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The policy scripts are compiled together to create a single executable
policy. The policy can act on the multimedia object with which
it is associated through the various interfaces discussed earlier:
transformation, reduction, scaling and compression.
When the policy is activated (asked to transform its media), it goes
through a number of parsing and resolution phases. These phases
determine which rules are relevant to the associated multimedia
object, which rules can be removed or overridden by others and they
establish a definitive precedence order between rules from different
policy sources.
Initially any rule which applies to media other than that of the
attached multimedia object are removed. Then rules which clash are
resolved according to the following criteria:
- user rules have precedence over authorial and default rules,
except when 3. or 4. is in place,
- authorial rule have precedence over default rules, except when
3. is in place.
- user and authorial rules can specify that they submit to default
rules.
- user rules can specify that they submit to authorial rules.
Four phases of multi-pass rule activation then take place:
- Transformation
- involves firing rules that transform the
multimedia object from one type in the media hierarchy (figure
2). Rules that utilise the transformation
interface are key to the writing of policies that cope with
difficult display attributes and location dependant data.
- Reduction
- involves firing rules that utilise the reduce
interface; thus reducing the quality of the attached multimedia
object and so improving down load time. The reduction phase is a
multi-pass operation. Each pass of the rules further reduces the
multimedia object. Passes are repeated until no rule is fired, that
is, all of the media's size and quality requirements are met. Of
course this may never happen! So, the multi-pass mechanism is
constrained by a set of heuristics which can identify looping, the
exhaustive limits of compression and rules which reduce the
multimedia object to the edge of our perception.
- Scaling
- is the process of altering the dimension of the Media.
Images are scaled in the X,Y dimension as are Video (although they
also have the frames per second dimension). For some media types
scaling is not meaningful or takes on a different meaning. For
example it is not meaning to scale ASCII and it only meaning to
scale audio in terms of amplitude or tone - qualities that can be
adjusted to suit the user, but do not effect the download time.
- compression
- is a simple one-pass treatment of using non-lossy
compression where possible. Media that benefit from this are those
which, unlike JPEG and MPEG, do not support their own compression
technique. Anything under the mime type mime:text/* can
benefit from the use of non-destructive compression
.
This multi-phase multi-pass process of resolution is at the heart of
what makes the policy work. It combines the disparate needs of user,
author and the system designers.
Next: Setting the Fixed Trajectories:
Up: Run-time Policies: The Media
Previous: Run-time Policies: The Media
Malcolm McIlhagga
Thursday June 11 16:17:19 BST 1998