I'm not sure why this isn't being discussed on www-svg as it seems a more appropriate forum - please feel free to redirect there.
Peter Sorotokin wrote:
At 07:00 PM 2/1/2004 +0100, Chris Lilley wrote:
TD> yes I consider the changes detrimental, the new formulation TD> will be harder to author correctly, and _much_ harder to implement.
So you said. Your comments, along with other folks comments, will of course be taken into account.
I would really like to see the details on why the new formulation is harder to implement and author. It certainly seems much, much easier to me than the old one - on both implementation and authoring side.
The reason it is harder to implement is because these new test attributes have to be made available on every element in SVG. Thus this new 'rendering contextual' behavior must be all the classes supporting graphical elements in SVG. Some of this may be able to be moved into base classes but certainly not all of it.
It may be important to note that one of the main purposes of the
resolution switching code is to avoid the generation of the associated
rendering tree until needed - in hand with this is avoiding fetching
external resources until a particular rendering branch is selected.
So this is not a simple matter of building the entire rendering tree
and just deciding if a particular node should paint it's self or not,
the rendering tree needs to morph as the user navigates. This
add another layer of complexity to the whole 'dirty' region management
because sometimes elements added to the rendering tree are coming
from DOM manipulations and sometimes from render context changes
in the first case you want to generate dirty regions for the next render for the later case you don't want to generate dirty regions
(because the change to render context will cause everything to
repaint anyways).
Add to this issues like a use element that references the same multi-resolution content with different scale factors, or the 'overview' pane in Squiggle (provides a thumbnail view of document for navigation) and this get's extremely complex to manage when it's allowed to happen on any element anywhere.
By having a single element that does resolution dependent switching this complex code is much more centralized. The current proposal essentially requires adding the functionality of this element to every element in SVG.
There are also deeper architectural issues for Batik because it generally builds it's rendering tree up front (this is nice for error handling as you know if the document is good or not before you go to display it). This change would strongly push all implementations to building the rendering tree 'on the fly' for the first rendering.
As to authoring, there are two major issues. First for multi-resolution data you almost always want boundary information tied to the resolution selecting information. The 'special purpose' element can enforce this (in the few cases where you don't really want bounds you can override 'overflow'). By having two independent test attributes it is easy to forget the requiredView attribute - the effect of this would be a general loss of performance something that is easy to miss when all files are local but a big mistake when it goes 'live'.
Secondly because the requiredView attribute doesn't clip - "small" errors in the specification would be _incredibly_ difficult to detect, they would generally show up when panning over the document as 'missing' data pan a little more 'boing' it pops in - ugg!
Also I find it ironic that requiredView is considered a test attribute for 'switch' as in at least one of it's major use cases (tiling content) it can't be used in a switch it must be used outside (otherwise at the seams you would lose one or the other tile).
Incidentally the boundary information error is not just a 'what if' this was exactly one of the bugs I ran into - I changed some stuff that accidentally disabled the viewport testing, everything rendered just fine - it was just a bit slow (I was working with local files), it wasn't until I checked in the debugger and noticed it fetching/drawing all these 'offscreen' high resolution images that I realized why it was slow. I don't know how an end user would detect this, he would likely just think that it was slow. Now this was a bug in my implementation not a bug in content but it made it very clear to me that it is important that content authors not be allowed to easily make the same mistake!
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