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Subject: Action item: New 5.4 section
Here is the new section 5.4 of the specification. I had an action item
to re-write it...
Some comments about the new section:
- The old one was referencing to CSS 2.1 (but that's only in last
call, we can't reference it)
- There's a conflict with 'inherit' on Style Properties and attempting
to inherit Specified Values! 'inherit' never gets resolve if you
inherit specified values...
- For the wording to work, I think we need to say that Computed Values
are inherited but that the 'Computed Value line' can define how the
computed value is resolve.
---
5.4 Inheritance of APS Attributes and of Style Properties
This section describes how @@APS attributes@@ are inherited in a
WebCGM structure tree. It also describes how @@Style Properties@@ are
inherited, which is similar but differs in some key details. The
inheritance models are based closely on the inheritance model of @@CSS
2.0@@. Some details have been adapted to the particulars of the WebCGM
format. This chapter is the normative reference for inheritance of
@@APS Attributes@@ and of @@Style Properties@@ in WebCGM.
5.4.1 Specified, computed, and actual values of Style Properties
WebCGM 2.0 user agents are required to support the inheritance model
of Style Properties 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
Properties.
Very similar to the CSS model, the final value of a Style Properties is
the result of a four-step calculation: the value is determined through
specification (the "Specified Value"), then resolved into a value that
may be 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").
5.4.1.1 Specified Values of Style Properties
User agents must first assign a Specified Value to each Style
Properties based on the following mechanisms (in order of precedence):
1. If the Style Property is assigned a value (via a DOM call, or via
an XCF), use it.
2. Otherwise, if the Style Property is inherited (i.e., its
definition includes "Inherited:yes") 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 Property's Initial Value. The Initial
Value of each Property is indicated in the Style Property's
definition.
Note: In the context of WebCGM inheritance, the root of the document
tree is the Picture node.
5.4.1.2 Computed Values of Style Properties
Specified Values are resolved to Computed Values after the document
tree is created; for example relative units ('%') are computed to
absolute values, that is mostly likely into RGB colors or NVDC values.
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 Property is determined as specified by the Computed Value line
in the definition of the Property. The 'inherit' keyword must be
replaced according to its definition. 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.
5.4.1.3 Used Values of Style Properties
In the CSS2 model from which the WebCGM model is derived, Computed
Values can be relative to each other; for example a width could be set
as a percentage, which is dependent on the containing block's width.
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. In this version of WebCGM, there are no examples where
Used Value differs from the Computed Value. This may change in a
future version of the specification.
5.4.1.4 Actual Values of Style Properties
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.
5.4.2 Specified, computed, and actual values of Application Structure
Attributes
WebCGM 2.0 user agents are required to support the inheritance model
of Application Structure Attributes defined in this section. Once a
user agent has loaded a document and constructed a document tree, it
must resolve, for every Application Structure in the tree, if an
Attribute has a value (i.e., no value is possible for some attributes).
Very similar to the CSS model, the final value of a Style Properties is
the result of a four-step calculation: the value is determined through
specification (the "Specified Value"), then resolved into a value that
may be 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").
5.4.1.1 Specified Values of Application Structure Attributes
User agents must first assign a Specified Value to each Style
Properties based on the following mechanisms (in order of precedence):
1. If the Attribute is assigned a value in the CGM document or
assigned a new value via a DOM call or an XCF, use it.
2. Otherwise, if the Attribute is inherited (i.e., its
definition includes "Inherited:yes") 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 Attribute's Initial Value. The Initial
Value of each Attribute is indicated in the Attribute's
definition.
Note: In the context of WebCGM inheritance, the root of the document
tree is the Picture node.
5.4.2.2 Computed Values of Application Structure Attributes
In this specification, Computed Values of Application Structure
Attributes, with the exception of the 'inherit' value are identical to
the Specified Values. The 'inherit' keyword must be replaced according
to its definition. See the @@section on inheritance@@ for the
definition of computed values when the specified value is 'inherit'.
The Computed Value exists even when the Attribute doesn't apply, as
defined by the 'Applies To' line.
5.4.2.3 Used Values of Application Structure Attributes
In the CSS2 model from which the WebCGM model is derived, Computed
Values can be relative to each other; for example a width could be set
as a percentage, which is dependent on the containing block's width.
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. In this version of WebCGM, there are no examples where
Used Value differs from the Computed Value. This may change in a
future version of the specification.
5.4.2.4 Actual Values of Application Structure Attributes
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. In this version of
WebCGM, there are no examples where Actual Value differs from the
Actual Value. This may change in a future version of the
specification.
5.4.3 Inheritance
Some values are inherited by the children of an Application Structure
in the document tree, as described above. Each Style Property and
Application Structure Attribute defines whether it is inherited or
not. As a general rule, when inheritance occurs, Application
Structures inherit Computed values of Style Properties and Application
Structure Attributes (unless implicitly stated in the Property or
Attribute definition).
5.4.3.1 The 'inherit' value
All Style Properties as well as the 'visibility' and 'interactivity'
Application Structure Attribute may also have a Specified Value of
'inherit'; which means that, for a given Application Structure, the
Property or Attribute takes the same Computed value of the Application
Structure's parent. The 'inherit' value can be used to strengthen
inherited values, and it can also be used on Style Properties that are
not normally inherited.
--
Benoit mailto:benoit@itedo.com
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