Benefits of the Microsoft Office Open XML File Formats

An XML Node Tree
An XML Node Tree

This article discusses the distinctions between the old and new Microsoft Office file formats, and explains the advantages of choosing the new formats, which are called the Office Open XML file formats. Confusingly, the Office Open XML formats are not the same as the OpenOffice XML file formats. The naming similarity causes much confusion, and arises from the fact that the goals of both definitions were similar, even though the implementations are distinct.

This article also explains how you can decompress a Word file that has been stored using the new XML format and examine its content directly. This can sometimes be helpful if you find that you cannot open a file because it has been corrupted, in which case you may be able to fix the error and make the file once again editable.

In an earlier post, I mentioned the new file format for Microsoft Word files (i.e., files with a .docx extension), which stores data using XML, instead of the binary data coding that was used by the original Microsoft Word format (files with a .doc extension). In fact, that is true not only for .docx files, but also for various other file types created using recent versions of Microsoft’s Office suite of programs. For example, Microsoft Excel files have a new .xlsx format, replacing the older .xls format.

In my earlier post, I also mentioned the general dangers of using proprietary file formats (for any application), because the data contained in the files can only be accessed via the one specific application that’s designed to open files in that format. If the application becomes unavailable, or if the manufacturer changes the program to the point where it is no longer able to open files that use its own older formats, you may have no way to access data in files with the proprietary format. This could result in a severe loss of data at some future time.

To avoid this situation, it’s better whenever possible to store data using open file formats.

Just in case you think that, by extolling the advantages of the Office Open XML file formats here, I’m acting as a “shill” for Microsoft, rest assured that I’m not. In fact, if you read on, you’ll discover why using these new formats can actually free you from dependence on Microsoft’s applications.

Office Productivity Suites

Over the years, it has become apparent that certain types of application program have widespread usefulness in office environments across many industries. The exact list varies, but in general the following program types are used by typical office computer users:

Typical Components of an Office Software Suite
Typical Components of an Office Software Suite
  • Word Processor
  • Spreadsheet
  • Email Client
  • Slide Show Generator
  • Vector Drawing

Software manufacturers have grouped together these commonly-used programs, and offer them as “office productivity suites” with varying levels of integration between the component programs within the suite.

Most computer users will be aware that the Microsoft Office suite is still the most widely-used office productivity suite in the world (see, for example, http://www.cmswire.com/cms/document-management/microsoft-office-is-still-the-productivity-suite-leader-022908.php).

The continued popularity of Microsoft Office is perhaps surprising, because the software is by no means free, and in fact there are good-quality free alternatives available. In this article, I won’t discuss the psychology of why so many people continue to pay to use a piece of software when there are equivalent free alternatives available. However, I will mention some of the alternatives, and show you how the Open XML file formats allow you to use those more easily.

Incidentally, it’s not my intention in this article to discuss the general use of Office suite software in “IT” environments. I don’t work in the field of “IT” (in the sense that the term is typically used), but I do use Office suite software in my roles as author and programmer.

Why were the XML Formats Developed?

I haven’t found any clear statement of the motivation that prompted Microsoft to consider replacing its long-standing binary file formats with XML-based formats. However, I suspect that the primary motivations were competition and pressure from large users of the Office suite.

Given the prevalence of Microsoft Office on computer systems around the world, around the year 2000, many government and official bodies were becoming concerned about the amount of vital information that was being stored in files using the Microsoft binary formats. The problem wasn’t merely the risk that files could become corrupt or unreadable. There was also concern that it was impossible to be certain that the proprietary data formats didn’t include “back doors” that would permit the reading of content that was supposed to be secure.

At the same time, open-source software was being developed to provide free alternatives to the more popular applications in the Microsoft Office suite. The most prominent of these open-source suites was OpenOffice, developed by Sun Microsystems. Although OpenOffice supported the Microsoft binary file formats, it also had its own set of XML-based formats, conforming to the public OpenOffice XML standards.

As a result of these developments, Microsoft offered its own version of open XML-based format specifications, and sought international certification of those formats. The result is that both sets of standards are now publicly available.

Advantages of the XML Formats

  • Files are more compact. In most cases, if you compare the size of an Office file saved in the binary format, with the same file saved in the equivalent Open XML format, the XML-formatted file will be smaller. This is largely because of the compression applied to the Open XML files. However, files that contain a large number of graphics may not be smaller, because the graphics cannot be further compressed by the zip algorithm.
  • Easier corrupted file recovery.
  • Easier to locate and parse content.
  • Files can be opened and edited with any XML editor.
  • Files containing macros are easier to identify.

