet {
     /// Encode the spritesheet to the in-memory PNG image.
     ///
     /// The `spritesheet` `Pixmap` is converted to an in-memory PNG, optimised using the [`oxipng`]
-    /// library.
+    /// library with the default optimization level.
     ///
     /// The spritesheet will match an index that can be retrieved with [`Self::get_index`].
     ///
     /// [`oxipng`]: https://github.com/shssoichiro/oxipng
     pub fn encode_png(&self) -> SpreetResult<Vec<u8>> {
+        self.encode_png_at(Optlevel::default())
+    }
+
+    /// Encode the spritesheet to the in-memory PNG image with the specified optimization level.
+    ///
+    /// The `spritesheet` `Pixmap` is converted to an in-memory PNG, optimised using the [`oxipng`]
+    /// library.
+    ///
+    /// The spritesheet will match an index that can be retrieved with [`Self::get_index`].
+    ///
+    /// [`oxipng`]: https://github.com/shssoichiro/oxipng
+    pub fn encode_png_at(&self, optlevel: Optlevel) -> SpreetResult<Vec<u8>> {
+        let options = match optlevel {
+            Optlevel::Oxipng { level } => oxipng::Options::from_preset(level.min(6)),
+            Optlevel::Zopfli { iterations } => oxipng::Options {
+                deflate: oxipng::Deflaters::Zopfli { iterations },
+                ..oxipng::Options::max_compression()
+            },
+        };
+
         Ok(optimize_from_memory(
             self.sheet.encode_png()?.as_slice(),
-            &oxipng::Options::default(),
+            &options,
         )?)
     }
 
     /// Saves the spritesheet to a local file named `path`.
     ///
     /// A spritesheet, called an [image file] in the Mapbox Style Specification, is a PNG image
     /// containing all the individual sprite images. The `spritesheet` `Pixmap` is converted to an
-    /// in-memory PNG, optimised using the [`oxipng`] library, and saved to a local file.
+    /// in-memory PNG, optimised using the [`oxipng`] library with the default optimization level,
+    /// and saved to a local file.
     ///
     /// The spritesheet will match an index file that can be saved with [`Self::save_index`].
     ///
@@ -526,6 +554,24 @@ impl Spritesheet {
         Ok(std::fs::write(path, self.encode_png()?)?)
     }
 
+    /// Saves the spritesheet to a local file named `path` with the specified optimization level.
+    ///
+    /// A spritesheet, called an [image file] in the Mapbox Style Specification, is a PNG image
+    /// containing all the individual sprite images. The `spritesheet` `Pixmap` is converted to an
+    /// in-memory PNG, optimised using the [`oxipng`] library, and saved to a local file.
+    ///
+    /// The spritesheet will match an index file that can be saved with [`Self::save_index`].
+    ///
+    /// [image file]: https://docs.mapbox.com/mapbox-gl-js/style-spec/sprite/#image-file
+    /// [`oxipng`]: https://github.com/shssoichiro/oxipng
+    pub fn save_spritesheet_at<P: AsRef<Path>>(
+        &self,
+        path: P,
+        optlevel: Optlevel,
+    ) -> SpreetResult<()> {
+        Ok(std::fs::write(path, self.encode_png_at(optlevel)?)?)
+    }
+
     /// Get the `sprite_index` that can be serialized to JSON.
     ///
     /// An [index file] is defined in the Mapbox Style Specification as a JSON document containing a
@@ -560,6 +606,12 @@ impl Spritesheet {
     }
 }
 
+impl Default for Optlevel {
+    fn default() -> Self {
+        Optlevel::Oxipng { level: 2 }
+    }
+}
+
 /// Returns the name (unique id within a spritesheet) taken from a file.
 ///
 /// The unique sprite name is the relative path from `path` to `base_path`