Tales of LibreOffice interoperability: the missing files

I’m welcoming the release of LibreOffice 4.3.0 in the name of Igalia with the last post in this series. This time we will talk about the preservation of embeddings in OOXML text documents, and devote some lines to the support of Standard Document Tags.

Embedded content in documents

Our goal this time was the preservation of embedded content in OOXML text documents, as a first step towards full support like we did with other features. The insertion of new embeddings in .docx documents or the edition of existing ones will have to come later in the future.

An embedded document usually consists of two files; one of them is a preview picture to be shown in the parent document, and the other one is the actual embedded document. For the case of a spreadsheet embedded in a text document, the most common case, you will find these two files in the document:


With the corresponding entries in the relations file:

<Relationship Id="rId2"
  Target="embeddings/oleObject1.xlsx" />
  <Relationship Id="rId3"
  Target="media/image1.emf" />

The relevant bits in the document.xml file are below. Notice a w:object consists of one shape, which is filled with data from an image file, and the OLE object itself, linked to the embedded spreadsheet. Also notice the attribute ProgID, which defines the program, document type and version:

  <v:shape id="ole_rId2"
  style="width:362.25pt;height:146.25pt" o:ole="">
    <v:imagedata r:id="rId3" o:title="" />
  <o:OLEObject Type="Embed" ProgID="Excel.Sheet.12"
  ShapeID="ole_rId2" DrawAspect="Content"
  ObjectID="_570182397" r:id="rId2" />

There is one more element that allows the embedded file to be properly detected by Word, it’s a content type definition in the content types file:

<Override PartName="/word/embeddings/oleObject1.xlsx"
ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet" />

As you can see there are three elements that determine the kind of embedding we are dealing with, and Word requires the right combination of the three of them:

  • The properties in the tag in document.xml
  • The ContentType for the file defined in [Content_Types].xml
  • The Type of the Relationship defined in document.xml.rels

The most convenient way to achieve our goal was using the grab bag technique to store the ProgID attribute of the object, and infer the correct content type and relation type. Some examples:

  • An object with ProgID Excel.Sheet.12 is a OOXML spreadsheet. Its media type must be application/vnd.openxmlformats-officedocument.spreadsheetml.sheet and the relation type is http://schemas.openxmlformats.org/officeDocument/2006/relationships/package.
  • If the ProgID is Excel.Sheet.8, this is an old Office spreadsheet. Now the media type must be application/vnd.ms-excel and the relation type http://schemas.openxmlformats.org/officeDocument/2006/relationships/oleObject.

If you detect a particular type of embedding in your documents that isn’t being preserved, drop us a line in the Bugzilla. A patch to add new relations of this kind should be quick and easy.

Bonus track: Structured Document Tags

Structured Document Tags (SDTs) is a family of document objects that contains form-like controls, citations, contents tables or bibliography tables among many other. This variety of uses means that they can live inside a paragraph or they can be a high-level element that contains several paragraphs and even shapes, which of course is tricky to implement.

For 4.3.0 we have worked on some of these tags, and we can say we properly implemented the import and export of combo, date and check boxes. We also wrote some code to preserve generic SDTs and now most of the tags are preserved but there are formatting issues. The proper way to support every kind of SDT is translating them to the equivalent objects in LibreOffice on import and translate them back to SDTs on export, but that will require time and work. Any volunteers? 😉


Despite the 6-month development cycles, I’m feeling like the development of 4.3 line started a long time ago and I may have forgotten to write about some little feature or fix… Anyway, it’s time to close this batch of blog posts about interoperability features, all of them developed by Igalia and sponsored by CloudOn.

Enjoy our shiny new LibreOffice, and happy hacking!

Tales of LibreOffice interoperability: shape effects

We continue introducing features that will be part of the 4.3.0 release of LibreOffice, which is coming soon. After having worked in the preservation of color in shapes, we worked on the different effects that can be applied to shapes and bitmaps.

There are three types of effects that are managed separately in the DrawingML specification.

General shape effects

Examples of these effects are inner or outer shadows, reflections, glow… They can be applied both to vectorial shapes or bitmaps, and several of them can be applied at the same time.