Formats & Applications

The Office Open XML formats correspond to the Office applications as shown in the table below:

Office File Formats
Office File Formats

How To Examine the Contents of a Word docx File

When you use Word (or an equivalent word processor) to open a file that uses one of the XML file formats, such as a Word docx file, all you see is a view of the document itself, complete with all its formatting. There seems to be no evidence of any XML structure.

If this is an XML-encoded file, then where is the XML? How do you actually access the XML that defines the document?

In fact, all files that use any of the Office XML formats compress all the XML and component parts into one “zip” file. You can, of course, compress other files into “zip” files, but, when you do, the resulting file typically has the extension .zip.

In fact, Office XML files are indeed zip files, and can have a valid .zip extension. To be able to view and even extract the internal XML and other components, you simply have to open the file using a zip extraction program, instead of a Microsoft Office program. In Windows, the easiest way to do that is to give the Office file a .zip extension.

The following procedure explains exactly how to do this under Windows. Note that this is not an “undocumented hack”; Microsoft encourages you to access the components of the documents this way. These instructions are available from Microsoft at: https://msdn.microsoft.com/en-us/library/aa982683(v=office.12).aspx

  1. Add a .zip extension to the end of the file name, before the .docx
  2. Double-click the file. It will open in the ZIP application. You can see the parts that comprise the file.
  3. Extract the parts to the folder that you created previously.

Non-Microsoft Support for the XML Formats

Many people seem to assume that, if they receive a file in one of the Microsoft Office file formats (either the older proprietary formats or the newer XML formats), then they must use Microsoft Office to open and edit it.

In fact, that’s not true, because the available competitor office suites can handle many of the Microsoft formats well. OpenOffice and Libre Office can both edit files in many of the Microsoft Office formats. Additionally, modern versions of Microsoft Office can at least open files in many of the OpenOffice XML formats, even if it does not fully support them. (In all cases there may be minor formatting differences, and you shouldn’t swap between formats unnecessarily.)

Thus, using the new Office Open XML file formats does not restrict you to using only Microsoft-supplied applications. Files in these formats can be expected to be reasonably “future-proof” for a long time to come.

Deficiencies of the Office Open XML Formats

I am not aware of any major deficiencies of the new formats that would dissuade anyone from using them in preference to the previous binary formats. Here are some relatively minor issues to consider:

  • Some files containing large quantities of graphics may be larger than in the equivalent binary format.
  • Files in the new Open XML formats cannot be opened using old (pre-2007) versions of Office.
  • The XML structure is such that it’s not easy to parse the content of the files in useful ways.

Structure Example

Here’s an example of the actual Word XML markup for a very simple text document. The example shows how revisions to the markup are stored in the file, which can make it difficult to parse the XML content to extract meaningful information.

I wrote a very simple text file, which includes the line of “Normal”-styled text: “This is just a test.”.

In the WordML XML, this appears as:

<w:r><w:t>This is just a test.</w:t></w:r>

Next, I deliberately introduced a revision, by typing some extra characters before the final “t” of “test”, then deleting the extra characters and saving the result. The resulting XML looks like this:

<w:r><w:t>This is just a tes</w:t></w:r><w:bookmarkStart w:id="0" w:name="_GoBack"/><w:bookmarkEnd w:id="0"/><w:r w:rsidR="0000364D"><w:t>t</w:t></w:r><w:r w:rsidR="003F3CE4"><w:t>.</w:t></w:r>

As you can see, the final “t” and the period are now in separate <w:t> elements, and a new bookmark has been inserted. This type of element-splitting makes it difficult to extract the actual text from the XML.

Therefore, before attempting any processing of an Office Open XML-formatted file, you should always “Accept all changes” to eliminate version-tracking markup.

Recommendations

  • Always use the new XML Office formats rather than the old binary formats when possible.
  • Even if you have Microsoft Office installed, consider installing LibreOffice, etc., on the same computer. You’ve nothing to lose.

Digital Color Palettes: the Essential Concepts

 

Fantasy Illustration of Computer Artist using Palette for Digital Color Palettes
An Unlikely Computer Artist

The word “palette” (or “pallet”) has several meanings: it can refer to a tray used to transport items, or to a board used by artists to mix colors (as shown in the fantasy illustration above, which I produced many years ago for a talk on Computer Artwork). In this article, I’ll discuss the principles of Digital Color Palettes. If you’re working with digital graphics files, you’re likely to encounter “palettes” sooner or later. Even though the use of palettes is less necessary and less prevalent in graphics now than it was years ago, it’s still helpful to understand them, and the pros and cons of using them.

