Add link tags for pdfs

Co-Authored-By: Caleb Hearon <caleb@chearon.net>

CHANGELOG.md

@@ -12,6 +12,8 @@ project adheres to [Semantic Versioning](http://semver.org/).
 * `ctx.font` has a new C++ parser and is 2x-400x faster. Please file an issue if you experience different results, as caching has been removed.
 
 ### Added
+* Support for accessibility and links in PDFs
+
 ### Fixed
 
 3.0.1

Readme.md

@@ -515,6 +515,26 @@ ctx.addPage(400, 800)
 ctx.fillText('Hello World 2', 50, 80)
 ```
 
+It is possible to add hyperlinks using `.beginTag()` and `.endTag()`:
+
+```js
+ctx.beginTag('Link', "uri='https://google.com'")
+ctx.font = '22px Helvetica'
+ctx.fillText('Hello World', 50, 80)
+ctx.endTag('Link')
+```
+
+Or with a defined rectangle:
+
+```js
+ctx.beginTag('Link', "uri='https://google.com' rect=[50 80 100 20]")
+ctx.endTag('Link')
+```
+
+Note that the syntax is unique to Cairo. See [cairo_tag_begin](https://www.cairographics.org/manual/cairo-Tags-and-Links.html#cairo-tag-begin) for the full documentation.
+
+You can create areas on the canvas using the "cairo.dest" tag, and then link to them using the "Link" tag with the `dest=` attribute. You can also define PDF structure for accessibility by using tag names like "P", "H1", and "TABLE". The standard tags are defined in §14.8.4 of the [PDF 1.7](https://opensource.adobe.com/dc-acrobat-sdk-docs/pdfstandards/PDF32000_2008.pdf) specification.
+
 See also:
 
 * [Image#dataMode](#imagedatamode) for embedding JPEGs in PDFs

examples/pdf-link.js

@@ -0,0 +1,20 @@
+const fs = require('fs')
+const path = require('path')
+const Canvas = require('..')
+
+const canvas = Canvas.createCanvas(400, 300, 'pdf')
+const ctx = canvas.getContext('2d')
+
+ctx.beginTag('Link', 'uri=\'https://google.com\'')
+ctx.font = '22px Helvetica'
+ctx.fillText('Text link to Google', 110, 50)
+ctx.endTag('Link')
+
+ctx.fillText('Rect link to node-canvas below!', 40, 180)
+
+ctx.beginTag('Link', 'uri=\'https://github.com/Automattic/node-canvas\' rect=[0 200 400 100]')
+ctx.endTag('Link')
+
+fs.writeFile(path.join(__dirname, 'pdf-link.pdf'), canvas.toBuffer(), function (err) {
+  if (err) throw err
+})

index.d.ts

@@ -232,6 +232,8 @@ export class CanvasRenderingContext2D {
 	createPattern(image: Canvas|Image, repetition: 'repeat' | 'repeat-x' | 'repeat-y' | 'no-repeat' | '' | null): CanvasPattern
 	createLinearGradient(x0: number, y0: number, x1: number, y1: number): CanvasGradient;
 	createRadialGradient(x0: number, y0: number, r0: number, x1: number, y1: number, r1: number): CanvasGradient;
+	beginTag(tagName: string, attributes?: string): void;
+	endTag(tagName: string): void;
 	/**
 	 * _Non-standard_. Defaults to 'good'. Affects pattern (gradient, image,
 	 * etc.) rendering quality.

src/CanvasRenderingContext2d.cc

@@ -135,6 +135,10 @@ Context2d::Initialize(Napi::Env& env, Napi::Object& exports) {
     InstanceMethod<&Context2d::CreatePattern>("createPattern", napi_default_method),
     InstanceMethod<&Context2d::CreateLinearGradient>("createLinearGradient", napi_default_method),
     InstanceMethod<&Context2d::CreateRadialGradient>("createRadialGradient", napi_default_method),
+    #if CAIRO_VERSION >= CAIRO_VERSION_ENCODE(1, 16, 0)
+    InstanceMethod<&Context2d::BeginTag>("beginTag", napi_default_method),
+    InstanceMethod<&Context2d::EndTag>("endTag", napi_default_method),
+    #endif
     InstanceAccessor<&Context2d::GetFormat>("pixelFormat", napi_default_jsproperty),
     InstanceAccessor<&Context2d::GetPatternQuality, &Context2d::SetPatternQuality>("patternQuality", napi_default_jsproperty),
     InstanceAccessor<&Context2d::GetImageSmoothingEnabled, &Context2d::SetImageSmoothingEnabled>("imageSmoothingEnabled", napi_default_jsproperty),
@@ -419,7 +423,7 @@ Context2d::fill(bool preserve) {
         width = cairo_image_surface_get_width(patternSurface);
         height = y2 - y1;
       }
-      
+
       cairo_new_path(_context);
       cairo_rectangle(_context, 0, 0, width, height);
