Text.Pandoc.Options: modifications for image attributes.

* Added `Ext_common_link_attributes` constructor to `Extension`
  (for link and image attributes).
* Added this to `pandocExtensions` and `phpMarkdownExtraExtensions`.
* Added `writerDpi` to `WriterOptions`.
* pandoc.hs:  Added `--dpi` option.
* Updated README for `--dpi` and `common_link_attributes` extension.

Patch due to mb21, with some modifications: `writerDpi` is now an
`Int` rather than a `Double`.

3 files changed, 88 insertions, 7 deletions

diff --git a/README b/README
index 3ecfaa813..3bb211341 100644
--- a/README
+++ b/README
@@ -420,14 +420,22 @@ General writer options
 :   Print a system default data file.  Files in the user data directory
     are ignored.
 
+`--dpi`=*NUMBER*
+:   Specify the dpi (dots per inch) value for conversion from pixels
+    to inch/centimeters and vice versa. The default is 96dpi.
+    Technically, the correct term would be ppi (pixels per inch).
+
 `--no-wrap`
 
 :   Disable text wrapping in output. By default, text is wrapped
-    appropriately for the output format.
+    appropriately for the output format. This affects only the
+    generated source code, not the layout on the rendered page.
 
 `--columns=`*NUMBER*
 
 :   Specify length of lines in characters (for text wrapping).
+    This affects only the generated source code, not the layout on
+    the rendered page.
 
 `--toc`, `--table-of-contents`
 
@@ -2634,6 +2642,53 @@ nonbreaking space after the image:
 
     ![This image won't be a figure](/url/of/image.png)\
 
+#### Extension: `common_link_attributes` ####
+
+Attributes can be set on images:
+
+    An inline ![image](foo.jpg){#id .class width=30 height=20px}
+    and a reference ![image][ref] with attributes.
+
+    [ref]: foo.jpg "optional title" {#id .class key=val key2="val 2"}
+
+(This syntax is compatible with [PHP Markdown Extra] when only `#id`
+and `.class` are used.)
+
+For HTML and EPUB, all attributes except `width` and `height` (but
+including `srcset` and `sizes`) are passed through as is. The other
+writers ignore attributes that are not supported by their output
+format.
+
+The `width` and `height` attributes on images are treated specially. When
+used without a unit, the unit is assumed to be pixels. However, any of
+the following unit identifiers can be used: `px`, `cm`, `mm`, `in`, `inch`
+and `%`. There must not be any spaces between the number and the unit.
+For example:
+
+```
+![](file.jpg){width=50%}
+```
+
+- Dimensions are converted to inches for output in page-based formats like
+  LaTeX. Dimensions are converted to pixels for output in HTML-like
+  formats.  Use the `--dpi` option to specify the number of pixels per
+  inch.  The default is 96dpi.
+- The `%` unit is generally relative to some available space.
+  For example the above example will render to
+  `<img href="file.jpg" style="width: 50%;" />` (HTML),
+  `\includegraphics[width=0.5\textwidth]{file.jpg}` (LaTeX), or
+  `\externalfigure[file.jpg][width=0.5\textwidth]` (ConTeXt).
+- Some output formats have a notion of a class
+  ([ConTeXt](http://wiki.contextgarden.net/Using_Graphics#Multiple_Image_Settings))
+  or a unique identifier (LaTeX `\caption`), or both (HTML).
+- When no `width` or `height` attributes are specified, the fallback
+  is to look at the image resolution and the dpi metadata embedded in
+  the image file.
+
+Note that while attributes are also parsed on links, pandoc's internal
+document model provides nowhere to put them, so they are presently
+just ignored.
+
 
 Footnotes
 ---------
@@ -2908,9 +2963,15 @@ letters are omitted.
 
 #### Extension: `link_attributes` ####
 
-Parses multimarkdown style key-value attributes on link and image references.
-Note that pandoc's internal document model provides nowhere to put
-these, so they are presently just ignored.
+Parses multimarkdown style key-value attributes on link
+and image references. (Since pandoc's internal document model
+provides nowhere to put these for links, they are presently just
+ignored, but they work for images.)
+
+    This is a reference ![image][ref] with multimarkdown attributes.
+
+    [ref]: http://path.to/image "Image title" width=20px height=30px
+           id=myId class="myClass1 myClass2"
 
 #### Extension: `mmd_header_identifiers` ####
 
@@ -2953,7 +3014,8 @@ variants are supported:
 `markdown_phpextra` (PHP Markdown Extra)
 :   `footnotes`, `pipe_tables`, `raw_html`, `markdown_attribute`,
     `fenced_code_blocks`, `definition_lists`, `intraword_underscores`,
-    `header_attributes`, `abbreviations`, `shortcut_reference_links`.
+    `header_attributes`, `common_link_attributes`, `abbreviations`,
+    `shortcut_reference_links`.
 
