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Improving borders of merged table cells in Writer

Estimated read time: 2 minutes

Writer now has better support for Word-compatible border rending when it comes to vertically merged cells in tables.

First, thanks Docmosis who made this work by Collabora possible.


Both Word and Writer allow specifying borders for any kind of table cells. When the user creates a vertically merged cell, there is a covering cell and there is one or more covered table cells.

The Writer approach is to render the cell borders according to the properties of the covering cell. This has the benefit that each edge of the table cell has a single border style (e.g. dashed or hairline).

The Word approach is to render the cell borders as if there would be no vertical merge, according to the properties of the covered cell. This has the benefit that merging the content of cells vertically doesn’t change the border rendering, but it also requires more complex code for painting.

Results so far

Writer can now detect that your tables originate from Word formats and render table borders the Word way in that case.

Here is how the new rendering result look like:
Figure 1. Writer rendering in compatibility mode, new output

And here is how it used to look like:
Figure 2. Writer rendering in compatibility mode, old output

And finally the reference rendering is:
Figure 3. Writer rendering in compatibility mode, reference output

You can see that the B4 and B5 cells are covered, they had some unwanted border on their left side and this is now gone.

How is this implemented?

If you would like to know a bit more about how this works, continue reading… :-)

  • First, some building blocks were introduced: SwCellFrame::GetCoveredCellInRow() can look up a covered cell in a certain row, provided that this cell covers it

  • Building on top of this, SwCellFrame::GetCoveredCells() can provide a list of cell frames which are covered by the current cell, due to vertical merge. This is needed, because previously the layout didn’t have to consider properties of covered cells, so while the document model had this information, it was not visible to the layout in a convenient way

  • Using the above functionality, SwTabFramePainter::Insert() can suppress painting of certain border lines in Word compatibility mode

  • Finally, the code change can be covered with a test by recording the rendering and asserting the vertical positions of border points: we can check that all the positions belong to the 1st, 2nd or 3rd rows, and not to a row below them

Want to start using this?

Collabora intends to continue supporting and contributing to LibreOffice, the code is merged so we expect all of this work will be available in TDF’s next release (7.2).

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