Enable option for multiple alpha transformations

CHANGELOG.md

@@ -1,6 +1,12 @@
 ### Version 0.17.0 (unreleased)
  - [SEMVER_MAJOR] Bump minimum required rustc version to 1.19.0
- - Change all transparent pixels to `rgba(0, 0, 0, 0)` to improve compression 
+ - [SEMVER_MINOR] Oxipng will now, by default, attempt to change all transparent pixels to `rgba(0, 0, 0, 0)` to improve compression.
+    It does fast trials with filters 0 and 5 to see if this is an improvement over
+    the existing alpha channel.
+ - [SEMVER_MINOR] Add a `-a` option to the command line (`alphas` in the struct) which enables 6 different
+    trials for optimizing the alpha channel, using the previously mentioned fast heuristic.
+    This option will make optimization of images with transparency somewhat slower,
+    but may improve compression.
 
 ### Version 0.16.3
  - Fix command-line help text ([#70](https://github.com/shssoichiro/oxipng/issues/70))

benches/reductions.rs

@@ -4,6 +4,7 @@ extern crate test;
 extern crate oxipng;
 
 use oxipng::png;
+use oxipng::colors::AlphaOptim;
 use test::Bencher;
 use std::path::PathBuf;
 
@@ -254,3 +255,81 @@ fn reductions_palette_full_reduction(b: &mut Bencher) {
         safe_png.reduce_palette();
     });
 }
+
+#[bench]
+fn reductions_alpha_black(b: &mut Bencher) {
+    let input = test::black_box(PathBuf::from(
+        "tests/files/rgba_8_reduce_alpha_black.png",
+    ));
+    let png = png::PngData::new(&input, false).unwrap();
+
+    b.iter(|| {
+        let mut safe_png = png.clone();
+        safe_png.reduce_alpha_channel(AlphaOptim::Black);
+    });
+}
+
+#[bench]
+fn reductions_alpha_white(b: &mut Bencher) {
+    let input = test::black_box(PathBuf::from(
+        "tests/files/rgba_8_reduce_alpha_white.png",
+    ));
+    let png = png::PngData::new(&input, false).unwrap();
+
+    b.iter(|| {
+        let mut safe_png = png.clone();
+        safe_png.reduce_alpha_channel(AlphaOptim::White);
+    });
+}
+
+#[bench]
+fn reductions_alpha_left(b: &mut Bencher) {
+    let input = test::black_box(PathBuf::from(
+        "tests/files/rgba_8_reduce_alpha_left.png",
+    ));
+    let png = png::PngData::new(&input, false).unwrap();
+
+    b.iter(|| {
+        let mut safe_png = png.clone();
+        safe_png.reduce_alpha_channel(AlphaOptim::Left);
+    });
+}
+
+#[bench]
+fn reductions_alpha_right(b: &mut Bencher) {
+    let input = test::black_box(PathBuf::from(
+        "tests/files/rgba_8_reduce_alpha_right.png",
+    ));
+    let png = png::PngData::new(&input, false).unwrap();
+
+    b.iter(|| {
+        let mut safe_png = png.clone();
+        safe_png.reduce_alpha_channel(AlphaOptim::Right);
+    });
+}
+
+#[bench]
+fn reductions_alpha_up(b: &mut Bencher) {
+    let input = test::black_box(PathBuf::from(
+        "tests/files/rgba_8_reduce_alpha_up.png",
+    ));
+    let png = png::PngData::new(&input, false).unwrap();
+
+    b.iter(|| {
+        let mut safe_png = png.clone();
+        safe_png.reduce_alpha_channel(AlphaOptim::Up);
+    });
+}
+
+#[bench]
+fn reductions_alpha_down(b: &mut Bencher) {
+    let input = test::black_box(PathBuf::from(
+        "tests/files/rgba_8_reduce_alpha_down.png",
+    ));
+    let png = png::PngData::new(&input, false).unwrap();
+
+    b.iter(|| {
+        let mut safe_png = png.clone();
+        safe_png.reduce_alpha_channel(AlphaOptim::Down);
+    });
+}