Even within the scope of digital graphics, there are several types of palette, including Aesthetic Palettes and Technical Palettes.

I discussed the distinction between bitmap and vector representations in a previous post [The Two Types of Computer Graphics]. Although digital color palettes are more commonly associated with bitmap images, vector images can also use them.

The Basic Concept

A digital color palette is essentially just an indexed table of color values. Using a palette in conjunction with a bitmap image permits a type of compression that reduces the size of the stored bitmap image.

In A Trick of the Light, I explained how the colors you see on the screen of a digital device display, such as a computer or phone, are made up of separate red, green and blue components. The pixels comprising the image that you see on-screen are stored in a bitmap matrix somewhere in the device’s memory.

In most modern bitmap graphic systems, each of the red, green and blue components of each pixel (which I’ll also refer to here as an “RGB Triple” for obvious reasons) is represented using 8 bits. This permits each pixel to represent one of 224 = 16,777,216 possible color values. Experience has shown that this range of values is, in most cases, adequate to allow images to display an apparently continuous spectrum of color, which is important in scenes that require smooth shading (for example, sky scenes). Computers are generally organized to handle data in multiples of bytes (8 bits), so again this definition of an RGB triple is convenient. (About twenty years ago, when memory capacities were much smaller, various smaller types of RGB triple were used, such as the “5-6-5” format, where the red and blue components used 5 bits and the green component 6 bits. This allowed each RGB triple to be stored in a 16-bit word instead of 24 bits. Now, however, such compromises are no longer worthwhile.)

There are, however, many bitmap images that don’t require the full gamut of 16,777,216 available colors. For example, a monochrome (grayscale) image requires only shades of gray, and in general 256 shades of gray are adequate to create the illusion of continuous gradation of color. Thus, to store a grayscale image, each pixel only needs 8 bits (since 28 = 256), instead of 24. Storing the image with 8 bits per pixel (instead of 24 bits) reduces the file size by two-thirds, which is a worthwhile size reduction.

Even full-color images may not need the full gamut of 16,777,216 colors, because they have strong predominant colors. In these cases, it’s useful to make a list of only the colors that are actually used in the image, treat the list as an index, and then store the image using the index values instead of the actual RGB triples.

The indexed list of colors is then called a “palette”. Obviously, if the matrix of index values is to be meaningful, you also have to store the palette itself somewhere. The palette can be stored as part of the file itself, or somewhere else.

To restate, whether implemented in hardware or software, an image that uses a palette does not store the color value of each pixel as an actual RGB triple. Instead, each color value is stored as an index to a single entry in the palette. The palette itself stores the RGB triples. You specify the pixels of a palettized* image by creating a matrix of index values, rather than a matrix of the actual RGB triples. Because each index value is significantly smaller than a single triple, the size the resulting bitmap is much smaller than it would be if each RGB triple were stored.

The table below shows the index values and colors for a real-world (albeit obsolete) color palette; the standard palette for the IBM CGA (Color Graphics Adapter), which was the first color graphics card for the IBM PC. This palette specified only 16 colors, so it’s practical to list the entire palette here.

CGA Color Palette Values
CGA Color Palette Table

(* For the action associated with digital images, this is the correct spelling. If you’re talking about placing items on a transport pallet, then the correct spelling is “palletize”.)

Aesthetic Palettes*

In this context, a palette is a range of specific colors that can be used by an artist creating a digital image. The usual reason for selecting colors from a palette, instead of just choosing any one of the millions of available colors, is to achieve a specific “look”, or to conform to a branding color scheme. Thus, the palette has aesthetic significance, but there is no technical requirement for its existence. The use of aesthetic palettes is always optional.

(* As I explained in Ligatures in English, this section heading could have been spelled “Esthetic Palettes”, but I personally prefer the spelling used here, and it is acceptable in American English.)

Technical Palettes

This type of palette is used to achieve some technological advantage in image display, such as a reduction of the amount of hardware required, or of the image file size. Some older graphical display systems require the use of a color palette, so their use is not optional.

Displaying a Palettized Image

The image below shows how a palettized bitmap image is displayed on a screen. The screen could be any digital bitmap display, such as a computer, tablet or smartphone.

