Set interlacing to 0 by default, add tests and improve docs

index.js

@@ -40,8 +40,8 @@ module.exports = options => async buffer => {
 		arguments_.push('-nb');
 	}
 
-	if (options.interlaced) {
-		arguments_.push('-i', '1');
+	if (typeof options.interlaced === 'boolean') {
+		arguments_.push('-i', options.interlaced ? '1' : '0');
 	}
 
 	if (!options.colorTypeReduction) {

readme.md

@@ -80,10 +80,10 @@ Apply palette reduction.
 
 ##### interlaced
 
-Type: `boolean`<br>
+Type: `boolean | undefined | null`<br>
 Default: `false`
 
-Make images interlaced (progressive rendering). Interlaced images look better when they're loaded partially, but usually interlace makes compression less efficient.
+Enable [Adam7](https://en.wikipedia.org/wiki/Adam7_algorithm) PNG interlacing on any images that are processed. Interlaced images look better when they're loaded partially, but usually interlace makes compression less efficient. Set to `undefined  or `null` to keep the same interlacing as the input image.
 
 ##### errorRecovery
 

test.js

@@ -42,3 +42,24 @@ test('errorRecovery explicit', async t => {
 test('errorRecovery is set to false', async t => {
 	await t.throwsAsync(optipng({errorRecovery: false})(fixtureBroken));
 });
+
+test('interlaced is set to true', async t => {
+	const [data1, data2] = await Promise.all([
+		optipng({interlaced: true})(fixture),
+		optipng()(fixture)
+	]);
+
+	t.true(isPng(data1));
+	t.true(data1.length > data2.length);
+});
+
+test('interlaced is set to undefined and null', async t => {
+	const [data1, data2, data3] = await Promise.all([
+		optipng({interlaced: undefined})(fixture),
+		optipng({interlaced: null})(fixture),
+		optipng({interlaced: true})(fixture)
+	]);
+
+	t.true(isPng(data1) && isPng(data2));
+	t.true(data1.length === data2.length && data1.length < data3.length);
+});