src/colors.rs

@@ -103,3 +103,33 @@ impl BitDepth {
         }
     }
 }
+
+#[derive(Debug, PartialEq, Clone, Copy, Eq, Hash)]
+/// Potential optimization methods for alpha channel
+pub enum AlphaOptim {
+    NoOp,
+    Black,
+    White,
+    Up,
+    Right,
+    Down,
+    Left,
+}
+
+impl fmt::Display for AlphaOptim {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(
+            f,
+            "{}",
+            match *self {
+                AlphaOptim::NoOp => "_",
+                AlphaOptim::Black => "B",
+                AlphaOptim::White => "W",
+                AlphaOptim::Up => "U",
+                AlphaOptim::Right => "R",
+                AlphaOptim::Down => "D",
+                AlphaOptim::Left => "L",
+            }
+        )
+    }
+}

src/lib.rs

@@ -90,6 +90,8 @@ pub struct Options {
     /// 8-15 are valid values
     /// Default: `15`
     pub window: u8,
+    /// Alpha filtering strategies to use
+    pub alphas: HashSet<colors::AlphaOptim>,
     /// Whether to attempt bit depth reduction
     /// Default: `true`
     pub bit_depth_reduction: bool,
@@ -241,6 +243,9 @@ impl Default for Options {
         for i in 0..4 {
             strategies.insert(i);
         }
+        let mut alphas = HashSet::new();
+        alphas.insert(colors::AlphaOptim::NoOp);
+        alphas.insert(colors::AlphaOptim::Black);
 
         // Default to 1 thread on single-core, otherwise use threads = 1.5x CPU cores
         let num_cpus = num_cpus::get();
@@ -265,6 +270,7 @@ impl Default for Options {
             memory: memory,
             strategies: strategies,
             window: 15,
+            alphas,
             bit_depth_reduction: true,
             color_type_reduction: true,
             palette_reduction: true,
@@ -429,7 +435,7 @@ fn optimize_png(
     original_data: &[u8],
     opts: &Options,
 ) -> Result<Vec<u8>, PngError> {
-    type TrialWithData = (u8, u8, u8, u8, Vec<u8>);
+    type TrialWithData = (TrialOptions, Vec<u8>);
 
     let original_png = png.clone();
 
@@ -495,7 +501,7 @@ fn optimize_png(
         } else {
             filter.len()
         };
-        let mut results: Vec<(u8, u8, u8, u8)> = Vec::with_capacity(combinations);
+        let mut results: Vec<TrialOptions> = Vec::with_capacity(combinations);
         if opts.verbosity.is_some() {
             eprintln!("Trying: {} combinations", combinations);
         }
@@ -505,24 +511,34 @@ fn optimize_png(
                 for zc in compression {
                     for zm in memory {
                         for zs in &strategies {
-                            results.push((*f, *zc, *zm, *zs));
+                            results.push(TrialOptions {
+                                filter: *f,
+                                compression: *zc,
+                                memory: *zm,
+                                strategy: *zs,
+                            });
                         }
                     }
                 }
             } else {
                 // Zopfli compression has no additional options
-                results.push((*f, 0, 0, 0));
+                results.push(TrialOptions {
+                    filter: *f,
+                    compression: 0,
+                    memory: 0,
+                    strategy: 0,
+                });
             }
         }
 
-        let mut filters_tmp: Vec<(u8, Vec<u8>)> = Vec::with_capacity(filter.len());
-        filter
+        let filters: HashMap<u8, Vec<u8>> = filter
             .par_iter()
             .with_max_len(1)
-            .map(|f| (*f, png.filter_image(*f)))
-            .collect_into(&mut filters_tmp);
-
-        let filters: HashMap<u8, Vec<u8>> = filters_tmp.into_iter().collect();
+            .map(|f| {
+                let png = png.clone();
+                (*f, png.filter_image(*f))
+            })
+            .collect();
 
