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SVG 1.2 Comment: about Multiple Pages, Streaming and Progressive Rendering

From: Cyril Concolato <cyril.concolato@enst.fr>
Date: Fri, 05 Nov 2004 10:48:03 +0100
Message-ID: <418B4C53.6030301@enst.fr>
To: www-svg@w3.org

Dear SVG WG,

So far, we have reviewed only parts of the WD (chapters 5, 7 and 8) and would 
like to make some comments on it. Other comments may follow.

Chapter 5: Multiple Pages
-------------------------
General Comment:
It seems to us that most of the features of the page/pageSet elements can be 
emulated using g, set to animate the display, streamedContents on g. Our 
understanding is that the advantage in defining these elements is that it allows 
the user agent to easily identify the pages and provides a mechanism to go from 
one page to another. We believe such a sentence could be useful in the 
specification to understand and guide the author.

Editorial Comments:
* Definition of page: The second sentence (The "ordered list of elements ...") 
seems to be a definition of pageSet and not of page.
* Typo:
    "The pageSet and page elements faciliate streamed animation" ->
       "The pageSet and page elements facili_t_ate streamed animation"
    "pageSeq" -> "pageSe_t_"

Technical Comments:
1) Scoping and discard Rules:
  The mechanism which defines the scoping rules on the page elements and how its 
content may be discarded seems unsufficient for important applications 
(cartoons, slide presentations). The three examples below illustrate such 
applications.
  - Example 1: Cartoon
    Imagine a long cartoon sequence made of several pages. Usually in cartoons, 
many elements are reused in many pages but not all. With the current scoping and 
discard mechanisms, it is impossible to discard these elements, because either 
they would have to be global or repeated in all the pages.
  - Example 2: Navigation
   Consider the following example: Page 1 contains a rectangle and a circle. A 
click on the rectangle displays page 2. A click on the circle jumps to page 3. 
The rectangle and circle are usefull only in the display of page 1 but, 
according to the scoping, they have to be global to handle the beginning of page 
2 and 3. If the scoping rule was relaxed, an explicit discard could be used 
after page 1.
  - Example 3: Reuse of pages:
  Consider the following example:
  <page id="page1">...page 1 content ...</page>
  <page id="page2"><use xlink:href="page1"></page>
  This case which is very frequent in long presentations (power-point like) 
seems to be disallowed. So a way to do this with the current draft would be the 
following:
  <defs>
    <g id="page1content">...page 1 content ...</g>
  </defs>
  <page id="page1"><use xlink:href="page1content"></page>
  <page id="page2"><use xlink:href="page1content"></page>
  Again, this leads to the global definition of all the content of the page 1. 
This is highly inefficient.

2) How to display an object on top of all the pages ? It is not allowed to place 
objects after the last page end tag and before the pageSet end tag.
Proposal: allow SVG elements after the last page end tag and before the pageSet 
end tag.

3) Can there be several pageSet elements in an SVG document ? Seems no since 
nothing can appear between the pageSet end tag and the SVG end tag and since 
only pages can be in pageSet.
=> Proposal: Clarify the fact that only ONE pageSet element is allowed in the 
document.
Consequence: An SVG document using pageSet becomes different from another SVG 
document. It implies a particular timing model and the rendering could be 
improved for this type of content. So, identifying easily this kind of document 
is important and not possible currently.
=> Proposal: Mandate that the pageSet element is the first and only children of 
the svg root. All the the other SVG elements should be placed before the first 
page or after the last page. This way, the SVG viewer can easily determine that 
the document has to be treated in a special way (possibly using streaming).

4) Timing and pages:
  Example: Consider an SVG document containing a global animation of a rectangle 
during 10s and a pageSet with 2 pages: page 1 lasts 5s, page2 lasts 5s. Now 
imagine that the user, viewing page 1, presses page down.
  Question: What happens to the animation of the rectangle ? In other words, how 
is the pageSet timeline linked to the document timeline. What is the 
relationship between the pageSet timeline and the document timeline ?

5) Clarify the last paragraph:
"Once this example has been played and the begin and end times resolved, seeking 
to a page will not reset these values. The element state will reset when the 
parent time container repeats or restarts."
What happens in the following scenario:
The user has clicked quickly (after 0.1 sec) on all pages to reach page 10. Once 
page 10 is viewed, the user seeks back to page 5 (using pageUp). How long will 
page 5 be displayed ?
The fact that the ending of the page is resolved forever (until restart or 
repeat) seems to contradict the SMIL animation specification. The handling of 
begin and end in SMIL uses a list of interval in which an instance of the 
unresolved values is kept even if the interval values have been resolved once.

Chapter 7: Streaming
--------------------
  6) General Comment:
  There seems to be a strong relationship between streaming and progressive 
rendering in the WD. A paragraph clarifying this relationship is needed.

  7) "Streamed Content" : What is the definition of this term ? What is the 
notion of streaming ? What is an SVG stream ? We believe it is important to 
define a streaming model. Such a model should define what is the stream, what it 
is made of, the timing associated to the parts of the stream (presentation time 
stamp, reception time stamp, parsing time stamp)

  8) streamedContents:
    - What are the elements on which this attribute is allowed. On the svg root 
only ? On the time container ? The RNG does not specify it.
    - The specification of 'discard' is completely unclear:
        In the sentence:
        "The contents of the document may be discarded as they are used. It is 
not a requirement that the contents are discarded."
        Why does it refer to 'the document' ? Shouldn't it be 'The contents of 
the element to which the attribute belongs, may be discarded as they are used. 
It is not a requirement that the contents are discarded.'
        Moreover, what does "as they are used" mean?

  9) As we suggested in the comments on the Multiple Page, we believe the 
discard mechanism is underspecified. We propose to remove the streamedContents 
attribute, to rename it to 'discard' whose type would be the same as the 'begin' 
attribute.
      discard
         Value: = "never" | offset-value | syncbase-value | event-value | 
accessKey-value | wallclock-sync-value
         Initial: 	 "never"
         Applies to: 	all elements
         Inherited: 	no
         Percentages: 	N/A
         Animatable: 	no
     Using this definition may mean that the document become in error in a way 
difficult to foresee, but it gives a lot more flexibility.
      An alternative would be to define an element with the following syntax
     <discard xlink:href="elementtobediscarded" begin="page1.end"/>

   10) timelineBegin attribute:
   This notion is not linked to the streaming but also applies in progressive 
rendering cases, therefore it should be in the progressive rendering section.

Chapter 8: Progressive Rendering
--------------------------------
General Comments:
11) This chapter is quite unclear with respect to the Streaming Chapter. It 
should be clarified when a progressive rendering should be done, if the author 
has any possibility to indicate that he/she wants a progressive rendering. For 
instance, it seems important to write that streaming of SVG should imply 
Progressive Rendering. In particular, if the timelineBegin attribute is set to 
onStart, then progressive rendering should be turned on.
12) This chapter seems to imply that progressive rendering can only be done when 
a SAX parser is used. This is very very restrictive. An ad-hoc mechanism could 
update the tree without using SAX.
13) The following sentence: "The user agent updates the rendering following each 
startElement and/or endElement SAX event." implies an important overhead in the 
rendering phase. If the reading rate is high this would mean updating the 
rendering with a rate higher than usual (30fps ...).

Best regards,

Cyril Concolato, Jean-Claude Moissinac

-- 
Ecole Nationale Supérieure des Télécommunications, Paris
46, rue Barrault 75013 Paris
Received on Friday, 5 November 2004 09:48:05 UTC

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