Shape effects sample

These effects are indicated with the a:effectLst tag inside the shape properties tag spPr. I won’t explain their specification in detail because you can find a very good description in this website, but you can get an idea by taking a look at the following example where three effects are applied: glow, inner shadow and reflection:

  <a:glow rad="63500">
    <a:schemeClr val="accent2">
      <a:satMod val="175000" />
      <a:alpha val="40000" />
  <a:innerShdw blurRad="63500" dist="50800"
    <a:prstClr val="black">
      <a:alpha val="50000" />
  <a:reflection blurRad="6350" stA="52000"
  endA="300" endPos="35000" dir="5400000"
  sy="-100000" algn="bl"
  rotWithShape="0" />

Notice that some effects only have some attributes while others contain color specifications as child elements like the ones explained in the previous post.

3D effects

Shapes and bitmaps can be transformed into 3D objects and get lighting and camera modifications applied to them.

Shape 3D effects sample

These effects are basically controlled by two children tags of spPr. One of them is a:scene3d and controls the camera and lighting, and the other one is a:sp3d which controls the transformation of the shape in a 3D object adding extrusion, bevels and a material effect to the surface. In the same website I linked before, you can read a description of scene3d and sp3d tags and their children. Find an example of their combined use below:

  <a:camera prst="perspectiveRelaxedModerately"
    <a:rot lat="19490639" lon="0"
    rev="12900001" />
  <a:lightRig rig="threePt" dir="t">
    <a:rot lat="0" lon="0" rev="4800000" />
<a:sp3d z="488950" extrusionH="63500"
  <a:bevelT w="165100" prst="coolSlant" />
    <a:schemeClr val="tx2" />

Artistic effects

Effects from the last category act like the filters found in image manipulation programs (blur, grain or background removal among others) and that’s why they only can be applied to bitmaps. This is actually not a part of DrawingML spec but an extension over it.

There is an important difference with other filters; these ones come pre-calculated in the document. The bitmap linked by the DrawingML shape already comes with the effect, and the effect specification links a second bitmap that contains the original picture so the effect can be undone. This second bitmap is saved in the relatively new loss-less Windows Media Photo format.

Writer screenshot showing artistic effects

Check the following example of a blip-filled shape; the actual filling comes from the file linked as rId6, while the effect definition is linked to rId7 which is a copy of the original image before the filter was applied:

<a:blip r:embed="rId6" cstate="print">
    <a:ext uri="{BEBA8EAE-BF5A-486C-A8C5-ECC9F3942E4B}">
      <a14:imgProps xmlns:a14="http://schemas.microsoft.com/office/drawing/2010/main">
        <a14:imgLayer r:embed="rId7">
            <a14:artisticLightScreen trans="10000" gridSize="6" />

These are the relations between the ids and the files contained in the document, as specified at document.xml.rels:

<Relationship Id="rId6"
  Target="media/image2.png" />
<Relationship Id="rId7"
  Target="media/hdphoto1.wdp" />

The funny thing of this approach is that LO was able to render these effects with no effort, although the program was not aware of the effect parameters or the original bitmap and these were being lost on save.


We use again the grab bag technique to save all the tags and attributes related with the effects as a hidden property that will be used later in the export phase to re-build the effect definitions. In the case of artistic effects, we additionally need to make sure that the original bitmap is preserved; LibreOffice doesn’t support the Windows Media Photo format yet, but we can keep the raw stream of data and output it to a properly named file in the exported document. A small cache table is maintained by the exporter code to prevent that the same original file is saved more than once when two or more pictures apply effects to the same image.

We have finished with the improvements related to shapes and pictures, but there are a few interoperability features not yet mentioned which will be covered in a future post. Like the current and previous ones, they were developed by Igalia and sponsored by CloudOn.

Happy hacking!

Tales of LibreOffice interoperability: shape theme colors and styles

This is the latest chapter of the series about interoperability features that will be part of LibreOffice 4.3, brought to you by Igalia and sponsored by CloudOn, like the previous ones.

Last week we explained our work with theme color preservation for fonts and paragraphs, and now we move to a very close topic: the preservation of theme colors and styles on shapes.