         let original_len = original_png.idat_data.len();
         let added_interlacing = opts.interlace == Some(1) && original_png.ihdr_data.interlaced == 0;
@@ -532,9 +548,15 @@ fn optimize_png(
                 .into_par_iter()
                 .with_max_len(1)
                 .filter_map(|trial| {
-                    let filtered = &filters[&trial.0];
+                    let filtered = &filters[&trial.filter];
                     let new_idat = if opts.deflate == Deflaters::Zlib {
-                        deflate::deflate(filtered, trial.1, trial.2, trial.3, opts.window)
+                        deflate::deflate(
+                            filtered,
+                            trial.compression,
+                            trial.memory,
+                            trial.strategy,
+                            opts.window,
+                        )
                     } else {
                         deflate::zopfli_deflate(filtered)
                     }.unwrap();
@@ -542,32 +564,33 @@ fn optimize_png(
                     if opts.verbosity == Some(1) {
                         eprintln!(
                             "    zc = {}  zm = {}  zs = {}  f = {}        {} bytes",
-                            trial.1,
-                            trial.2,
-                            trial.3,
-                            trial.0,
+                            trial.compression,
+                            trial.memory,
+                            trial.strategy,
+                            trial.filter,
                             new_idat.len()
                         );
                     }
 
                     if new_idat.len() < original_len || added_interlacing || opts.force {
-                        Some((trial.0, trial.1, trial.2, trial.3, new_idat))
+                        Some((trial, new_idat))
                     } else {
                         None
                     }
                 })
-                .reduce_with(|i, j| if i.4.len() <= j.4.len() { i } else { j });
+                .reduce_with(|i, j| if i.1.len() <= j.1.len() { i } else { j });
 
         if let Some(better) = best {
-            png.idat_data = better.4;
+            png.idat_data = better.1;
             if opts.verbosity.is_some() {
+                let opts = better.0;
                 eprintln!("Found better combination:");
                 eprintln!(
                     "    zc = {}  zm = {}  zs = {}  f = {}        {} bytes",
-                    better.1,
-                    better.2,
-                    better.3,
-                    better.0,
+                    opts.compression,
+                    opts.memory,
+                    opts.strategy,
+                    opts.filter,
                     png.idat_data.len()
                 );
             }
@@ -673,8 +696,6 @@ fn perform_reductions(png: &mut png::PngData, opts: &Options) -> bool {
         report_reduction(png);
     }
 
-    png.reduce_alpha_channel();
-
     if let Some(interlacing) = opts.interlace {
         if png.change_interlacing(interlacing) {
             png.ihdr_data.interlaced = interlacing;
@@ -682,6 +703,8 @@ fn perform_reductions(png: &mut png::PngData, opts: &Options) -> bool {
         }
     }
 
+    png.try_alpha_reduction(&opts.alphas);
+
     reduction_occurred
 }
 
@@ -744,3 +767,12 @@ fn perform_strip(png: &mut png::PngData, opts: &Options) {
 fn is_fully_optimized(original_size: usize, optimized_size: usize, opts: &Options) -> bool {
     original_size <= optimized_size && !opts.force && opts.interlace.is_none()
 }
+
+#[derive(Debug, PartialEq, Clone, Copy)]
+/// Defines options to be used for a single compression trial
+struct TrialOptions {
+    pub filter: u8,
+    pub compression: u8,
+    pub memory: u8,
+    pub strategy: u8,
+}

src/main.rs

@@ -12,6 +12,7 @@ extern crate clap;
 extern crate regex;
 
 use clap::{App, Arg, ArgMatches};
+use oxipng::colors::AlphaOptim;
 use oxipng::deflate::Deflaters;
 use oxipng::headers::Headers;
 use oxipng::Options;
@@ -156,6 +157,10 @@ fn main() {
             .possible_value("8k")
             .possible_value("16k")
             .possible_value("32k"))
+        .arg(Arg::with_name("alpha")
+            .help("Perform additional alpha optimizations")
+            .short("a")
+            .long("alpha"))
         .arg(Arg::with_name("no-bit-reduction")
             .help("No bit depth reduction")
             .long("nb"))
@@ -336,6 +341,14 @@ fn parse_opts_into_struct(matches: &ArgMatches) -> Result<Options, String> {
         opts.stdout = true;
     }
 