       cairo_clip(_context);
@@ -3348,3 +3352,55 @@ Context2d::Ellipse(const Napi::CallbackInfo& info) {
   }
   cairo_set_matrix(ctx, &save_matrix);
 }
+
+#if CAIRO_VERSION >= CAIRO_VERSION_ENCODE(1, 16, 0)
+
+void
+Context2d::BeginTag(const Napi::CallbackInfo& info) {
+  std::string tagName = "";
+  std::string attributes = "";
+
+  if (info.Length() == 0) {
+    Napi::TypeError::New(env, "Tag name is required").ThrowAsJavaScriptException();
+    return;
+  } else {
+    if (!info[0].IsString()) {
+      Napi::TypeError::New(env, "Tag name must be a string.").ThrowAsJavaScriptException();
+      return;
+    } else {
+      tagName = info[0].As<Napi::String>().Utf8Value();
+    }
+
+    if (info.Length() > 1) {
+      if (!info[1].IsString()) {
+        Napi::TypeError::New(env, "Attributes must be a string matching Cairo's attribute format").ThrowAsJavaScriptException();
+        return;
+      } else {
+        attributes = info[1].As<Napi::String>().Utf8Value();
+      }
+    }
+  }
+
+  cairo_t *ctx = context();
+  cairo_tag_begin(ctx, tagName.c_str(), attributes.c_str());
+}
+
+void
+Context2d::EndTag(const Napi::CallbackInfo& info) {
+  if (info.Length() == 0) {
+    Napi::TypeError::New(env, "Tag name is required").ThrowAsJavaScriptException();
+    return;
+  }
+
+  if (!info[0].IsString()) {
+    Napi::TypeError::New(env, "Tag name must be a string.").ThrowAsJavaScriptException();
+    return;
+  }
+
+  std::string tagName = info[0].As<Napi::String>().Utf8Value();
+
+  cairo_t *ctx = context();
+  cairo_tag_end(ctx, tagName.c_str());
+}
+
+#endif

src/CanvasRenderingContext2d.h

@@ -178,6 +178,10 @@ class Context2d : public Napi::ObjectWrap<Context2d> {
     void SetFont(const Napi::CallbackInfo& info, const Napi::Value& value);
     void SetTextBaseline(const Napi::CallbackInfo& info, const Napi::Value& value);
     void SetTextAlign(const Napi::CallbackInfo& info, const Napi::Value& value);
+    #if CAIRO_VERSION >= CAIRO_VERSION_ENCODE(1, 16, 0)
+    void BeginTag(const Napi::CallbackInfo& info);
+    void EndTag(const Napi::CallbackInfo& info);
+    #endif
     inline void setContext(cairo_t *ctx) { _context = ctx; }
     inline cairo_t *context(){ return _context; }
     inline Canvas *canvas(){ return _canvas; }

test/canvas.test.js

@@ -16,7 +16,8 @@ const {
   loadImage,
   registerFont,
   Canvas,
-  deregisterAllFonts
+  deregisterAllFonts,
+  cairoVersion
 } = require('../')
 
 function assertApprox(actual, expected, tol) {
@@ -755,6 +756,11 @@ describe('Canvas', function () {
         assertPixel(0xffff0000, 5, 0, 'first red pixel')
       })
     })
+
+    it('Canvas#toBuffer("application/pdf")', function () {
+      const buf = createCanvas(200, 200, 'pdf').toBuffer('application/pdf')
+      assert.equal('PDF', buf.slice(1, 4).toString())
+    })
   })
 
   describe('#toDataURL()', function () {
@@ -2000,4 +2006,36 @@ describe('Canvas', function () {
       })
     }
   })
+
+  describe('Context2d#beingTag()/endTag()', function () {
+    before(function () {
+      const canvas = createCanvas(20, 20, 'pdf')
+      const ctx = canvas.getContext('2d')
+      if (!('beginTag' in ctx)) {
+        this.skip()
+      }
+    })
+
+    it('generates a pdf', function () {
+      const canvas = createCanvas(20, 20, 'pdf')
+      const ctx = canvas.getContext('2d')
+      ctx.beginTag('Link', "uri='http://example.com'")
+      ctx.strokeText('hello', 0, 0)
+      ctx.endTag('Link')
+      const buf = canvas.toBuffer('application/pdf')
+      assert.equal('PDF', buf.slice(1, 4).toString())
+    })
+
+    it('requires tag argument', function () {
+      const canvas = createCanvas(20, 20, 'pdf')
+      const ctx = canvas.getContext('2d')
+      assert.throws(() => { ctx.beginTag() })
+    })
+
+    it('requires attributes to be a string', function () {
+      const canvas = createCanvas(20, 20, 'pdf')
+      const ctx = canvas.getContext('2d')
+      assert.throws(() => { ctx.beginTag('Link', {}) })
+    })
+  })
 })