Diagram of Palettized Image Display for Digital Color Palettes
Palette-based Display System

The system works as follows (the step numbers below correspond to the callout numbers in the image):

  1. As the bitmap image in memory is scanned sequentially, each index value in the bitmap is used to “look up” a corresponding entry in the palette.
  2. Each index value acts as a lookup to an RGB triple value in the palette. The correct RGB triple value for each pixel is presented to the Display Drivers.
  3. The Display Drivers (which may be Digital-to-Analog Converters, or some other circuity, depending on the screen technology) create red, green and blue signals to illuminate the pixels of the device screen.
  4. The device screen displays the full-color image reconstituted from the index bitmap and the palette.

Hardware Palette

In the early days of computer graphics, memory was expensive and capacities were small. It made economic sense to maximize the use of digital color palettes where possible, to minimize the amount and size of memory required. This was particularly important in the design of graphics display cards, which required sufficient memory to store at least one full frame of the display. By adding a small special area of memory on the card for use as a palette, it was possible to reduce the size of the main frame memory substantially. This was achieved at the expense of complexity, because now every image that was displayed had to have a palette. To avoid having to create a special palette for every image, Standard color palettes and then Adaptive color palettes were developed; for more details, see Standard vs. Adaptive Palettes below.

One of the most famous graphics card types that (usually) relied on hardware color palettes was the IBM VGA (Virtual or Video Graphics Array) for PCs (see https://en.wikipedia.org/wiki/Video_Graphics_Array).

As the cost of memory has fallen, and as memory device capacities have increased, the use of hardware palettes has become unnecessary. Few, if any, modern graphics cards implement hardware palettes. However, there are still some good reasons to use software palettes.

Software Palette

Generally, the software palette associated with an image is included in the image file itself. The palette and the image matrix form separate sections within one file. Some image formats, such as GIF, require the use of a software palette, whereas others, such as BMP, don’t support palettes at all.

Modern bitmap image formats, such as PNG, usually offer the option to use a palette, but do not require it.

Standard & Adaptive Palettes

Back when most graphics cards implemented hardware palettes, rendering a photograph realistically on screen was a significant problem. For example, a photograph showing a cloud-filled sky would include a large number of pixels whose values are various shades of blue, and the color transitions across the image would be smooth. If you were to try to use a limited color palette to encode the pixel values in the image, it’s unlikely that the palette would include every blue shade that you’d need. In that case, you were faced with the choice of using a Standard Palette plus a technique called Dithering, or else using an Adaptive Palette, as described below.

Standard Palette

Given that early graphics cards could display only palettized images, it simplified matters to use a Standard palette, consisting of only the most commonly-used colors. If you were designing a digital image, you could arrange to use only colors in the standard palette, so that it would be rendered correctly on-screen. However, the standard palette could not, in general, render a photograph realistically—the only way to approximate that was to apply Dithering.

The most commonly-used Standard palette for the VGA graphics card was that provided by BIOS Mode 13H.

Dithering

One technique that was often applied in connection with palettized bitmap images is dithering. The origin of the term “dithering” seems to go back to World War II. When applied to palettized bitmap images, the dithering process essentially introduces “noise” in the vicinity of color transitions, in order to disguise abrupt color changes. Dithering creates patterns of interpolated color values, using only colors available in the palette, that, to the human eye, appear to merge and create continuous color shades. For a detailed description of this technique, see https://en.wikipedia.org/wiki/Dither.

While dithering can improve the appearance of a palettized image (provided that you don’t look too closely), it achieves its results at the expense of reduced image resolution, because of the fact that the dithering of pixel values introduces “noise” into the image. Therefore, you should never dither an image that you want to keep as a “master”.

Adaptive Palette

Instead of specifying a Standard Palette that includes entries for any image, you can instead specify a palette that is restricted only to colors that are most appropriate for the image that you want to palettize. Such palettes are called Adaptive Palettes. Most modern graphics software can create an Adaptive Palette for any image automatically, so this is no longer a difficult proposition.

A significant problem with Adaptive Palettes is that a display device that relies on a hardware palette can typically use only one palette at a time. This makes it difficult or impossible to display more than one full-color image on the screen. You can set the device’s palette to be correct for the first photograph and the image will look great. However, as soon as you change the palette to that for the second photograph, the colors in the first image are likely to become completely garbled.

Fortunately, the days when graphical display devices used hardware palettes are over, so you can use Adaptive Palettes where appropriate, without having to worry about rendering conflicts.