+    if matches.is_present("alpha") {
+        opts.alphas.insert(AlphaOptim::White);
+        opts.alphas.insert(AlphaOptim::Up);
+        opts.alphas.insert(AlphaOptim::Down);
+        opts.alphas.insert(AlphaOptim::Left);
+        opts.alphas.insert(AlphaOptim::Right);
+    }
+
     if matches.is_present("backup") {
         opts.backup = true;
     }

src/png.rs

@@ -1,6 +1,6 @@
 use bit_vec::BitVec;
 use byteorder::{BigEndian, WriteBytesExt};
-use colors::{BitDepth, ColorType};
+use colors::{BitDepth, ColorType, AlphaOptim};
 use crc::crc32;
 use deflate;
 use error::PngError;
@@ -15,6 +15,12 @@ use std::io::Read;
 use std::iter::Iterator;
 use std::path::Path;
 
+const STD_COMPRESSION: u8 = 8;
+const STD_MEMORY: u8 = 9;
+const STD_STRATEGY: u8 = 2; // Huffman only
+const STD_WINDOW: u8 = 15;
+const STD_FILTERS: [u8; 2] = [0, 5];
+
 #[derive(Debug, Clone)]
 /// An iterator over the scan lines of a PNG image
 pub struct ScanLines<'a> {
@@ -737,7 +743,36 @@ impl PngData {
         changed
     }
 
