A Mr. Harry Potter from Fort Lee, New Jersey writes in an asks:
Why did Autodesk abandon the use of RML files in favor of DWF?
Autodesk View was the first Autodesk product to provide a redline capability. When this application got combined with WHIP! to form Volo View, the markup information was saved in what was called a redline markup language (RML) file. To keep file sizes small, the RML file contained just the markup information. The design author and reviewer were both assumed to have the original drawing file. The RML file contained the name of the drawing that the markup applied to. This allowed Volo View to load the drawing when the RML file was loaded.
Having just the drawing file name in the RML file proved problematic in that the RML file did not identify which version of a drawing had been used. In many cases, the drawing had changed between the time it was marked up and the time that the recipient of the RML wanted to respond to a reviewer's comments. The problem was exacerbated by the fact that the markup was done based on all of what the user could see when viewed in Volo View. This included XREFs and other files, but none of the names of the XREF references appeared in the RML file. So if an XREF had been added or dropped since the markup was created, there was no way to tell.
So rather than produce an RML file that just contained the markup and have to try to determine what it had been created against, we went the other direction. We decided to capture what was being viewed (source, XREFs, and all) in addition to the markup. We used a DWF file which captured the original drawing and allowed Autodesk Design Review to add markup information to that DWF file. It actually adds the markup information at the end - leaving the original DWF file in tact. For updating the original drawing file, we created a round trip process where the markup information could be culled from the DWF file and aligned with the source drawing. This removed all doubt as to what a drawing looked like when it was marked up but still allowed the markup to be viewed against the latest version of a drawing.
The markup information in a DWF is actually a combination of XML, an industry standard, and typical DWF geometry. The RML format predated XML and was proprietary.
One of the benefits of the RML file was that Volo View loaded it as an XREF against the original drawing. Thus you could load more than one reviewer's comments at the same time. This is also possible with Autodesk Design Review. Using Autodesk Design Review, you can combine multiple marked up sheets from several DWF files into one DWF file. You then load the combined DWF file in the Markup Set Manager of AutoCAD. The duplicated DWF geometry is ignored and only the markup items are shown. The result is that you see all of the reviewers comments against the original geometry in the drawing.