Add Text.Pandoc.Image with unexported svgToPng.

pandoc.cabal

@@ -632,6 +632,7 @@ library
                    Text.Pandoc.UUID,
                    Text.Pandoc.Translations,
                    Text.Pandoc.Slides,
+                   Text.Pandoc.Image,
                    Paths_pandoc
   autogen-modules: Paths_pandoc
   buildable:       True

src/Text/Pandoc/Image.hs

@@ -0,0 +1,40 @@
+{-# LANGUAGE NoImplicitPrelude #-}
+{-# LANGUAGE OverloadedStrings, ScopedTypeVariables, CPP #-}
+{-# LANGUAGE ViewPatterns      #-}
+{- |
+Module      : Text.Pandoc.Image
+Copyright   : Copyright (C) 2020 John MacFarlane
+License     : GNU GPL, version 2 or above
+
+Maintainer  : John MacFarlane <jgm@berkeley.edu>
+Stability   : alpha
+Portability : portable
+
+Functions for converting images.
+-}
+module Text.Pandoc.Image ( svgToPng ) where
+import Prelude
+import Text.Pandoc.Options (WriterOptions(..))
+import Text.Pandoc.Process (pipeProcess)
+import qualified Data.ByteString.Lazy as L
+import System.Exit
+import Data.Text (Text)
+import Text.Pandoc.Shared (tshow)
+import qualified Control.Exception as E
+
+-- | Convert svg image to png. rsvg-convert
+-- is used and must be available on the path.
+svgToPng :: WriterOptions
+         -> L.ByteString  -- ^ Input image as bytestring
+         -> IO (Either Text L.ByteString)
+svgToPng opts bs = do
+  let dpi = show $ writerDpi opts
+  E.catch
+       (do (exit, out) <- pipeProcess Nothing "rsvg-convert"
+                          ["-f","png","-a","--dpi-x",dpi,"--dpi-y",dpi]
+                          bs
+           if exit == ExitSuccess
+              then return $ Right out
+              else return $ Left "conversion from SVG failed")
+       (\(e :: E.SomeException) -> return $ Left $
+           "check that rsvg-convert is in path.\n" <> tshow e)