Text.Pandoc.ImageSize: Parse EXIF format JPGs.

Note:  For now we just assign them all 72 dpi.  It wasn't
clear to me how to extract the resolution information.
At least the aspect ratio will be right, and 72 dpi is
the most common setting.

Closes #976.

src/Text/Pandoc/ImageSize.hs

@@ -53,7 +53,8 @@ imageType :: ByteString -> Maybe ImageType
 imageType img = case B.take 4 img of
                      "\x89\x50\x4e\x47" -> return Png
                      "\x47\x49\x46\x38" -> return Gif
-                     "\xff\xd8\xff\xe0" -> return Jpeg
+                     "\xff\xd8\xff\xe0" -> return Jpeg  -- JFIF
+                     "\xff\xd8\xff\xe1" -> return Jpeg  -- Exif
                      "%PDF"             -> return Pdf
                      "%!PS"
                        | (B.take 4 $ B.drop 1 $ B.dropWhile (/=' ') img) == "EPSF"
@@ -139,8 +140,14 @@ gifSize img = do
 jpegSize :: ByteString -> Maybe ImageSize
 jpegSize img = do
   let (hdr, rest) = B.splitAt 4 img
-  guard $ hdr == "\xff\xd8\xff\xe0"
   guard $ B.length rest >= 14
+  case hdr of
+       "\xff\xd8\xff\xe0" -> jfifSize rest
+       "\xff\xd8\xff\xe1" -> exifSize rest
+       _                  -> mzero
+
+jfifSize :: ByteString -> Maybe ImageSize
+jfifSize rest = do
   let [dpiDensity,dpix1,dpix2,dpiy1,dpiy2] = map fromIntegral
                                            $ unpack $ B.take 5 $ B.drop 9 $ rest
   let factor = case dpiDensity of
@@ -149,11 +156,11 @@ jpegSize img = do
                     _ -> const 72
   let dpix = factor (shift dpix1 8 + dpix2)
   let dpiy = factor (shift dpiy1 8 + dpiy2)
-  (w,h) <- findJpegSize rest
+  (w,h) <- findJfifSize rest
   return $ ImageSize { pxX = w, pxY = h, dpiX = dpix, dpiY = dpiy }
 
-findJpegSize :: ByteString -> Maybe (Integer,Integer)
-findJpegSize bs = do
+findJfifSize :: ByteString -> Maybe (Integer,Integer)
+findJfifSize bs = do
   let bs' = B.dropWhile (=='\xff') $ B.dropWhile (/='\xff') bs
   case B.uncons bs' of
        Just (c,bs'') | c >= '\xc0' && c <= '\xc3' -> do
@@ -165,8 +172,23 @@ findJpegSize bs = do
               [c1,c2] -> do
                 let len = shift c1 8 + c2
                 -- skip variables
-                findJpegSize $ B.drop len bs''
+                findJfifSize $ B.drop len bs''
               _       -> fail "JPEG parse error"
        Nothing -> fail "Did not find length record"
 
+exifSize :: ByteString -> Maybe ImageSize
+exifSize rest = do
+  let bs' = B.takeWhile (/='\xff') $ B.drop 8 rest  -- exif data
+  let (_,bs'') = B.breakSubstring "\xa0\x02" bs' -- width
+  let rawWidth = B.take 2 $ B.drop 10 bs''
+  let (_,bs''') = B.breakSubstring "\xa0\x03" bs' -- height
+  let rawHeight = B.take 2 $ B.drop 10 bs'''
+  let tonum bs =  case map fromIntegral $ unpack bs of
+                       [x,y]  -> Just $ shift x 8 + y
+                       _      -> Nothing
+  case (tonum rawWidth, tonum rawHeight) of
+       (Just w, Just h) ->
+                 return $ ImageSize { pxX = w, pxY = h, dpiX = 72, dpiY = 72 }
+       _      -> fail "Could not determine exif image size"
+  -- some day, figure out how to parse dpi from exif