Should you Use Digital Color Palettes?

Palettization of an image is usually a lossy process. As I explained in a previous post [How to Avoid Mosquitoes], you should never apply lossy processes to “master” files. Thus, if your master image is full-color (e.g., a photograph), you should always store it in a “raw” state, without a palette.

However, if you want to transmit an image as efficiently as possible, it may reduce the file size if you palettize the image. This also avoids the necessity to share the high-quality unpalettized master image, which could be useful if you’re posting the image to a public web page.

If it’s obvious that your image uses only a limited color range, such as a monochrome photograph, then you can palettize it without any loss of color resolution. In the case of monochrome images, you don’t usually have to create a custom palette, because most graphics programs allow you to store the image “as 8-bit Grayscale”, which achieves the same result.

In summary, then, in general it’s best not to use palettes for full-color images. However, if you know that your image is intended to contain only a limited color range, then you may be able to save file space by using a palette. Experimentation is sometimes necessary in such cases. You may also want to palettize an image so that you don’t have to make the high-quality original available publicly. If you’re an artist who has created an image that deliberately uses a limited palette of colors, and you want to store or communicate those choices, then that would also be a good reason to use a palettized image.

Converting Between Absolute & Relative Paths in MadCap Flare: Sample C# Code

I regularly use MadCap Flare for the production of technical documentation. Flare is a sophisticated content authoring tool, which stores all its topic and control files using XML. This makes it relatively easy to process the content of the files programmatically, as in the example of CSS class analysis that I described in a previous post.

The Flare software is based on Microsoft’s .NET framework, so the program runs only under Windows. For that reason, this discussion will be restricted to Windows file systems.

In Windows, the “path” to a file consists of a hierarchical list of subfolders beneath a root volume, for example:

c:\MyRoot\MyProject\Content\MyFile.htm

Sometimes, however, it’s convenient to specify a path relative to another location. For example, if the file at:

c:\MyRoot\MyProject\Content\Subsection\MyTopic.htm

contained a link to MyFile.htm as above, the relative path could be specified as:

..\MyTopic.htm

In the syntax of relative paths, “..” means “go up one folder level”. Similarly, “.” means “this folder level”, so .\MyFile.htm refers to a file that’s in the same folder as the file containing the relative path.

If you’ve ever examined the markup in Flare files, you’ll have noticed that extensive use is made of “relative paths”. For example, a Flare topic may contain a hyperlink to another topic in the same project, such as:

<MadCap:xref href="..\MyTopic.htm">Linked Topic</MadCap:xref>

Similarly, Flare’s Table-Of-Contents (TOC) files (which have .fltoc extensions) are XML files that contain trees of TocEntry elements. Each TocEntry element has a Link attribute that contains the path to the topic or sub-TOC that appears at that point in the TOC. All the Link attribute paths start at the project’s Content (for linked topics) or Project (for linked sub-TOCs) folder, so in that sense they are relative paths.

An example of a TocEntry element would be:

<TocEntry Title="Sample Topic" Link="/Content/Subsection/MyTopic.htm" />

When I’m writing code to process these files (for example to open and examine each topic in a Flare TOC file), I frequently have to convert Flare’s relative paths into absolute paths (because the XDocument.Load() method, as described in my previous post, will accept only an absolute path), and vice versa if I want to insert a path into a Flare file. Therefore, I’ve found it very useful to create “library” functions in C# to perform these conversions. I can then call the functions AbsolutePathToRelativePath() and RelativePathToAbsolutePath() without having to think again about the details of how to convert from one format to the other.

I’m sure that there are probably similar functions available in other programming languages. For example, I’m told that Python includes a built-in conversion function called os.path.relpath, which would make it unnecessary to create custom code. Anyway, my experience as a programmer suggests that you can never have too many code samples, so I’m offering my own versions here to add to the available set. I have tested both functions extensively and they do work as listed.

The methods below are designed as static methods for inclusion in a stringUtilities class. You could place them in any class, or make them standalone functions.

AbsolutePathToRelativePath

This static method converts an absolute file path specified by strTargFilepath to its equivalent path relative to strRootDir. strRootDir must be a directory tree only, and must not include a file name.

For example, if the absolute path strTargFilepath is:

c:\folder1\folder2\subfolder1\filename.ext

And the root directory strRootDir is:

c:\folder1\folder2\folder3\folder4

The method returns the relative file path:

..\..\subfolder1\filename.ext

Note that there must be some commonality between the folder tree of strTargFilepath and strRootDir. If there is no commonality, then the method just returns strTargFilepath unchanged.

