Support reducing the variation space of individual axes (partial instancing)

README.md

@@ -1,6 +1,23 @@
 # subset-font
 
-Create a subset font from an existing font in SFNT (TrueType/OpenType), WOFF, or WOFF2 format. Uses [`harfbuzzjs`](https://github.com/harfbuzz/harfbuzzjs), which is a WebAssembly build of [HarfBuzz](https://harfbuzz.github.io/).
+Create a subset font from an existing font in SFNT (TrueType/OpenType), WOFF, or WOFF2 format. When subsetting a variable font, you can also reduce the variation space at the individual axis level.
+
+These operations are implemented using [`harfbuzzjs`](https://github.com/harfbuzz/harfbuzzjs), which is a WebAssembly build of [HarfBuzz](https://harfbuzz.github.io/).
+
+## Basic example
+
+```js
+const subsetFont = require('subset-font');
+
+const mySfntFontBuffer = Buffer.from(/*...*/);
+
+// Create a new font with only the characters required to render "Hello, world!" in WOFF2 format:
+const subsetBuffer = await subsetFont(mySfntFontBuffer, 'Hello, world!', {
+  targetFormat: 'woff2',
+});
+```
+
+## Reducing the variation space
 
 ```js
 const subsetFont = require('subset-font');
@@ -10,6 +27,15 @@ const mySfntFontBuffer = Buffer.from(/*...*/);
 // Create a new font with only the characters required to render "Hello, world!" in WOFF2 format:
 const subsetBuffer = await subsetFont(mySfntFontBuffer, 'Hello, world!', {
   targetFormat: 'woff2',
+  variationAxes: {
+    // Pin the axis to 200:
+    wght: 200,
+    // Reduce the variation space, explicitly setting a new default value:
+    GRAD: { min: -50, max: 50, default: 25 },
+    // Reduce the variation space. A new default value will be inferred by clamping the old default to the new range:
+    slnt: { min: -9, max: 0 },
+    // The remaining axes will be kept as-is
+  },
 });
 ```
 
@@ -25,6 +51,7 @@ Options:
 
 - `targetFormat` - the format to output, can be either `'sfnt'`, `'woff'`, or `'woff2'`.
 - `preserveNameIds` - an array of numbers specifying the extra name ids to preserve in the `name` table. By default the harfbuzz subsetter drops most of these. Use case described [here](https://github.com/papandreou/subset-font/issues/7).
+- `variationAxes` - an object specifying a full or partial instancing of variation axes in the font. Only works with variable fonts. See the example above.
 
 For backwards compatibility reasons, `'truetype'` is supported as an alias for `'sfnt'`.
 

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} to ${value}, indicating failure. Maybe the axis does not exist in the font?`
+          );
+        }
+      } else if (value && typeof value === 'object') {
+        // Complex case: Reduce the variation space of the axis
+        if (
+          typeof value.min === 'undefined' ||
+          typeof value.max === 'undefined'
+        ) {
+          exports.hb_face_destroy(face);
+          exports.free(fontBuffer);
+          throw new Error(
+            `${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 ${axisName} to [${value.min}; ${value.max}] and a default value of ${value.default}, indicating failure. Maybe the axis does not exist in the font?`
+          );
+        }
+      }
     }
   }
 

package.json

@@ -5,7 +5,7 @@
   "main": "index.js",
   "dependencies": {
     "fontverter": "^2.0.0",
-    "harfbuzzjs": "^0.3.4",
+    "harfbuzzjs": "^0.3.5",
     "lodash": "^4.17.21",
     "p-limit": "^3.1.0"
   },

test/index.js

@@ -407,9 +407,108 @@ 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);
       });
+
+      describe('when leaving out a min value', function () {
+        it('should error', async function () {
+          await expect(
+            () =>
+              subsetFont(this.variableRobotoFont, 'abcd', {
+                variationAxes: {
+                  wght: { max: 300 },
+                },
+              }),
+            'to error',
+            'wght: You must provide both a min and a max value when setting the axis range'
+          );
+        });
+      });
+
+      describe('when leaving out a max value', function () {
+        it('should error', async function () {
+          await expect(
+            () =>
+              subsetFont(this.variableRobotoFont, 'abcd', {
+                variationAxes: {
+                  wght: { min: 300 },
+                },
+              }),
+            'to error',
+            'wght: You must provide both a min and a max value when setting the axis range'
+          );
+        });
+      });
+
+      describe('when pinning a non-existent axis', function () {
+        it('should error', async function () {
+          await expect(
+            () =>
+              subsetFont(this.variableRobotoFont, 'abcd', {
+                variationAxes: {
+                  foob: 123,
+                },
+              }),
+            'to error',
+            'hb_subset_input_pin_axis_location (harfbuzz) returned zero when pinning foob to 123, indicating failure. Maybe the axis does not exist in the font?'
+          );
+        });
+      });
+
+      describe('when reducing the variation space of a non-existent axis', function () {
+        it('should error', async function () {
+          await expect(
+            () =>
+              subsetFont(this.variableRobotoFont, 'abcd', {
+                variationAxes: {
+                  foob: {
+                    min: 123,
+                    max: 456,
+                  },
+                },
+              }),
+            'to error',
+            'hb_subset_input_set_axis_range (harfbuzz) returned zero when setting the range of foob to [123; 456] and a default value of undefined, indicating failure. Maybe the axis does not exist in the font?'
+          );
+        });
+      });
     });
   });
 });