diff --git a/Cargo.lock b/Cargo.lock
index fdcfe114..c1bc15dc 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -251,7 +251,7 @@ dependencies = [
 
 [[package]]
 name = "oxipng"
-version = "2.1.8"
+version = "2.2.0"
 dependencies = [
  "bit-vec 0.5.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "byteorder 1.3.1 (registry+https://github.com/rust-lang/crates.io-index)",
diff --git a/Cargo.toml b/Cargo.toml
index 1e70eb06..5c03d859 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -15,7 +15,7 @@ homepage = "https://github.com/shssoichiro/oxipng"
 license = "MIT"
 name = "oxipng"
 repository = "https://github.com/shssoichiro/oxipng"
-version = "2.1.8"
+version = "2.2.0"
 
 [badges]
 travis-ci = { repository = "shssoichiro/oxipng", branch = "master" }
diff --git a/README.md b/README.md
index 7d883db6..ec291121 100644
--- a/README.md
+++ b/README.md
@@ -94,29 +94,29 @@ Oxipng is open-source software, distributed under the MIT license.
 
 ## Benchmarks
 
-Tested oxipng 2.1.7 (compiled on rustc 1.30.1 (1433507eb 2018-11-07)) against OptiPNG version 0.7.7 on Intel(R) Core(TM) i7-6700HQ CPU @ 2.60GHz with 8 logical cores
+Tested oxipng 2.2.0 (compiled on rustc 1.32.0 (9fda7c223 2019-01-16)) against OptiPNG version 0.7.7 on Intel(R) Core(TM) i7-6700HQ CPU @ 2.60GHz with 8 logical cores
 
 
 
 Benchmark #1: ./target/release/oxipng -P ./tests/files/rgb_16_should_be_grayscale_8.png
-  Time (mean ± σ):     104.5 ms ±   2.0 ms    [User: 169.4 ms, System: 23.9 ms]
-  Range (min … max):   101.3 ms … 108.9 ms
+  Time (mean ± σ):     117.2 ms ±   5.6 ms    [User: 269.2 ms, System: 17.2 ms]
+  Range (min … max):   106.6 ms … 126.0 ms    25 runs
  
 Benchmark #2: optipng -simulate ./tests/files/rgb_16_should_be_grayscale_8.png
-  Time (mean ± σ):     272.3 ms ±   3.1 ms    [User: 271.3 ms, System: 1.1 ms]
-  Range (min … max):   269.1 ms … 278.6 ms
+  Time (mean ± σ):     280.5 ms ±   3.8 ms    [User: 278.0 ms, System: 2.4 ms]
+  Range (min … max):   275.7 ms … 284.4 ms    10 runs
  
 Summary
   './target/release/oxipng -P ./tests/files/rgb_16_should_be_grayscale_8.png' ran
-    2.60 ± 0.06 times faster than 'optipng -simulate ./tests/files/rgb_16_should_be_grayscale_8.png'
+    2.39 ± 0.12 times faster than 'optipng -simulate ./tests/files/rgb_16_should_be_grayscale_8.png'
 Benchmark #1: ./target/release/oxipng -o4 -P ./tests/files/rgb_16_should_be_grayscale_8.png
-  Time (mean ± σ):     128.1 ms ±   4.1 ms    [User: 404.8 ms, System: 27.3 ms]
-  Range (min … max):   123.1 ms … 140.4 ms
+  Time (mean ± σ):     144.7 ms ±   6.3 ms    [User: 497.5 ms, System: 25.5 ms]
+  Range (min … max):   134.7 ms … 159.4 ms    19 runs
  
 Benchmark #2: optipng -o 4 -simulate ./tests/files/rgb_16_should_be_grayscale_8.png
-  Time (mean ± σ):     914.8 ms ±   3.7 ms    [User: 913.2 ms, System: 1.2 ms]
-  Range (min … max):   910.5 ms … 923.7 ms
+  Time (mean ± σ):     945.8 ms ±   8.2 ms    [User: 942.4 ms, System: 2.7 ms]
+  Range (min … max):   932.5 ms … 964.6 ms    10 runs
  
 Summary
   './target/release/oxipng -o4 -P ./tests/files/rgb_16_should_be_grayscale_8.png' ran
-    7.14 ± 0.23 times faster than 'optipng -o 4 -simulate ./tests/files/rgb_16_should_be_grayscale_8.png'
+    6.54 ± 0.29 times faster than 'optipng -o 4 -simulate ./tests/files/rgb_16_should_be_grayscale_8.png'