84aeb6c80b
- Rename all .txt doc files to .rst to make it more clear that they are restructured text. - Add requirements.txt for Sphinx - Rerun sphinx-quickstart to make compatible with the latest Sphinx version. - Fix module names from gaphor to gaphas - Fix warnings about incorrect indent following :mod: lines - Fix TypeError for class Segmant: function object has no attribute '__mro__' Signed-off-by: Dan Yeaw <dan@yeaw.me>
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2.0 KiB
ReStructuredText
63 lines
2.0 KiB
ReStructuredText
Constraints
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===========
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Introduction
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------------
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There are problems related to canvas items, which can be solved in
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`declarative way <http://en.wikipedia.org/wiki/Declarative_programming>`_
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allowing for simpler and less error prone implementation of canvas item.
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For example, if an item should be a rectangle, then it could be declared
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that
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- bottom-right vertex should be below and on the right side of top-left
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vertex
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- two top rectangle vertices should be always at the same y-axis
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- two left rectangle vertices should be always at the same x-axis
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- ...
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Above rules are constraints, which need to be applied to a rectangular
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item. The rules can be satisfied (constraints can be solved) using
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`constraint solver <http://en.wikipedia.org/wiki/Constraint_satisfaction_problem>`_.
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Gaphas implements its own constraint solver (`gaphas.solver` module).
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Items can be constrained using APIs defined in `Canvas` and `Item` classes.
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Constraints API
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---------------
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The `Canvas` class constraints API supports adding a constraint to
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constraint solver. Instance of a constraint has to be created and then
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added using `Canvas.add_constraint` method. For example, it allows to
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declare that two variables should be equal.
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The `Item` class constraint API is more abstract, it allows to constraint
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positions, i.e.
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- positions of two item handles should be on the same x-axis
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- position should be always on a line
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If this API does not provide some constraint declaration, then one can
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fallback to `Canvas` class constraint API.
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Examples
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--------
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Item API
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^^^^^^^^
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Canvas API
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^^^^^^^^^^
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Further Reading
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---------------
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Theory and examples related to constraint solving
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- http://en.wikipedia.org/wiki/Declarative_programming
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- http://en.wikipedia.org/wiki/Constraint_satisfaction_problem
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- http://norvig.com/sudoku.html
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There are other projects providing constraint solvers
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- http://adaptagrams.sourceforge.net/
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- http://minion.sourceforge.net/
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- http://labix.org/python-constraint
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- http://www.cs.washington.edu/research/constraints/cassowary/
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