The path separator character that will be used in the returned relative path is specified by strPreferredSeparator. The default value is correct for Windows.

using System.IO;

public static string AbsolutePathToRelativePath(string strRootDir, string strTargFilepath, string strPreferredSeparator = "\\")
{
	if (strRootDir == null || strTargFilepath == null)
		return null;

 	string[] strSeps = new string[] { strPreferredSeparator };

 	if (strRootDir.Length == 0 || strTargFilepath.Length == 0)
		return strTargFilepath;

 	// Convert to arrays
	string[] strRootFolders = strRootDir.Split(strSeps, StringSplitOptions.None);
	string[] strTargFolders = strTargFilepath.Split(strSeps, StringSplitOptions.None);
	if (string.Compare(strRootFolders[0], strTargFolders[0], StringComparison.OrdinalIgnoreCase) != 0)
		return strTargFilepath;

 	// Count common root folders
	int i = 0;
	List<string> listRelFolders = new List<string>();
	for (i = 0; i < strRootFolders.Length; i++)
	{
		if (string.Compare(strRootFolders[i], strTargFolders[i], StringComparison.OrdinalIgnoreCase) != 0)
			break;
	}
	
	for (int k = i; k < strTargFolders.Length; k++)
		listRelFolders.Add(strTargFolders[k]);

	System.Text.StringBuilder sb = new System.Text.StringBuilder();
	if (i > 0)
	{
		// Note: the last element of strTargFolders is actually the filename, so must adjust count for that
		for (int j = 0; j < strRootFolders.Length - i; j++)
		{
			sb.Append("..");
			sb.Append(strPreferredSeparator);
		}
	}

	return sb.Append(string.Join(strPreferredSeparator, listRelFolders.ToArray())).ToString();
}

RelativePathToAbsolutePath

This static method converts a relative file path specified by strTargFilepath to its equivalent absolute path using strRootDir. strRootDir must be a directory tree only, and must not include a file name.

For example, if the relative path strTargFilepath is:

..\..\subfolder1\filename.ext

And the root directory strRootDir is:

c:\folder1\folder2\folder3\folder4

The method returns the absolute file path:

c:\folder1\folder2\subfolder1\filename.ext

If strTargFilepath starts with “.\” or “\”, then strTargFilepath is simply appended to strRootDir

The path separator character that will be used in the returned relative path is specified by strPreferredSeparator. The default value is correct for Windows.

using System.IO;

public static string RelativePathToAbsolutePath(string strRootDir, string strTargFilepath, string strPreferredSeparator = "\\")
{
	if (string.IsNullOrEmpty(strRootDir) || string.IsNullOrEmpty(strTargFilepath))
		return null;
	
	string[] strSeps = new string[] { strPreferredSeparator };

 	// Convert to lists
	List<string> listTargFolders = strTargFilepath.Split(strSeps, StringSplitOptions.None).ToList<string>();
	List<string> listRootFolders = strRootDir.Split(strSeps, StringSplitOptions.None).ToList<string>();

	// If strTargFilepath starts with .\ or \, delete initial item
	if (string.IsNullOrEmpty(listTargFolders[0]) || (listTargFolders[0] == "."))
		listTargFolders.RemoveAt(0);
	while (listTargFolders[0] == "..")
	{
		listRootFolders.RemoveAt(listRootFolders.Count - 1);
		listTargFolders.RemoveAt(0);
	}
	if ((listRootFolders.Count == 0) || (listTargFolders.Count == 0))
		return null;

 	// Combine root and subfolders
	System.Text.StringBuilder sb = new System.Text.StringBuilder();
	foreach (string str in listRootFolders)
	{
		sb.Append(str);
		sb.Append(strPreferredSeparator);
	}
	for (int i = 0; i < listTargFolders.Count; i++)
	{
		sb.Append(listTargFolders[i]);
		if (i < listTargFolders.Count - 1)
			sb.Append(strPreferredSeparator);
	}

	return sb.ToString();
}

[7/1/16] Note that the method above does not check for the case where a relative path contains a partial overlap with the specified absolute path. If required, you would need to add code to handle such cases.

For example, if the relative path strTargFilepath is:

folder4\subfolder1\filename.ext

and the root directory strRootDir is:

c:\folder1\folder2\folder3\folder4

the method will not detect that folder4 is actually already part of the root path.