-    pub fn reduce_alpha_channel(&mut self) -> bool {
+    pub fn try_alpha_reduction(&mut self, alphas: &HashSet<AlphaOptim>) {
+        assert!(!alphas.is_empty());
+        let best = alphas
+            .iter()
+            .map(|alpha| {
+                let mut image = self.clone();
+                image.reduce_alpha_channel(*alpha);
+                let size = STD_FILTERS
+                    .iter()
+                    .map(|f| {
+                        deflate::deflate(
+                            &image.filter_image(*f),
+                            STD_COMPRESSION,
+                            STD_MEMORY,
+                            STD_STRATEGY,
+                            STD_WINDOW,
+                        ).unwrap()
+                            .len()
+                    })
+                    .min()
+                    .unwrap();
+                (size, image)
+            })
+            .min_by_key(|&(size, _)| size)
+            .unwrap();
+
+        self.raw_data = best.1.raw_data;
+    }
+
+    pub fn reduce_alpha_channel(&mut self, optim: AlphaOptim) -> bool {
         let (bpc, bpp) = match self.ihdr_data.color_type {
             ColorType::RGBA => {
                 match self.ihdr_data.bit_depth {
@@ -757,20 +792,121 @@ impl PngData {
                 return false;
             }
         };
-
         let mut reduced = Vec::with_capacity(self.raw_data.len());
-        for line in self.scan_lines() {
-            reduced.push(line.filter);
-            for pixel in line.data.chunks(bpp) {
-                if pixel.iter().skip(bpp - bpc).fold(0, |sum, i| sum | i) == 0 {
-                    for _ in 0..bpp {
-                        reduced.push(0);
+
+        match optim {
+            AlphaOptim::NoOp => {
+                return false;
+            }
+            AlphaOptim::Black => {
+                for line in self.scan_lines() {
+                    reduced.push(line.filter);
+                    for pixel in line.data.chunks(bpp) {
+                        if pixel.iter().skip(bpp - bpc).fold(0, |sum, i| sum | i) == 0 {
+                            for _ in 0..bpp {
+                                reduced.push(0);
+                            }
+                        } else {
+                            reduced.extend_from_slice(pixel);
+                        }
+                    }
+                }
+            }
+            AlphaOptim::White => {
+                for line in self.scan_lines() {
+                    reduced.push(line.filter);
+                    for pixel in line.data.chunks(bpp) {
+                        if pixel.iter().skip(bpp - bpc).fold(0, |sum, i| sum | i) == 0 {
+                            for _ in 0..(bpp - bpc) {
+                                reduced.push(255);
+                            }
+                            for _ in 0..bpc {
+                                reduced.push(0);
+                            }
+                        } else {
+                            reduced.extend_from_slice(pixel);
+                        }
+                    }
+                }
+            }
+            AlphaOptim::Up => {
+                let mut lines = Vec::new();
+                let scan_lines = self.scan_lines().collect::<Vec<ScanLine>>();
+                let mut last_line = vec![0; scan_lines[0].data.len()];
+                let mut current_line = Vec::with_capacity(last_line.len());
+                for line in scan_lines.into_iter().rev() {
+                    current_line.push(line.filter);
+                    for (pixel, last_pixel) in line.data.chunks(bpp).zip(last_line.chunks(bpp)) {
+                        if pixel.iter().skip(bpp - bpc).fold(0, |sum, i| sum | i) == 0 {
+                            current_line.extend_from_slice(&last_pixel[0..(bpp - bpc)]);
+                            for _ in 0..bpc {
+                                current_line.push(0);
+                            }
+                        } else {
+                            current_line.extend_from_slice(pixel);
+                        }
+                    }
+                    last_line = current_line.clone();
+                    lines.push(current_line.clone());
+                    current_line.clear();
+                }
+                reduced.extend(lines.into_iter().rev().flatten());
+            }
+            AlphaOptim::Down => {
+                let mut last_line = vec![0; self.scan_lines().next().unwrap().data.len()];
+                for line in self.scan_lines() {
+                    reduced.push(line.filter);
+                    for (pixel, last_pixel) in line.data.chunks(bpp).zip(last_line.chunks(bpp)) {
+                        if pixel.iter().skip(bpp - bpc).fold(0, |sum, i| sum | i) == 0 {
+                            reduced.extend_from_slice(&last_pixel[0..(bpp - bpc)]);
+                            for _ in 0..bpc {
+                                reduced.push(0);
+                            }
+                        } else {
+                            reduced.extend_from_slice(pixel);
+                        }
+                    }
+                    last_line = reduced.clone();
+                }
+            }
+            AlphaOptim::Left => {
+                for line in self.scan_lines() {
+                    let mut line_bytes = Vec::with_capacity(line.data.len());
+                    let mut last_pixel = vec![0; bpp];
+                    for pixel in line.data.chunks(bpp).rev() {
+                        if pixel.iter().skip(bpp - bpc).fold(0, |sum, i| sum | i) == 0 {
+                            line_bytes.extend_from_slice(&last_pixel[0..(bpp - bpc)]);
+                            for _ in 0..bpc {
+                                line_bytes.push(0);
+                            }
+                        } else {
+                            line_bytes.extend_from_slice(pixel);
+                        }
+                        last_pixel = pixel.to_owned();
+                    }
+                    reduced.push(line.filter);
+                    reduced.extend(line_bytes.chunks(bpp).rev().flatten());
+                }
+            }
+            AlphaOptim::Right => {
+                for line in self.scan_lines() {
+                    reduced.push(line.filter);
+                    let mut last_pixel = vec![0; bpp];
+                    for pixel in line.data.chunks(bpp) {
+                        if pixel.iter().skip(bpp - bpc).fold(0, |sum, i| sum | i) == 0 {
+                            reduced.extend_from_slice(&last_pixel[0..(bpp - bpc)]);
+                            for _ in 0..bpc {
+                                reduced.push(0);
+                            }
+                        } else {
+                            reduced.extend_from_slice(pixel);
+                        }
+                        last_pixel = pixel.to_owned();
                     }
-                } else {
-                    reduced.extend_from_slice(pixel);
                 }
             }
         }
+
         self.raw_data = reduced;
         true
     }

tests/reduction.rs

@@ -1,6 +1,6 @@
 extern crate oxipng;
 
-use oxipng::colors::{BitDepth, ColorType};
+use oxipng::colors::{BitDepth, ColorType, AlphaOptim};
 use oxipng::png;
 use std::collections::HashSet;
 use std::error::Error;
@@ -1096,7 +1096,6 @@ fn rgba_16_reduce_alpha_black() {
     );
 }
 
-
 #[test]
 fn rgba_8_reduce_alpha_black() {
     let input = PathBuf::from("tests/files/rgba_8_reduce_alpha_black.png");
@@ -1131,7 +1130,6 @@ fn grayscale_alpha_16_reduce_alpha_black() {
     );
 }
 
