diff --git a/README.md b/README.md
index b4402f7a..97f58a26 100644
--- a/README.md
+++ b/README.md
@@ -42,6 +42,7 @@ Create image differential between two images.
   expectedFilename: string;
   diffFilename?: string;
   options?: {
+    preprocess?: ([Image, Image]) => void | Promise<[Image, Image]>;
     threshold?: number;   // default 0.1
     includeAA?: boolean;  // default false
   }
@@ -51,7 +52,18 @@ Create image differential between two images.
 - `actualFilename` - *Required* - Path to actual image file.
 - `expectedFilename` - *Required* - Path to expected image file.
 - `diffFilename` - *Optional* - Path to differential image file. If omitted, `imgDiff` does not output image file.
-- `options` - *Optional* - An object to pass through [pixelmatch](https://github.com/mapbox/pixelmatch#api).
+- `options` - *Optional* 
+- `options.preprocess` - *Optional* - Preprocess function. It's called with decoded images and also can process them.
+- `options.threshold`, `options.includeAA` - *Optional* - Parameters using by [pixelmatch](https://github.com/mapbox/pixelmatch#api).
+
+#### `Image`
+```ts
+{
+  width: number;
+  height: number;
+  data: Uint8Array;
+}
+```
 
 #### `ImgDiffResult`
 
diff --git a/lib/index.js b/lib/index.js
index 6d137359..fb3b98cc 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -68,7 +68,19 @@ function imgDiff(opt) {
     decode(opt.actualFilename),
     decode(opt.expectedFilename),
   ]).then(imgs => {
-    return compare(imgs[0], imgs[1], opt.diffFilename, opt.options);
+    const options = opt.options || { };
+    let pp;
+    if (options.preprocess && typeof options.preprocess === "function") {
+      const ret = options.preprocess(imgs);
+      if (typeof ret === "object" && ret.then) {
+        pp = ret;
+      } else {
+        pp = Promise.resolve(imgs);
+      }
+    } else {
+      pp = Promise.resolve(imgs);
+    }
+    return pp.then(imgs => compare(imgs[0], imgs[1], opt.diffFilename, options));
   })
   ;
 }
diff --git a/test/index.test.js b/test/index.test.js
index 9091a7cc..0b8a97ed 100644
--- a/test/index.test.js
+++ b/test/index.test.js
@@ -61,3 +61,32 @@ test("compare with 2 jpeg files", async t => {
   });
   t.truthy(fs.statSync(path.resolve(__dirname, "images/diff_generated.jpg.png")));
 });
+
+test("call preprocess hook", async t => {
+  let called = false;
+  await imgDiff({
+    actualFilename: path.resolve(__dirname, "images/expected.png"),
+    expectedFilename: path.resolve(__dirname, "images/expected.png"),
+    options: {
+      preprocess: ([img1, img2]) => {
+        called = !!img1 && !!img2;
+      },
+    },
+  });
+  t.is(called, true);
+});
+
+test("call preprocess hook with promiss", async t => {
+  let called = false;
+  await imgDiff({
+    actualFilename: path.resolve(__dirname, "images/expected.png"),
+    expectedFilename: path.resolve(__dirname, "images/expected.png"),
+    options: {
+      preprocess: ([img1, img2]) => {
+        called = !!img1 && !!img2;
+        return Promise.resolve([img2, img1]);
+      },
+    },
+  });
+  t.is(called, true);
+});