Add usage example and further unit test for new max option

Simplify max vs crop logic

README.md

@@ -85,25 +85,25 @@ sharp('input.jpg').resize(300, 200).write('output.jpg', function(err) {
 ```
 
 ```javascript
-sharp('input.jpg').resize(null, 200).progressive().toBuffer(function(err, buffer) {
+sharp('input.jpg').resize(null, 200).progressive().toBuffer(function(err, outputBuffer) {
   if (err) {
     throw err;
   }
-  // buffer contains progressive JPEG image data, 200 pixels high
+  // outputBuffer contains progressive JPEG image data, 200 pixels high
 });
 ```
 
 ```javascript
-sharp('input.png').resize(300).sharpen().quality(90).webp(function(err, buffer) {
+sharp('input.png').resize(300).sharpen().quality(90).webp(function(err, outputBuffer) {
   if (err) {
     throw err;
   }
-  // buffer contains 300 pixels wide, sharpened, 90% quality WebP image data
+  // outputBuffer contains 300 pixels wide, sharpened, 90% quality WebP image data
 });
 ```
 
 ```javascript
-sharp(buffer).resize(200, 300).embedWhite().write('output.tiff', function(err) {
+sharp(inputBuffer).resize(200, 300).embedWhite().write('output.tiff', function(err) {
   if (err) {
     throw err;
   }
@@ -113,15 +113,25 @@ sharp(buffer).resize(200, 300).embedWhite().write('output.tiff', function(err) {
 ```
 
 ```javascript
-sharp('input.gif').resize(200, 300).embedBlack().webp(function(err, buffer) {
+sharp('input.gif').resize(200, 300).embedBlack().webp(function(err, outputBuffer) {
   if (err) {
     throw err;
   }
-  // buffer contains WebP image data of a 200 pixels wide and 300 pixels high image
+  // outputBuffer contains WebP image data of a 200 pixels wide and 300 pixels high
   // containing a scaled version, embedded on a black canvas, of input.gif
 });
 ```
 
+```javascript
+sharp(inputBuffer).resize(200, 200).max().jpeg(function(err, outputBuffer) {
+  if (err) {
+    throw err;
+  }
+  // outputBuffer contains JPEG image data no wider than 200 pixels and no higher
+  // than 200 pixels regardless of the inputBuffer image dimensions
+});
+```
+
 ## API
 
 ### sharp(input)
@@ -147,6 +157,8 @@ Crop the resized image to the exact size specified, the default behaviour.
 
 Preserving aspect ratio, resize the image to the maximum width or height specified.
 
+Both `width` and `height` must be provided via `resize` otherwise the behaviour will default to `crop`.
+
 ### embedWhite()
 
 Embed the resized image on a white background of the exact size specified.

src/sharp.cc

@@ -142,9 +142,9 @@ class ResizeWorker : public NanAsyncWorker {
       // if max is set, we need to compute the real size of the thumb image
       if (baton->max) {
         if (xfactor > yfactor) {
-          baton->height = round(in->Ysize/factor);
+          baton->height = round(static_cast<double>(in->Ysize) / xfactor);
         } else {
-          baton->width = round(in->Xsize/factor);
+          baton->width = round(static_cast<double>(in->Xsize) / yfactor);
         }
       }
     } else if (baton->width > 0) {
@@ -235,8 +235,8 @@ class ResizeWorker : public NanAsyncWorker {
     // Crop/embed
     VipsImage *canvased = vips_image_new();
     if (affined->Xsize != baton->width || affined->Ysize != baton->height) {
-      if (baton->crop || baton->max) {
-        // Crop
+      if (baton->crop) {
+        // Crop/max
         int width = std::min(affined->Xsize, baton->width);
         int height = std::min(affined->Ysize, baton->height);
         int left = (affined->Xsize - width + 1) / 2;
@@ -355,13 +355,11 @@ NAN_METHOD(resize) {
   if (canvas->Equals(NanSymbol("c"))) {
     baton->crop = true;
   } else if (canvas->Equals(NanSymbol("w"))) {
-    baton->crop = false;
     baton->extend = VIPS_EXTEND_WHITE;
   } else if (canvas->Equals(NanSymbol("b"))) {
-    baton->crop = false;
     baton->extend = VIPS_EXTEND_BLACK;
   } else if (canvas->Equals(NanSymbol("m"))) {
-    baton->crop = false;
+    baton->crop = true;
     baton->max = true;
   }
   baton->sharpen = args[6]->BooleanValue();

tests/unit.js

@@ -133,5 +133,17 @@ async.series([
         done();
       });
     });
+  },
+  // Attempt to resize to max but only provide one dimension, so should default to crop
+  function(done) {
+    sharp(inputJpg).resize(320).max().write(outputJpg, function(err) {
+      if (err) throw err;
+      imagemagick.identify(outputJpg, function(err, features) {
+        if (err) throw err;
+        assert.strictEqual(320, features.width);
+        assert.strictEqual(261, features.height);
+        done();
+      });
+    });
   }
 ]);