-
 #[test]
 fn grayscale_alpha_8_reduce_alpha_black() {
     let input = PathBuf::from("tests/files/grayscale_alpha_8_reduce_alpha_black.png");
@@ -1148,3 +1146,383 @@ fn grayscale_alpha_8_reduce_alpha_black() {
         BitDepth::Eight,
     );
 }
+
+#[test]
+fn rgba_16_reduce_alpha_white() {
+    let input = PathBuf::from("tests/files/rgba_16_reduce_alpha_white.png");
+    let mut opts = get_opts(&input);
+    opts.alphas = HashSet::with_capacity(1);
+    opts.alphas.insert(AlphaOptim::White);
+    let output = opts.out_file.clone();
+
+    test_it_converts(
+        &input,
+        &output,
+        &opts,
+        ColorType::RGBA,
+        BitDepth::Sixteen,
+        ColorType::RGBA,
+        BitDepth::Eight,
+    );
+}
+
+#[test]
+fn rgba_8_reduce_alpha_white() {
+    let input = PathBuf::from("tests/files/rgba_8_reduce_alpha_white.png");
+    let mut opts = get_opts(&input);
+    opts.alphas = HashSet::with_capacity(1);
+    opts.alphas.insert(AlphaOptim::White);
+    let output = opts.out_file.clone();
+
+    test_it_converts(
+        &input,
+        &output,
+        &opts,
+        ColorType::RGBA,
+        BitDepth::Eight,
+        ColorType::RGBA,
+        BitDepth::Eight,
+    );
+}
+
+#[test]
+fn grayscale_alpha_16_reduce_alpha_white() {
+    let input = PathBuf::from("tests/files/grayscale_alpha_16_reduce_alpha_white.png");
+    let mut opts = get_opts(&input);
+    opts.alphas = HashSet::with_capacity(1);
+    opts.alphas.insert(AlphaOptim::White);
+    let output = opts.out_file.clone();
+
+    test_it_converts(
+        &input,
+        &output,
+        &opts,
+        ColorType::GrayscaleAlpha,
+        BitDepth::Sixteen,
+        ColorType::GrayscaleAlpha,
+        BitDepth::Eight,
+    );
+}
+
+#[test]
+fn grayscale_alpha_8_reduce_alpha_white() {
+    let input = PathBuf::from("tests/files/grayscale_alpha_8_reduce_alpha_white.png");
+    let mut opts = get_opts(&input);
+    opts.alphas = HashSet::with_capacity(1);
+    opts.alphas.insert(AlphaOptim::White);
+    let output = opts.out_file.clone();
+
+    test_it_converts(
+        &input,
+        &output,
+        &opts,
+        ColorType::GrayscaleAlpha,
+        BitDepth::Eight,
+        ColorType::GrayscaleAlpha,
+        BitDepth::Eight,
+    );
+}
+
+#[test]
+fn rgba_16_reduce_alpha_down() {
+    let input = PathBuf::from("tests/files/rgba_16_reduce_alpha_down.png");
+    let mut opts = get_opts(&input);
+    opts.alphas = HashSet::with_capacity(1);
+    opts.alphas.insert(AlphaOptim::Down);
+    let output = opts.out_file.clone();
+
+    test_it_converts(
+        &input,
+        &output,
+        &opts,
+        ColorType::RGBA,
+        BitDepth::Sixteen,
+        ColorType::RGBA,
+        BitDepth::Eight,
+    );
+}
+
+#[test]
+fn rgba_8_reduce_alpha_down() {
+    let input = PathBuf::from("tests/files/rgba_8_reduce_alpha_down.png");
+    let mut opts = get_opts(&input);
+    opts.alphas = HashSet::with_capacity(1);
+    opts.alphas.insert(AlphaOptim::Down);
+    let output = opts.out_file.clone();
+
+    test_it_converts(
+        &input,
+        &output,
+        &opts,
+        ColorType::RGBA,
+        BitDepth::Eight,
+        ColorType::RGBA,
+        BitDepth::Eight,
+    );
+}
+
+#[test]
+fn grayscale_alpha_16_reduce_alpha_down() {
+    let input = PathBuf::from("tests/files/grayscale_alpha_16_reduce_alpha_down.png");
+    let mut opts = get_opts(&input);
