Add testing section of docs

site/dev/testing/fonts.md

+Fonts and GM Tests
+==================
+
+Overview
+--------
+
+Each test in the gm directory draws a reference image. Their primary purpose is
+to detect when images change unexpectedly, indicating that a rendering bug has
+been introduced.
+
+The gm tests have a secondary purpose: they detect when rendering is different
+across platforms and configurations.
+
+The dm \(Diamond Master\) tool supports flags that minimize or eliminate the
+differences introduced by the font scaler native to each platform.
+
+
+Portable fonts
+--------------
+
+The most portable font format uses Skia to draw characters directly from paths,
+and contains a idealized set of font metrics. This does not exercise platform
+specific fonts at all, but does support specifying the font name, font size,
+font style, and attributes like fakeBold. The paths are generated on a reference
+platform \(currently a Mac\) and are stored as data in
+'tools/test_font_data.cpp' .
+
+To use portable fonts, pass '\-\-portableFonts' to dm.
+
+
+Resource fonts
+--------------
+
+The '\-\-resourceFonts' flag directs dm to use font files present in the resources
+directory. By using the same font set on all buildbots, the generated gm images
+become more uniform across platforms.
+
+Today, the set of fonts used by gm, and present in my resources directory,
+include:
+
+  * Courier New Bold Italic.ttf
+  * Courier New Bold.ttf
+  * Courier New Italic.ttf
+  * Courier New.ttf
+  * LiberationSans-Bold.ttf
+  * LiberationSans-BoldItalic.ttf
+  * LiberationSans-Italic.ttf
+  * LiberationSans-Regular.ttf
+  * Papyrus.ttc
+  * Pro W4.otf
+  * Times New Roman Bold Italic.ttf
+  * Times New Roman Bold.ttf
+  * Times New Roman Italic.ttf
+  * Times New Roman.ttf 
+
+
+System fonts
+------------
+
+If neither '\-\-portableFonts' nor '\-\-resourceFonts' is specified, dm uses the fonts
+present on the system. Also, if '\-\-portableFonts' or '\-\-resourceFonts' is specified
+and the desired font is not available, the native font lookup algorithm is
+invoked.
+
+
+GM font selection
+-----------------
+
+Each gm specifies the typeface to use when drawing text. For now, to set the
+portable typeface on the paint, call:
+
+~~~~
+sk_tool_utils::set_portable_typeface(SkPaint* , const char* name = NULL,
+SkTypeface::Style style = SkTypeface::kNormal );
+~~~~
+
+To create a portable typeface, use:
+
+~~~~
+SkTypeface* typeface = sk_tool_utils::create_portable_typeface(const char* name,
+SkTypeface::Style style);
+~~~~
+
+Eventually, both 'set_portable_typeface()' and 'create_portable_typeface()' will be
+removed. Instead, a test-wide 'SkFontMgr' will be selected to choose portable
+fonts or resource fonts.
+
+
+Adding new fonts and glyphs to a GM
+-----------------------------------
+
+If a font is missing from the portable data or the resource directory, the
+system font is used instead. If a glyph is missing from the portable data, the
+first character, usually a space, is drawn instead.
+
+Running dm with '\-\-portableFonts' and '\-\-reportUsedChars' generates
+'tools/test_font_data_chars.cpp', which describes the fonts and characters used by
+all gm tests. Subsequently running the 'create_test_font' tool generates new paths
+and writes them into 'tools/test_font_data.cpp' .
+
+
+Future work
+-----------
+
+The font set used by gm tests today is arbitrary and not intended to be
+cross-platform. By choosing fonts without licensing issues, all bots can freely
+contain the same fonts. By narrowing the font selection, the size of the test
+font data will be more manageable.
+
+Adding support for selecting from multiple font managers at runtime permits
+removing manual typeface selection in the gm tests. Today, options to dm like
+'\-\-pipe' fail with '\-\-portableFonts' because we're hard-coded to using the default
+font manage when pictures are serialized.
+
+Some gm tests explicitly always want to use system fonts and system metrics;
+other gm tests use text only to label the drawing; yet other gm tests use text
+to generate paths for testing. Additional discrimination is needed to
+distinguish these cases.