Inheritance

[Benoit Bezaire <benoit@itedo.com>]

Hi,

I'm trying to come up with spec wording regarding inheritance.  I
copied the relevant pieces from the CSS specification.  I don't know
yet, if we have a need for the same, more elaborate, or simpler model
than what CSS defines. I tried to adapt the wording to WebCGM. Please
read it and provide comment on the questions I raised (bottom of
email). Also feel free to ask more questions.

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1 Style attributes

Local table of content goes here.

1.1 Specified, computed, and actual values

WebCGM 2.0 user agents are required to support the inheritance model
defined in this section. Once a user agent has loaded a document and
constructed a document tree, it must assign, for every Application
Structure in the tree, a value to every style attributes.

Very similar to the CSS model, the final value of a style attribute is
the result of a four-step calculation: the value is determined through
specification (the "specified value"), then resolved into a value that
is used for inheritance (the "computed value"), then converted into an
absolute value if necessary (the "used value"), and finally
transformed according to the limitations of the local environment (the
"actual value").

1.1.1 Specified values
User agents must first assign a specified value to each style
attributes based on the following mechanisms (in order of precedence):

 1) If the style attribute is assigned a value, use it.
 2) Otherwise, if the style attribute is inherited and the Application
 Structure is not the root of the document tree, use the computed
 value of the parent Application Structure.
 3) Otherwise use the style attributes's initial value. The initial
 value of each property is indicated in the style attribute's
 definition. 
  
1.1.2 Computed values
Specified values are resolved to computed values after the document
tree is create; for example relative units ('%') are computed to
absolute values (i.e., RGB color or NVDC). Computing a value never
requires the user agent to render the document. 

When the specified value is not 'inherit', the computed value of a
style attribute is determined as specified by the Computed Value line
in the definition of the property. See the section on inheritance for the
definition of computed values when the specified value is 'inherit'.   

The computed value exists even when the property doesn't apply, as
defined by the 'Applies To' line. However, some style attributes may
define the computed value of a style attribute for an Application
Structure to depend on whether the style attributes applies to that
Application Structure.

1.1.3 Used values
Computed values can be relative to each other; for example 'hmmm...
need to find one'. The used value is the result of taking the computed
value and resolving these dependencies into a final absolute value
used for the actual display.

1.1.4 Actual values
A used value is in principle the value used for rendering, but a user
agent may not be able to make use of the value in a given environment.
For example, a user agent may only be able to render borders with
integer pixel widths and may therefore have to approximate the
computed width, or the user agent may be forced to use only black and
white shades instead of full colour. The actual value is the used
value after any approximations have been applied.

1.2 Inheritance
Some values are inherited by the children of an element in the
document tree, as described above. Each style attribute defines
whether it is inherited or not. When inheritance occurs, elements
inherit computed values. The computed value from the parent element
becomes both the specified value and the computed value on the child.
 
1.2.1 The 'inherit' value
Each style attribute may also have a specified value of 'inherit',
which means that, for a given element, the property takes the same
computed value as the style attribute for the Application Structure's
parent. The 'inherit' value can be used to strengthen inherited
values, and it can also be used on style attributes that are not
normally inherited.

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Questions:

- Should we rename style attributes to become "properties", a la CSS?
- Should we allow the 'inherit' value? (ex: visibility="on | off |
inherit")
- Do we have the concept of Specified, computed, and actual values in
WebCGM? I'm not convinced we have computed values relative to each
other in WebCGM (ex: can a stroke-weight depend on character-height
which depends on intensity value?)
- I'm missing a line about inheritance and 'grnode'. Actually I think
that what we said about 'grnode' is that they behave 'as if' all its
style attributes were set to 'inherit'.

-- 
 Benoit                 mailto:benoit@itedo.com