+    opts.alphas = HashSet::with_capacity(1);
+    opts.alphas.insert(AlphaOptim::Down);
+    let output = opts.out_file.clone();
+
+    test_it_converts(
+        &input,
+        &output,
+        &opts,
+        ColorType::GrayscaleAlpha,
+        BitDepth::Sixteen,
+        ColorType::GrayscaleAlpha,
+        BitDepth::Eight,
+    );
+}
+
+#[test]
+fn grayscale_alpha_8_reduce_alpha_down() {
+    let input = PathBuf::from("tests/files/grayscale_alpha_8_reduce_alpha_down.png");
+    let mut opts = get_opts(&input);
+    opts.alphas = HashSet::with_capacity(1);
+    opts.alphas.insert(AlphaOptim::Down);
+    let output = opts.out_file.clone();
+
+    test_it_converts(
+        &input,
+        &output,
+        &opts,
+        ColorType::GrayscaleAlpha,
+        BitDepth::Eight,
+        ColorType::GrayscaleAlpha,
+        BitDepth::Eight,
+    );
+}
+
+#[test]
+fn rgba_16_reduce_alpha_up() {
+    let input = PathBuf::from("tests/files/rgba_16_reduce_alpha_up.png");
+    let mut opts = get_opts(&input);
+    opts.alphas = HashSet::with_capacity(1);
+    opts.alphas.insert(AlphaOptim::Up);
+    let output = opts.out_file.clone();
+
+    test_it_converts(
+        &input,
+        &output,
+        &opts,
+        ColorType::RGBA,
+        BitDepth::Sixteen,
+        ColorType::RGBA,
+        BitDepth::Eight,
+    );
+}
+
+#[test]
+fn rgba_8_reduce_alpha_up() {
+    let input = PathBuf::from("tests/files/rgba_8_reduce_alpha_up.png");
+    let mut opts = get_opts(&input);
+    opts.alphas = HashSet::with_capacity(1);
+    opts.alphas.insert(AlphaOptim::Up);
+    let output = opts.out_file.clone();
+
+    test_it_converts(
+        &input,
+        &output,
+        &opts,
+        ColorType::RGBA,
+        BitDepth::Eight,
+        ColorType::RGBA,
+        BitDepth::Eight,
+    );
+}
+
+#[test]
+fn grayscale_alpha_16_reduce_alpha_up() {
+    let input = PathBuf::from("tests/files/grayscale_alpha_16_reduce_alpha_up.png");
+    let mut opts = get_opts(&input);
+    opts.alphas = HashSet::with_capacity(1);
+    opts.alphas.insert(AlphaOptim::Up);
+    let output = opts.out_file.clone();
+
+    test_it_converts(
+        &input,
+        &output,
+        &opts,
+        ColorType::GrayscaleAlpha,
+        BitDepth::Sixteen,
+        ColorType::GrayscaleAlpha,
+        BitDepth::Eight,
+    );
+}
+
+#[test]
+fn grayscale_alpha_8_reduce_alpha_up() {
+    let input = PathBuf::from("tests/files/grayscale_alpha_8_reduce_alpha_up.png");
+    let mut opts = get_opts(&input);
+    opts.alphas = HashSet::with_capacity(1);
+    opts.alphas.insert(AlphaOptim::Up);
+    let output = opts.out_file.clone();
+
+    test_it_converts(
+        &input,
+        &output,
+        &opts,
+        ColorType::GrayscaleAlpha,
+        BitDepth::Eight,
+        ColorType::GrayscaleAlpha,
+        BitDepth::Eight,
+    );
+}
+
+#[test]
+fn rgba_16_reduce_alpha_left() {
+    let input = PathBuf::from("tests/files/rgba_16_reduce_alpha_left.png");
+    let mut opts = get_opts(&input);
+    opts.alphas = HashSet::with_capacity(1);
+    opts.alphas.insert(AlphaOptim::Left);
+    let output = opts.out_file.clone();
+
+    test_it_converts(
+        &input,
+        &output,
+        &opts,
+        ColorType::RGBA,
+        BitDepth::Sixteen,
+        ColorType::RGBA,
+        BitDepth::Eight,
+    );
+}
+
+#[test]
+fn rgba_8_reduce_alpha_left() {
+    let input = PathBuf::from("tests/files/rgba_8_reduce_alpha_left.png");
