[Compare] added vf vs v2.138 comparisons

Docs/Compare/vf_vs_2_138/README.md

@@ -0,0 +1,59 @@
+# [v2.138 unhinted offical](https://github.com/google/roboto/releases/tag/v2.138) vs VF
+
+Regression tests to determine whether the variable font match the previous unhinted v2.138 release.
+
+
+## Building ttfs
+
+master_ttf/ built with [fontmake v1.6.0](https://github.com/googlei18n/fontmake/releases/tag/v1.6.0) using the following cli:
+
+```
+sh build-min.sh
+```
+
+## Viewing diffs
+
+Diffs produced by using Python's HTTPSimple server on this repo and viewing vf_vs_v2_138.html in Browserstack.
+
+The html file contains every glyph in the font on a single line. Our goal is to ensure that both fonts overlay perfectly. The diff images show the very end of the line.
+
+Three further html test files are provided:
+
+1. master_ttf_vs_v2_138.html
+2. master_ttf_interpolatable_vs_2_138.html
+3. vf_vs_2_138_real_world.html
+
+
+### Starting Python's simple server to view test pages in browserstack
+
+```
+
+cd ./to/this/repo
+python -m SimpleHTTPServer
+# Enabling browserstack local testing https://www.browserstack.com/local-testing
+# view local site in browserstack
+```
+
+## Summary for commit e7ece644e5714951089cd6916db07fc055c9383d
+- Each OS and Browser produced a different result
+
+
+## Diff Notes
+
+Following summary has been made by M Foley by reviewing each diff image
+
+- Masters in the VF produced a closer match (images with 200 in filename)
+- Overall Chrome produced the closest match
+- Chrome's results across each platform seem almost identical
+- Glyphs which are assembled from components are affected the most
+
+It's interesting to note that each platform will produce different results. This means it's not just the fonts which determine the end result. Rasterizers and shaping engines such as CoreText play a critical role as well. Due to this, it's probably impossible to get a 100% match.
+
+The metrics may also differ between platforms because some browsers may not use the [hvar](https://docs.microsoft.com/en-us/typography/opentype/spec/hvar) table and others will (not tested, just a thought).
+
+The master_ttf_interpolatable fonts which were used to construct the VF match perfectly in Chrome. This is the same for the master_ttf fonts.
+
+This test is extreme since we're testing all glyphs without a single line break. If we compare typical samples of text, The results are much better.
+
+![Real World Test](diff_images/osx_safari11_real_world.png)
+