SkPDF/Documentation: backend limitations

NOTRY=true

Review-Url: https://codereview.chromium.org/2377473002

1 files changed, 58 insertions, 2 deletions

diff --git a/site/user/sample/pdf.md b/site/user/sample/pdf.md
index 1668f3939a..d54172bc52 100644
--- a/site/user/sample/pdf.md
+++ b/site/user/sample/pdf.md
@@ -1,8 +1,8 @@
 Using Skia's PDF Backend
 ========================
 
-Here is an example of using Skia's PDF backend in the recommended way:
-via the SkDocument and SkCanvas APIs.
+Here is an example of using Skia's PDF backend (SkPDF) via the
+SkDocument and SkCanvas APIs.
 
 <!--?prettify lang=cc?-->
 
@@ -39,3 +39,59 @@ via the SkDocument and SkCanvas APIs.
         }
         return pdfDocument->close();
     }
+
+* * *
+
+<span id="limits">SkPDF Limitations</span>
+------------------------------------------
+
+There are several corners of Skia's public API that SkPDF currently
+does not handle because either no known client uses the feature or
+there is no simple PDF-ish way to handle it.
+
+In this document:
+
+  + **drop** means to draw nothing.
+
+  + **ignore** mean to draw without the effect
+
+  + **expand** means to implement something in a non-PDF-ish way.
+    This may mean to rasterize vector graphics, to expand paths with
+    path effects into many individual paths, or to convert text to
+    paths.
+
+<style scoped><!--
+#pdftable {border-collapse:collapse;}
+#pdftable tr th, #pdftable tr td {border:#888888 2px solid;padding: 5px;}
+--></style>
+<table id="pdftable">
+<tr><th>Effect</th>                  <th>text</th>   <th>images</th> <th>everything
+                                                                         else</th></tr>
+<tr><th>SkMaskFilter</th>            <td>drop</td>   <td>ignore</td> <td>ignore</td></tr>
+<tr><th>SkPathEffect</th>            <td>ignore</td> <td>n/a</td>    <td>expand</td></tr>
+<tr><th>SkColorFilter</th>           <td>ignore</td> <td>expand</td> <td>ignore</td></tr>
+<tr><th>SkImageFilter</th>           <td>expand</td> <td>expand</td> <td>expand</td></tr>
+<tr><th>unsupported SkXferModes</th> <td>ignore</td> <td>ignore</td> <td>ignore</td></tr>
+<tr><th>non-gradient SkShader</th>   <td>expand</td> <td>n/a</td>    <td>expand</td></tr>
+</table>
+
+Notes:
+
+  - *SkImageFilter*: When SkImageFilter is expanded, text-as-text is lost.
+
+  - *SkXferMode*: The following transfer modes are not natively
+    supported by PDF: DstOver, SrcIn, DstIn, SrcOut, DstOut, SrcATop,
+    DstATop, and Modulate.
+
+Other limitations:
+
+  - *drawText with VerticalText* — drop. No known clients seem to make use
+    of the VerticalText flag.
+
+  - *drawTextOnPath* — expand. (Text-as-text is lost.)
+
+  - *drawVertices* — drop.
+
+  - *drawPatch* — drop.
+
+* * *