Support reducing the variation space of individual axes (partial inst…

…ancing)

index.js

@@ -84,12 +84,52 @@ async function subsetFont(
 
   if (variationAxes) {
     for (const [axisName, value] of Object.entries(variationAxes)) {
-      exports.hb_subset_input_pin_axis_location(
-        input,
-        face,
-        HB_TAG(axisName),
-        value
-      );
+      if (typeof value === 'number') {
+        // Simple case: Pin/instance the variation axis to a single value
+        if (
+          !exports.hb_subset_input_pin_axis_location(
+            input,
+            face,
+            HB_TAG(axisName),
+            value
+          )
+        ) {
+          exports.hb_face_destroy(face);
+          exports.free(fontBuffer);
+          throw new Error(
+            `hb_subset_input_pin_axis_location (harfbuzz) returned zero when pinning ${axisName} and a value of ${value}, indicating failure.`
+          );
+        }
+      } else if (value && typeof value === 'object') {
+        // Complex case: Reduce the variation space of the axis
+        if (
+          typeof value.min === 'undefined' ||
+          typeof value.max === 'undefined'
+        ) {
+          exports.hb_face_destroy(face);
+          exports.free(fontBuffer);
+          throw new Error(
+            `${axisName}: You must provide both a min and a max value when setting the axis range`
+          );
+        }
+        if (
+          !exports.hb_subset_input_set_axis_range(
+            input,
+            face,
+            HB_TAG(axisName),
+            value.min,
+            value.max,
+            // An explicit NaN makes harfbuzz use the existing default value, clamping to the new range if necessary
+            value.default ?? NaN
+          )
+        ) {
+          exports.hb_face_destroy(face);
+          exports.free(fontBuffer);
+          throw new Error(
+            `hb_subset_input_set_axis_range (harfbuzz) returned zero when setting the range of ${axisName} to [${min}; ${max}] and a default value of ${defaultValue}, indicating failure.`
+          );
+        }
+      }
     }
   }
 

test/index.js

@@ -407,6 +407,42 @@ describe('subset-font', function () {
           slnt: { name: 'slnt', min: -10, default: 0, max: 0 },
         });
 
+        // When not instancing the subset font is about 29 KB
+        expect(result.length, 'to be less than', 26000);
+      });
+    });
+
+    describe('when reducing the ranges of some variation axes', function () {
+      it('should perform a partial instancing', async function () {
+        const result = await subsetFont(this.variableRobotoFont, 'abcd', {
+          variationAxes: {
+            GRAD: { min: -50, max: 50, default: 25 },
+            slnt: { min: -9, max: 0 },
+            YTDE: { min: -100, max: -98 },
+            opsz: 14,
+            XTRA: 468,
+            XOPQ: 96,
+            YOPQ: 79,
+            YTLC: 514,
+            YTUC: 712,
+            YTAS: 750,
+            YTFI: 738,
+            // Leaving out wght and wdth so that the full variation space is preserved
+          },
+        });
+
+        expect(
+          fontkit.create(result).variationAxes,
+          'to exhaustively satisfy',
+          {
+            GRAD: { name: 'GRAD', min: -50, max: 50, default: 25 },
+            slnt: { name: 'slnt', min: -9, max: 0, default: 0 },
+            YTDE: { name: 'YTDE', min: -100, max: -98, default: -100 },
+            wght: { name: 'wght', min: 100, max: 1000, default: 400 },
+            wdth: { name: 'wdth', min: 25, max: 151, default: 100 },
+          }
+        );
+
         // When not instancing the subset font is about 29 KB
         expect(result.length, 'to be less than', 25000);
       });