+    let mut opts = get_opts(&input);
+    opts.alphas = HashSet::with_capacity(1);
+    opts.alphas.insert(AlphaOptim::Left);
+    let output = opts.out_file.clone();
+
+    test_it_converts(
+        &input,
+        &output,
+        &opts,
+        ColorType::RGBA,
+        BitDepth::Eight,
+        ColorType::RGBA,
+        BitDepth::Eight,
+    );
+}
+
+#[test]
+fn grayscale_alpha_16_reduce_alpha_left() {
+    let input = PathBuf::from("tests/files/grayscale_alpha_16_reduce_alpha_left.png");
+    let mut opts = get_opts(&input);
+    opts.alphas = HashSet::with_capacity(1);
+    opts.alphas.insert(AlphaOptim::Left);
+    let output = opts.out_file.clone();
+
+    test_it_converts(
+        &input,
+        &output,
+        &opts,
+        ColorType::GrayscaleAlpha,
+        BitDepth::Sixteen,
+        ColorType::GrayscaleAlpha,
+        BitDepth::Eight,
+    );
+}
+
+#[test]
+fn grayscale_alpha_8_reduce_alpha_left() {
+    let input = PathBuf::from("tests/files/grayscale_alpha_8_reduce_alpha_left.png");
+    let mut opts = get_opts(&input);
+    opts.alphas = HashSet::with_capacity(1);
+    opts.alphas.insert(AlphaOptim::Left);
+    let output = opts.out_file.clone();
+
+    test_it_converts(
+        &input,
+        &output,
+        &opts,
+        ColorType::GrayscaleAlpha,
+        BitDepth::Eight,
+        ColorType::GrayscaleAlpha,
+        BitDepth::Eight,
+    );
+}
+
+#[test]
+fn rgba_16_reduce_alpha_right() {
+    let input = PathBuf::from("tests/files/rgba_16_reduce_alpha_right.png");
+    let mut opts = get_opts(&input);
+    opts.alphas = HashSet::with_capacity(1);
+    opts.alphas.insert(AlphaOptim::Right);
+    let output = opts.out_file.clone();
+
+    test_it_converts(
+        &input,
+        &output,
+        &opts,
+        ColorType::RGBA,
+        BitDepth::Sixteen,
+        ColorType::RGBA,
+        BitDepth::Eight,
+    );
+}
+
+#[test]
+fn rgba_8_reduce_alpha_right() {
+    let input = PathBuf::from("tests/files/rgba_8_reduce_alpha_right.png");
+    let mut opts = get_opts(&input);
+    opts.alphas = HashSet::with_capacity(1);
+    opts.alphas.insert(AlphaOptim::Right);
+    let output = opts.out_file.clone();
+
+    test_it_converts(
+        &input,
+        &output,
+        &opts,
+        ColorType::RGBA,
+        BitDepth::Eight,
+        ColorType::RGBA,
+        BitDepth::Eight,
+    );
+}
+
+#[test]
+fn grayscale_alpha_16_reduce_alpha_right() {
+    let input = PathBuf::from("tests/files/grayscale_alpha_16_reduce_alpha_right.png");
+    let mut opts = get_opts(&input);
+    opts.alphas = HashSet::with_capacity(1);
+    opts.alphas.insert(AlphaOptim::Right);
+    let output = opts.out_file.clone();
+
+    test_it_converts(
+        &input,
+        &output,
+        &opts,
+        ColorType::GrayscaleAlpha,
+        BitDepth::Sixteen,
+        ColorType::GrayscaleAlpha,
+        BitDepth::Eight,
+    );
+}
+
+#[test]
+fn grayscale_alpha_8_reduce_alpha_right() {
+    let input = PathBuf::from("tests/files/grayscale_alpha_8_reduce_alpha_right.png");
+    let mut opts = get_opts(&input);
+    opts.alphas = HashSet::with_capacity(1);
+    opts.alphas.insert(AlphaOptim::Right);
+    let output = opts.out_file.clone();
+
+    test_it_converts(
+        &input,
+        &output,
+        &opts,
+        ColorType::GrayscaleAlpha,
+        BitDepth::Eight,
+        ColorType::GrayscaleAlpha,
+        BitDepth::Eight,
+    );
+}