Fix actualBoundingBoxLeft/Right with center/right alignment

This bug goes back 10 years to the original implementation.

Fixes #1909

Author: zbjornson

CHANGELOG.md

@@ -10,6 +10,7 @@ project adheres to [Semantic Versioning](http://semver.org/).
 ### Changed
 ### Added
 ### Fixed
+* Fix `actualBoundingBoxLeft` and `actualBoundingBoxRight` when `textAlign='center'` or `'right'` ([#1909](https://github.com/Automattic/node-canvas/issues/1909))
 
 2.10.0
 ==================

src/CanvasRenderingContext2d.cc

@@ -2748,7 +2748,7 @@ NAN_METHOD(Context2d::MeasureText) {
   double x_offset;
   switch (context->state->textAlignment) {
     case 0: // center
-      x_offset = logical_rect.width / 2;
+      x_offset = logical_rect.width / 2.;
       break;
     case 1: // right
       x_offset = logical_rect.width;
@@ -2766,10 +2766,10 @@ NAN_METHOD(Context2d::MeasureText) {
            Nan::New<Number>(logical_rect.width)).Check();
   Nan::Set(obj,
            Nan::New<String>("actualBoundingBoxLeft").ToLocalChecked(),
-           Nan::New<Number>(x_offset - PANGO_LBEARING(ink_rect))).Check();
+           Nan::New<Number>(PANGO_LBEARING(ink_rect) + x_offset)).Check();
   Nan::Set(obj,
            Nan::New<String>("actualBoundingBoxRight").ToLocalChecked(),
-           Nan::New<Number>(x_offset + PANGO_RBEARING(ink_rect))).Check();
+           Nan::New<Number>(PANGO_RBEARING(ink_rect) - x_offset)).Check();
   Nan::Set(obj,
            Nan::New<String>("actualBoundingBoxAscent").ToLocalChecked(),
            Nan::New<Number>(y_offset + PANGO_ASCENT(ink_rect))).Check();

test/canvas.test.js

@@ -20,6 +20,11 @@ const {
   deregisterAllFonts
 } = require('../')
 
+function assertApprox(actual, expected, tol) {
+  assert(Math.abs(expected - actual) <= tol,
+    "Expected " + actual + " to be " + expected + " +/- " + tol);
+}
+
 describe('Canvas', function () {
   // Run with --expose-gc and uncomment this line to help find memory problems:
   // afterEach(gc);
@@ -946,20 +951,46 @@ describe('Canvas', function () {
       let metrics = ctx.measureText('Alphabet')
       // Actual value depends on font library version. Have observed values
       // between 0 and 0.769.
-      assert.ok(metrics.alphabeticBaseline >= 0 && metrics.alphabeticBaseline <= 1)
+      assertApprox(metrics.alphabeticBaseline, 0.5, 0.5)
       // Positive = going up from the baseline
       assert.ok(metrics.actualBoundingBoxAscent > 0)
       // Positive = going down from the baseline
-      assert.ok(metrics.actualBoundingBoxDescent > 0) // ~4-5
+      assertApprox(metrics.actualBoundingBoxDescent, 5, 2)
 
       ctx.textBaseline = 'bottom'
       metrics = ctx.measureText('Alphabet')
       assert.strictEqual(ctx.textBaseline, 'bottom')
-      assert.ok(metrics.alphabeticBaseline > 0) // ~4-5
+      assertApprox(metrics.alphabeticBaseline, 5, 2)
       assert.ok(metrics.actualBoundingBoxAscent > 0)
       // On the baseline or slightly above
       assert.ok(metrics.actualBoundingBoxDescent <= 0)
     })
+
+    it('actualBoundingBox is correct for left, center and right alignment (#1909)', function () {
+      const canvas = createCanvas(0, 0)
+      const ctx = canvas.getContext('2d')
+
+      // positive actualBoundingBoxLeft indicates a distance going left from the
+      // given alignment point.
+
+      // positive actualBoundingBoxRight indicates a distance going right from
+      // the given alignment point.
+
+      ctx.textAlign = 'left'
+      const lm = ctx.measureText('aaaa')
+      assertApprox(lm.actualBoundingBoxLeft, -1, 4)
+      assertApprox(lm.actualBoundingBoxRight, 21, 4)
+
+      ctx.textAlign = 'center'
+      const cm = ctx.measureText('aaaa')
+      assertApprox(cm.actualBoundingBoxLeft, 9, 4)
+      assertApprox(cm.actualBoundingBoxRight, 11, 4)
+      
+      ctx.textAlign = 'right'
+      const rm = ctx.measureText('aaaa')
+      assertApprox(rm.actualBoundingBoxLeft, 19, 4)
+      assertApprox(rm.actualBoundingBoxRight, 1, 4)
+    })
   })
 
   it('Context2d#fillText()', function () {

test/public/tests.js

@@ -2658,7 +2658,8 @@ tests['measureText()'] = function (ctx) {
     const metrics = ctx.measureText(text)
     ctx.strokeStyle = 'blue'
     ctx.strokeRect(
-      x - metrics.actualBoundingBoxLeft + 0.5,
+      // positive numbers for actualBoundingBoxLeft indicate a distance going left
+      x + metrics.actualBoundingBoxLeft + 0.5,
       y - metrics.actualBoundingBoxAscent + 0.5,
       metrics.width,
       metrics.actualBoundingBoxAscent + metrics.actualBoundingBoxDescent
@@ -2677,8 +2678,19 @@ tests['measureText()'] = function (ctx) {
   drawWithBBox('Alphabet bottom', 20, 90)
 
   ctx.textBaseline = 'alphabetic'
+  ctx.save()
   ctx.rotate(Math.PI / 8)
   drawWithBBox('Alphabet', 50, 100)
+  ctx.restore()
+
+  ctx.textAlign = 'center'
+  drawWithBBox('Centered', 100, 195)
+
+  ctx.textAlign = 'left'
+  drawWithBBox('Left', 10, 195)
+
+  ctx.textAlign = 'right'
+  drawWithBBox('right', 195, 195)
 }
 
 tests['image sampling (#1084)'] = function (ctx, done) {