XML definitions

DrawingML includes new conventions to denominate colors, wider than the initial OOXML spec as used for paragraphs and fonts. Such color definitions can be used in the context of shape properties or style definitions to define fillings, line colors, gradients…

In first place, English color names (red, black, etc.) can be used besides RGB values and theme color names; and for any color, a set of transformations can be defined, similarly to the tint and shade properties in fonts and paragraph colors, but with more options. An important transformation is alpha to define the transparency of that color.

Find below one example of each color tag (for color names, RGB values or theme colors), with or without transformations:

<!-- color by name, with an alpha transform -->
<a:prstClr val="black">
  <a:alpha val="50000"/>

<!-- color by RGB value, no transforms -->
<a:srgbClr val="FF0000"/>

<!-- theme color, with two transforms -->
<a:schemeClr val="accent1">
  <a:satMod val="175000"/>
  <a:alpha val="40000"/>

Shapes in DrawingML receive their attributes from two different sources: the style definitions and the own shape properties. The former act as a fallback of the latter; attributes from style definitions will only be applied if they are not overwritten by shape properties. For example, in case a shape contains both a filling definition among its properties and a filling style, the filling definition prevails.

The theme file defines a list of filling and line styles so they can be used by shapes. This is an example of an area filling style which adds a couple of color transformations:

    <a:schemeClr val="phClr">
      <a:tint val="15000"/>
      <a:satMod val="350000"/>
  <!-- more fill style definitions -->

Notice it’s not using any particular color, phClr is an entry parameter of sorts for a color that will be indicated in the shape style definition. The style definition can add some transformations or even define a gradient fill based on that color.

A line style definition can be a bit more complex, because it has more format options: width, dash pattern, single or double… These options are available for shape properties too.

  <a:ln w="9525" cap="flat" cmpd="sng" algn="ctr">
      <a:schemeClr val="phClr"/>
    <a:prstDash val="solid"/>
  <!-- more line style definitions -->

This is how a shape from the document would be assigned to the first filling style and the second line style, specifying “accent1” and “accent2” colors as parameters:

  <a:lnRef idx="2"> <!-- use the second element from fillStyleLst -->
    <a:schemeClr val="accent1"/>
  <a:fillRef idx="1"> <!-- use the first element from lnStyleLst -->
    <a:schemeClr val="accent2"/>

Finally, this is how a shape can overwrite the style-defined attributes for its filling and line. The following XML chunk defines a solid filling for the area using “accent3” color and a solid color for the shape line using “accent4”:

<wps:spPr> <!-- shape properties tag -->
  <a:solidFill> <!-- area filling -->
    <a:schemeClr val="accent3"/>
  <a:ln> <!-- line properties -->
      <a:schemeClr val="accent4"/>

The importer code

The duty of the importer code is, in general, applying the style definitions, merging them with the shape properties and transforming the properties in a way that matches LibreOffice internal data model.

Applying the style definitions means getting the corresponding styles from the theme file and applying them to the specific colors indicated in the shape wps:style block. This results in a set of properties that has to be merged with the shape-specific ones, and this is done so the latter have priority, as explained before.

All properties must be transformed to match LibreOffice data model, and specially colors because of their complexity. Theme colors are translated to their RGB value checking the theme definition to match the color name with its value. As for color transformations, the importer applies all the transformations to the original color to get a final RGB value. The only exception is the alpha transformation, which is converted in a transparency value to be stored in the fill properties object (alpha and transparency are complementary; after homogenizing their units, alpha = 1 – transparency).

LibreOffice doesn’t have equivalent concepts to theme colors and color transformations other than alpha, and doesn’t have shape styles either. To prevent information loss, we will use a hidden property in the Shape object called the interop grab bag to fill it with any properties we need to rebuild this information on export:

  • To preserve a theme color, we need to store its name and the complete list of transformations. We will also need the RGB value of the color with all the transformations applied, so we can compare it with the final color to know if the user has changed it. We do it both for line and filling colors.
  • For preset and RGB colors, the loss of the transformations is not important so we don’t consider this special case.
  • To preserve the style definitions, we store the idx attribute of each and their color parameter, with all transformations if necessary. We also store the RGB value of the color including transformations because we will need it too.

