Improve compression of PNG images embedded in generated SVG files

[Rob--W]

Fixes issue #8560 for Node.js.

The first four commits are refactorings, the last one contains the actual fix (read its commit message for the motivation for the fix, test steps and suggestions for future improvements).

[yurydelendik]

/botio test

[pdfjsbot]

From: Bot.io (Linux m4)

---

Received

Command cmd_test from @yurydelendik received. Current queue size: 1

Live output at: http://54.67.70.0:8877/e1723c2f4aabe8d/output.txt

[pdfjsbot]

From: Bot.io (Windows)

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Received

Command cmd_test from @yurydelendik received. Current queue size: 1

Live output at: http://54.215.176.217:8877/6904598f8d26383/output.txt

[yurydelendik]

Thank you for the patch.

[pdfjsbot]

From: Bot.io (Linux m4)

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Success

Full output at http://54.67.70.0:8877/e1723c2f4aabe8d/output.txt

Total script time: 16.64 mins

• Font tests: Passed
• Unit tests: Passed
• Regression tests: Passed

[pdfjsbot]

From: Bot.io (Windows)

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Failed

Full output at http://54.215.176.217:8877/6904598f8d26383/output.txt

Total script time: 28.91 mins

• Font tests: Passed
• Unit tests: Passed
• Regression tests: FAILED

Image differences available at: http://54.215.176.217:8877/6904598f8d26383/reftest-analyzer.html#web=eq.log

[Rob--W]

FYI, I rebased the branch to account for the change from #8630.
@yurydelendik You've approved the changes. Is it good to merge?

[yurydelendik]

You've approved the changes. Is it good to merge?

Yes. (it was kind of r+)

[timvandermeij]

Awesome work!

[Snuffleupagus]

When running the unit-tests locally (in Firefox), either with gulp unittest or directly in the browser with http://localhost:8888/test/unit/unit_test.html, one of the newly added unit-tests isn't working as expected with SPEC HAS NO EXPECTATIONS should produce a svg:image even if zlib is unavailable displayed in the results.

My first guess was that it's simply a problem with the unit-tests being asynchronous, but missing the required done calls to make sure that we wait for each unit-test to complete properly; i.e. like so:

diff --git a/test/unit/display_svg_spec.js b/test/unit/display_svg_spec.js
index 515e59fe..ccd89f79 100644
--- a/test/unit/display_svg_spec.js
+++ b/test/unit/display_svg_spec.js
@@ -101,7 +101,7 @@ describe('SVGGraphics', function () {
       });
     }
 
-    it('should produce a reasonably small svg:image', function() {
+    it('should produce a reasonably small svg:image', function(done) {
       if (!isNodeJS()) {
         pending('zlib.deflateSync is not supported in non-Node environments.');
       }
@@ -117,10 +117,12 @@ describe('SVGGraphics', function () {
         // Without zlib (uncompressed), the size of the data URL was excessive
         // (80247).
         expect(imgUrl.length).toBeLessThan(367);
-      });
+        done();
+      }).catch(done.fail);
     });
 
-    it('should produce a svg:image even if zlib is unavailable', function() {
+    it('should produce a svg:image even if zlib is unavailable',
+        function(done) {
       withZlib(false, getSVGImage).then(function(svgImg) {
         expect(svgImg.nodeName).toBe('svg:image');
         expect(svgImg.getAttribute('width')).toBe('200px');
@@ -129,7 +131,8 @@ describe('SVGGraphics', function () {
         expect(imgUrl).toMatch(/^data:image\/png;base64,/);
         // The size of our naively generated PNG file is excessive :(
         expect(imgUrl.length).toBe(80247);
-      });
+        done();
+      }).catch(done.fail);
     });
   });
 });

Having made the above changes locally, which are necessary to ensure that the tests work correctly, the same test now fail consistently with Failed: Requesting object that isn't resolved yet [object Object].

@Rob--W I don't really have time to delve deeper into this right now, so I'd appreciate if you could take a look!