ARCHITECTURE.md: Finish documenting the CSS cascade process

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......@@ -69,6 +69,10 @@ appropriate subclass of an `RsvgNode` object, hooks the node to the
DOM tree, and tells the node to set its attributes from the key/value
pairs.
*Through this document we may use **node** and **element**
interchangeably:* a node is the struct we use to represent an SVG/XML
element.
While a node sets its key/value pairs in its `set_atts()` method, it
may encounter an invalid value, for example, a negative width where
only nonnegative ones are allowed. In this case the element may
......@@ -152,6 +156,34 @@ Each node draws itself in the following way:
configure the `cairo::Context`, and call actual Cairo functions to
draw paths/text/etc.
### What about referenced nodes which have a different cascade?
Sometimes, though, the node being considered in the recursive
traversal has to refer to some other node. For example, a shape like
a `rect`angle may reference a `linearGradient` for its `fill`
attribute. In this case, the `rect`'s cascaded values will contain
things like its fill opacity, or its stroke width and color. However,
the `linearGradient` has cascaded values that come from its own place
in the element tree, not from the `rect` that references it (multiple
objects may reference the same gradient; in each case, the gradient
has its own cascade derived only from its ancestors).
In such cases, the code that needs to resolve the referenced node's
CSS properties needs to do this:
* Create a temporary `state` with `rsvg_state_new()`, or grab the
temporary `draw_ctx.get_state()`.
* Call `state::reconstruct(state, node)`. This will walk the tree
from the root directly down to the node, reconstructing the CSS
cascade state for *that* node.
This is a rather ugly special case for elements that are referenced
outside the "normal" recursion used for rendering. We hope to move to
a model where all CSS properties are cascaded first, then bounding
boxes are propagated, and finally all rendering can happen in a single
pass in a fully-resolved tree.
# Comparing floating-point numbers
Librsvg sometimes needs to compute things like "are these points
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