The exporter code

The normal behavior of the exporter is translating every LibreOffice shape property to DrawingML to write it to the document. In the case of colors, that means using a a:srgbClr tag to save it in RGB format and optionally adding an alpha transformation calculated from the shape transparency value.

The preservation of shape styles, area and line colors increases a bit the complexity. The shape grab bag must be checked for interoperability information saved in the import phase which should be saved back to the document under certain circumstances.

If there are any style definitions in the shape grab bag, they are always written back to the document; we need no additional verification because style definitions act as a fallback as we already explained. On the other hand, the case of shape properties has some complexity because we must perform several checks before deciding which area and line information we want to save:

  • Does the final color match the original one? If it doesn’t, it means the user has changed it during edition, so the new one must be saved and the information in the grab bag discarded.
  • If the original and final colors match, is there any theme color information in the grab bag? If there is, we must write it to the shape properties.
  • If the original and final colors match and there is no theme color information in the grab bag, we must compare the color with the style color; if they match, it means the shape was using the style color so we must not write a shape property that would overwrite it.
  • Otherwise, the shape was using a custom color different from the style color and must be written to the shape properties.

Bonus track: multiple color gradients

While I was working on this topic, I noticed that DrawingML allows to specify gradient fillings with any number of steps, unlike in previous MS Office documents (and LibreOffice ones) where you could only use two. This is an example of gradient with three steps:

      <a:gs pos="0">
        <a:srgbClr val="ffff00" />
      <a:gs pos="50000">
        <a:srgbClr val="ffff33">
          <a:alpha val="20000" />
      <a:gs pos="100000">
        <a:srgbClr val="ff0000" />
    <a:lin ang="5400000" />

Every a:gs tag indicates a point in the gradient extension with the pos attribute, measured as a percentage, and the color of that point. The system should calculate the gradient between the colors of two consecutive points.

LibreOffice does an approximation of those gradients with more than two steps using only two colors when it imports the document. To prevent information loss, we use again the grab bag to store the complete gradient definition. In an analogous way, we store the original, approximated gradient information to be able to compare it with the gradient information in the moment of the save operation.

We can identify three situations in the exporter:

  • The original and final gradients don’t match: it means the user has changed it during edition, so the new one must be saved and the information in the grab bag discarded.
  • The original and final gradients match and there is a full gradient definition stored in the grab bag: we must write that gradient definition to the shape properties.
  • The original and final gradients match and there isn’t any gradient definition stored in the grab bag: the gradient definition comes from the shape style and in that case we must not write the shape property that would, again, overwrite it.

One more thing or two

As you can see, this feature relies on the hidden interop grab bag property which contains more and more information as we increase its use to preserve unsupported properties. We hope to reduce its weight as those properties become natively supported. I would also like to comment that we have complemented our work with a set of unit tests that will help to detect regressions appearing in the future.

We haven’t yet finished with shapes; next post will be related to the preservation of different kinds of shape effects. It will be ready soon!

Tales of LibreOffice interoperability: font and paragraph colors

Last week, LibreOffice community branched out the version 4.3 in preparation of the next release in July. We had already introduced one of the new interoperability features that will be part of that release in the previous post in this series, and now we will continue explaining how OOXML theme colors will be preserved in LibreOffice.

Theme colors have a prominent place in the color palette in the latest Microsoft Office versions, as you can see:
Word color elector

They consist on a palette of ten colors, each one of them with five more variations. When a user changes the document theme, a new palette of colors will be loaded and any objects in the document that used one of those colors will be updated with the corresponding one from the new palette.

In the XML document, a theme color is identified by a name, and its variations are implemented with two attributes: shade to lighten the color and tint to darken it.

This is an example of theme color applied to the characters in a text run – a chunk of text with the same properties. In the run properties (rPr) we find the following tag (reference):

  <w:color w:val="E5B8B7" w:themeColor="accent2" w:themeTint="66"/>

Theme color for character

This is the theme color named accent2 darkened a 66%. The w:val property indicates the RGB code of the final color, it’s ignored if w:themeColor is present.