 `markdown_github` (GitHub-flavored Markdown)
 :   `pipe_tables`, `raw_html`, `tex_math_single_backslash`,
diff --git a/pandoc.hs b/pandoc.hs
index fb9b9abbf..e6488e99e 100644
--- a/pandoc.hs
+++ b/pandoc.hs
@@ -196,6 +196,7 @@ data Opt = Opt
     , optIgnoreArgs        :: Bool    -- ^ Ignore command-line arguments
     , optVerbose           :: Bool    -- ^ Verbose diagnostic output
     , optReferenceLinks    :: Bool    -- ^ Use reference links in writing markdown, rst
+    , optDpi               :: Int     -- ^ Dpi
     , optWrapText          :: Bool    -- ^ Wrap text
     , optColumns           :: Int     -- ^ Line length in characters
     , optFilters           :: [FilePath] -- ^ Filters to apply
@@ -258,6 +259,7 @@ defaultOpts = Opt
     , optIgnoreArgs            = False
     , optVerbose               = False
     , optReferenceLinks        = False
+    , optDpi                   = 96
     , optWrapText              = True
     , optColumns               = 72
     , optFilters               = []
@@ -454,6 +456,16 @@ options =
                   "FILE")
                   "" -- "Print default data file"
 
+    , Option "" ["dpi"]
+                 (ReqArg
+                  (\arg opt ->
+                    case safeRead arg of
+                         Just t | t > 0 -> return opt { optDpi = t }
+                         _              -> err 31
+                                        "dpi must be a number greater than 0")
+                  "NUMBER")
+                 "" -- "Dpi (default 96)"
+
     , Option "" ["no-wrap"]
                  (NoArg
                   (\opt -> return opt { optWrapText = False }))
@@ -1012,8 +1024,8 @@ extractMedia media dir d =
           return $ walk (adjustImagePath dir fps) d
 
 adjustImagePath :: FilePath -> [FilePath] -> Inline -> Inline
-adjustImagePath dir paths (Image lab (src, tit))
-   | src `elem` paths = Image lab (dir ++ "/" ++ src, tit)
+adjustImagePath dir paths (Image attr lab (src, tit))
+   | src `elem` paths = Image attr lab (dir ++ "/" ++ src, tit)
 adjustImagePath _ _ x = x
 
 adjustMetadata :: M.Map String MetaValue -> Pandoc -> IO Pandoc
@@ -1087,6 +1099,7 @@ main = do
               , optIgnoreArgs            = ignoreArgs
               , optVerbose               = verbose
               , optReferenceLinks        = referenceLinks
+              , optDpi                   = dpi
               , optWrapText              = wrap
               , optColumns               = columns
               , optFilters               = filters
@@ -1304,6 +1317,7 @@ main = do
                             writerNumberOffset     = numberFrom,
                             writerSectionDivs      = sectionDivs,
                             writerReferenceLinks   = referenceLinks,
+                            writerDpi              = dpi,
                             writerWrapText         = wrap,
                             writerColumns          = columns,
                             writerEmailObfuscation = obfuscationMethod,
diff --git a/src/Text/Pandoc/Options.hs b/src/Text/Pandoc/Options.hs
index 17eb4a15c..060fa6c05 100644
--- a/src/Text/Pandoc/Options.hs
+++ b/src/Text/Pandoc/Options.hs
@@ -86,6 +86,7 @@ data Extension =
     | Ext_markdown_attribute      -- ^ Interpret text inside HTML as markdown
                                   --   iff container has attribute 'markdown'
     | Ext_escaped_line_breaks     -- ^ Treat a backslash at EOL as linebreak
+    | Ext_common_link_attributes  -- ^ link and image attributes
     | Ext_link_attributes     -- ^ MMD style reference link attributes
     | Ext_autolink_bare_uris  -- ^ Make all absolute URIs into links
     | Ext_fancy_lists         -- ^ Enable fancy list numbers and delimiters
@@ -154,6 +155,7 @@ pandocExtensions = Set.fromList
   , Ext_subscript
   , Ext_auto_identifiers
   , Ext_header_attributes
+  , Ext_common_link_attributes
   , Ext_implicit_header_references
   , Ext_line_blocks
   , Ext_shortcut_reference_links
@@ -187,6 +189,7 @@ phpMarkdownExtraExtensions = Set.fromList
   , Ext_definition_lists
   , Ext_intraword_underscores
   , Ext_header_attributes
+  , Ext_common_link_attributes
   , Ext_abbreviations
   , Ext_shortcut_reference_links
   ]
@@ -324,6 +327,7 @@ data WriterOptions = WriterOptions
   , writerSectionDivs      :: Bool   -- ^ Put sections in div tags in HTML
   , writerExtensions       :: Set Extension -- ^ Markdown extensions that can be used
   , writerReferenceLinks   :: Bool   -- ^ Use reference links in writing markdown, rst
+  , writerDpi              :: Int    -- ^ Dpi for pixel to/from inch/cm conversions
   , writerWrapText         :: Bool   -- ^ Wrap text to line length
   , writerColumns          :: Int    -- ^ Characters in a line (for text wrapping)
   , writerEmailObfuscation :: ObfuscationMethod -- ^ How to obfuscate emails
@@ -370,6 +374,7 @@ instance Default WriterOptions where
                       , writerSectionDivs      = False
                       , writerExtensions       = pandocExtensions
                       , writerReferenceLinks   = False
+                      , writerDpi              = 96
                       , writerWrapText         = True
                       , writerColumns          = 72
                       , writerEmailObfuscation = JavascriptObfuscation