fix resolution option for PNG export

Author: zbjornson

CHANGELOG.md

@@ -9,6 +9,8 @@ project adheres to [Semantic Versioning](http://semver.org/).
 ==================
 ### Changed
 ### Added
+* (Actually) added `resolution` option for `canvas.toBuffer("image/png")` and
+  `canvas.createPNGStream()`. This was documented since 2.0.0 but not working.
 ### Fixed
 * PDF metadata (added in 2.3.0) wasn't being set with `canvas.createPDFStream()`
 * Fix custom "inspect" function deprecation warnings (#1326)
@@ -172,8 +174,8 @@ context.quality = 'best' // new
  * Support for `canvas.toBuffer("image/jpeg")`
  * Unified configuration options for `canvas.toBuffer()`, `canvas.pngStream()`
    and `canvas.jpegStream()`
- * Added `resolution` option for `canvas.toBuffer("image/png")` and
-   `canvas.createPNGStream()`
+ * ~~Added `resolution` option for `canvas.toBuffer("image/png")` and
+   `canvas.createPNGStream()`~~ this was not working
  * Support for `canvas.toDataURI("image/jpeg")` (sync)
  * Support for `img.src = <url>` to match browsers
  * Support reading data URL on `img.src`

src/Canvas.cc

@@ -250,6 +250,12 @@ static void parsePNGArgs(Local<Value> arg, PngClosure& pngargs) {
       if (val <= 9) pngargs.compressionLevel = val;
     }
 
+    Local<Value> rez = obj->Get(Nan::New("resolution").ToLocalChecked());
+    if (rez->IsUint32()) {
+      uint32_t val = Nan::To<uint32_t>(rez).FromMaybe(0);
+      if (val > 0) pngargs.resolution = val;
+    }
+
     Local<Value> filters = obj->Get(Nan::New("filters").ToLocalChecked());
     if (filters->IsUint32()) pngargs.filters = Nan::To<uint32_t>(filters).FromMaybe(0);
 

test/canvas.test.js

@@ -533,11 +533,13 @@ describe('Canvas', function () {
     it('Canvas#toBuffer("image/png", {resolution: 96})', function () {
       const buf = createCanvas(200, 200).toBuffer('image/png', {resolution: 96});
       // 3780 ppm ~= 96 ppi
+      let foundpHYs = false;
       for (let i = 0; i < buf.length - 12; i++) {
         if (buf[i] === 0x70 &&
           buf[i + 1] === 0x48 &&
           buf[i + 2] === 0x59 &&
           buf[i + 3] === 0x73) { // pHYs
+          foundpHYs = true;
           assert.equal(buf[i + 4], 0);
           assert.equal(buf[i + 5], 0);
           assert.equal(buf[i + 6], 0x0e);
@@ -548,6 +550,7 @@ describe('Canvas', function () {
           assert.equal(buf[i + 11], 0xc4); // y
         }
       }
+      assert.ok(foundpHYs, "missing pHYs header");
     })
 
     it('Canvas#toBuffer("image/png", {compressionLevel: 5})', function () {