Soothed Foperators, I have been looking for a while, like everybody else, at keeps. I was looking for a way to unify the processing of keeps and breaks at the layout level. Breaks are easy enough; just create column-break and page-break pseudo-area and insert into the galley as a sibling of the affected object's area subtree. Once that's done, look-ahead and re-layout operate on the same stream of galley objects. However, keeps didn't fit.
It occurred to me that, conceptually, the keeps can all be expressed as a new keep-together pseudo-area. The keep-together property itself is expressed during layout by wrapping all of the generated areas in a keep-together area. Keep-with-previous on formatting object A becomes a keep-together area spanning the first non-blank normal area leaf node, L, generated by A or its offspring, and the last non-blank normal area leaf node preceding L in the area tree. Likewise, keep-with-next on formatting object A becomes a keep-together area spanning the last non-blank normal area leaf node, L, generated by A or its offspring, and the first non-blank normal area leaf node following L in the area tree. The obvious problem with this arrangement is that the keep-together area violate the hierarachical arrangement of the layout tree. They form a concurrent structure focussed on the leaf nodes. This seems to be the essential problem of handling keep-with-(previous/next); that it cuts across the naturally tree-structured flow of processing. Such problems are endemic in page layout. In any case, it seems that the relationships between areas that are of interest in keep processing need some form of direct expression, parallel to the layout tree itself. I have yet to examine inline relationships or the problem of changes in block-progression-direction, but just looking at the simple block stacking cases, you get a diagram like the attached PNG. In order to track the relationships through the tree, I think you need four sets of links. The basic links are: Leading edge to leading edge of first normal child. Trailing edge to leading edge of next normal sibling. Trailing edge to trailing edge of parent Superimposed on the basic links are bridging links which span adjacent sets of links. These spanning links are the tree violators, and give direct access to the areas which are of interest in keep processing. They could be implemented as double-linked lists, either within the layout tree nodes or as separate structures. Gaps in the spanning links are joined by simply reproducing the single links, as in the diagram. The whole layout tree for a page is effectively threaded in order of interest, as far as keeps are concerned. The bonus of this structure is that it looks like a superset of the stacking constraints. It gives direct access to all sets of adjacent edges and sets of edges whose space specifiers need to be resolved. Fences can be easily enough detected during the process of space resolution. I will be looking at what happens with changes of block-progression-direction and the related structures for inline keeps. How does this gel with what other people are doing with keeps? Peter
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