The preservation of this tag is simple:

  • On import, store w:themeColor, w:themeTint and w:themeShade properties as hidden properties in the text run.
  • Store the original color too!
  • On export, check if the original color has changed:
    • if it did, write only the new color in the w:val property.
    • if it didn’t, write the stored w:themeColor, w:themeTint and w:themeShade properties.

Now, an example of paragraph shade with theme colors (reference):

  <w:shd w:val="thinDiagStripe"
         w:color="215868" w:themeColor="accent5" w:themeShade="80"
         w:fill="DBE5F1" w:themeFill="accent1" w:themeFillTint="33" />

Paragraph filling with pattern

Paragraph shades can specify two colors, one for the background and another one for the pattern if present, with the type of pattern specified by the w:val attribute. The set of attributes w:color, w:themeColor, w:themeShade and w:themeTint work like in the case of font colors, specifying the RGB code, theme color name, shade and tint modifiers for the pattern; w:fill, w:themeFill w:themeFillTint and w:themeFillShade do the same for the background color.

The preservation of these properties works analogously; they are also stored as a hidden property of the text on import, and saved back on export in case the user hasn’t changed the fill color while editing the document with LibreOffice.

The w:shd tag can also be used in table or table cell properties with the same meaning. And speaking of tables, we also made sure that the table style property was preserved:

  <w:tblStyle w:val="Tablaconcuadrcula" />

We will continue next week with more shiny features brought to you by Igalia and CloudOn. Stay tuned!

Tales of LibreOffice interoperability: theme fonts

This is the first post in a series that explains the work that we’ve been doing lately in LibreOffice at Igalia, which hopefully will be part of the future 4.3 release.

Document themes is one of the features of the Microsoft Office suite that doesn’t have support in LibreOffice yet. While it happens, which is a hard work that includes extending the ODF standard, we are focusing on the preservation of the theme information when a document coming from Office is edited in LibreOffice and saved back to OOXML.

First of all, we needed to preserve the theme files contained in the document, which are stored at /word/theme/ inside the document. LibreOffice was able to read and parse the theme definition file to assign colors and fonts to the document elements, but it was discarded after that and not preserved on export. Miguel and Andrés took care of that part of the job when working in the preservation of Smart-Art information and it’s already present in 4.2 release.

Then it was my turn to identify and preserve the theme-related attributes in the document, starting with font attributes. From the user point of view, there are two types of theme fonts in Word: Heading, named major internally, and Body, named minor. Besides this distinction, there are three types of fonts that users can manage separately: Latin, Asian and Complex Script (for Hebrew, Arabic, etc.). Users can set one specific font, or a major or minor theme font, for each of the three types.

Word 2010 font selector

What actual font this mess translates to depends on several things:

  • Type of characters: the application decides if a portion of text is written in Latin, Asian or CS checking the range of characters it contains, and uses the font that the user set for that type.
  • The document language: the theme file can define one font per language besides a fallback font per type:
            <!-- default fonts per type -->
            <a:latin typeface="Trebuchet MS" />
            <a:ea typeface="" />
            <a:cs typeface="" />
            <!-- language-specific fonts -->
            <a:font script="Hans" typeface="宋体" />
            <a:font script="Hebr" typeface="Arial" />
  • The default language: the document settings file can define one default language for each font type:
    <w:themeFontLang w:val="en-US" w:eastAsia="zh-CN"
                     w:bidi="he-IL" />

How does it work altogether? With the above definition of the language settings, the default language for CS text is Hebrew (he-IL), and the theme defines the minor font Arial for that language. In the case of latin languages, it will be Trebuchet MS. This was a new behaviour in LibreOffice that we had to implement ourselves. Notice the mixed use of different naming conventions for languages and regions, which requires conversions.

LibreOffice font selector

My work on theme attributes comprises the preservation of font and shape colors too, but this post is already long enough; that will be part of the next chapter. Let me finish with a thank you to CloudOn for funding this development, and to Adam Fyne for his work detecting and triaging these theme preservation issues.