Re: Area tree -> renderer ("pr.fo" for structure renderers)

[Bertrand Delacretaz <bd...@codeconsult.ch>]

On Saturday 16 March 2002 14:52, Peter B. West wrote:
>. . .
>The last stage of the FOP process translates one page
> description (the area tree) into another (the input to the target
> renderer.)

ok

> So why would anyone want to interpose another translation step into
> this tightly coupled arrangement?  Who knows?  
>. . .
For page-based renderers where FOP has to compute the layout (like for 
PDF output), I don't see a need for another translation step either. 
IMHO asking "who knows" is usually a sign that a feature is not needed 
*at this stage of development*...

OTOH, for structure-based renderers like RTF and MIF, I think the only 
parts of FOP that are reusable are the first two stages: parsing and 
property resolution (apart from infrastructure like logging, config, 
etc. which might come from Avalon in the future?).

Last week we had a talk about using XML to communicate between FOP 
pipeline components, but for me this currently would only make sense 
between the "property resolver" and "structure renderers" components.

The deal is being able to reuse FOP's property resolution (p.res) in 
different contexts. I think the following usage scenarios could greatly 
benefit from reusing FOP's front-end (parser + p.res).

In the following I call "pr.fo" an XSL-FO document where all properties 
(fonts, sizes, etc.) are explicitely written, for example when 
inherited from parents:

a) XSL-FO to RTF conversion:
FOP parser -> FOP p.res -> pr.fo -> jfor converter

b) XSL-FO to MIF conversion
FOP parser -> FOP p.res -> pr.fo -> yet-to-write XML-to-MIF converter 

c) automated testing of first FOP stages
FOP parser -> FOP p.res -> pr.fo -> XML testing tool

Keeping in mind that RTF and MIF are formats where the page layout is 
left to the client software to compute (word processor or FrameMaker), 
keeping these converters independent of FOP instead of integrating them 
has several advantages:

b) Helps keep FOP focused on its main task: generating great PDF from 
XSL-FO documents

c) If FOP is ever rewritten in another language for performance, these 
converters, being much less compute-intensive, can stay in java and 
keep the same interface to the FOP components that they use

d) assuming I want to write a MIF converter, basing it on XSL-FO input 
instead of on a FOP API allows me to easily include MIF-specific 
constructs for applications where XSL-FO conformance is not needed but 
precise control of the generated MIF is (often a requirement for MIF 
when producing "half-finished" documents that are typographically 
reviewed before printing).

In conclusion, I think an interface based on XML documents (possibly 
this "pr.fo" discussed above) is the best choice to use between the FOP 
"property resolution" stage and the "structure renderers" like RTF and 
MIF renderers.

OTOH I agree that using XML between the layout and rendering stages 
doesn't make sense at this stage of FOP development.

Due to many other commitments, I don't have time right now (sorry, 
I know you're getting used to hear this) to implement this "pr.fo" 
interface, but if we agree on its usefulness I'll put this high on my 
list and hopefully give it a shot in the next few weeks...

-- 
 Bertrand Delacrétaz (codeconsult.ch, jfor.org)

 buzzwords: XML, java, XSLT, cocoon, mentoring/teaching/coding.
 disclaimer: eternity is very long. mostly towards the end. get ready.

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Re: Area tree -> renderer ("pr.fo" for structure renderers)

[Bertrand Delacretaz <bd...@codeconsult.ch>]

Hi Peter,

On Monday 18 March 2002 22:06, Peter B. West wrote:
>. . .
> There's another gotcha - markers.  The properties in markers are
> resolved relative to the retrieve-marker invocation point.
>. . .

Thanks - I'll keep this in mind when I get to play with this stuff..

-Bertrand

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Re: Area tree -> renderer ("pr.fo" for structure renderers)

["Peter B. West" <pb...@powerup.com.au>]

Bertrand,

There's another gotcha - markers.  The properties in markers are 
resolved relative to the retrieve-marker invocation point.

Peter

Bertrand Delacretaz wrote:

>On Monday 18 March 2002 13:37, Peter B. West wrote:
>
>>The big problem is in defining the p.res step.  How far do you need
>>to go with this?  If you require all of the relative lengths
>>resolved, e.g., you'll have to wait until the layout is done.  The
>>properties are only finalised as the area tree is being constructed. 
>>It's one of the things that makes this all so frustrating.
>>
>
>ok I see.
>I'll try to play with this for RTF rendering based on jfor, to get a 
>feel for how hard/useful this is. 
>
>In case of jfor, what is needed is mostly property inheritance, for 
>which AFAIK rules are well defined in FOP. 
>
>I guess relative lengths will probably stay relative in the RTF code, 
>but I'll have to play with it to be positive about this.
>



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Re: Area tree -> renderer ("pr.fo" for structure renderers)

[Bertrand Delacretaz <bd...@codeconsult.ch>]

On Monday 18 March 2002 13:37, Peter B. West wrote:
>. . .
> Bertrand Delacretaz wrote:
> >In conclusion, I think an interface based on XML documents (possibly
> >this "pr.fo" discussed above) is the best choice to use between the
> > FOP "property resolution" stage and the "structure renderers" like
> > RTF and MIF renderers.
>
> The big problem is in defining the p.res step.  How far do you need
> to go with this?  If you require all of the relative lengths
> resolved, e.g., you'll have to wait until the layout is done.  The
> properties are only finalised as the area tree is being constructed. 
> It's one of the things that makes this all so frustrating.

ok I see.
I'll try to play with this for RTF rendering based on jfor, to get a 
feel for how hard/useful this is. 

In case of jfor, what is needed is mostly property inheritance, for 
which AFAIK rules are well defined in FOP. 

I guess relative lengths will probably stay relative in the RTF code, 
but I'll have to play with it to be positive about this.

-Bertrand

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Re: Area tree -> renderer ("pr.fo" for structure renderers)

["Peter B. West" <pb...@powerup.com.au>]

Bertrand,

See below...

Bertrand Delacretaz wrote:

>On Saturday 16 March 2002 14:52, Peter B. West wrote:
>
>>. . .
>>The last stage of the FOP process translates one page
>>description (the area tree) into another (the input to the target
>>renderer.)
>>
>
>ok
>
>>So why would anyone want to interpose another translation step into
>>this tightly coupled arrangement?  Who knows?  
>>. . .
>>
>For page-based renderers where FOP has to compute the layout (like for 
>PDF output), I don't see a need for another translation step either. 
>IMHO asking "who knows" is usually a sign that a feature is not needed 
>*at this stage of development*...
>
This was purely for rhetorical effect.  My point, developed further in 
the original, was that shoe-horning an xml-based externalisation step in 
here is probably completely redundant.  You have to invent a method of 
expressing all of the information contained in the area tree, while 
flattening the tree into a series of drawing instructions.  If you need 
to externalise the area tree, why not externalise it using an already 
well-defined page description language.  Your externalisation is then
Area Tree -> Well defined PDL

Then attach any translators that you like to the end of that PDL; PDL -> 
PDF, PDL -> awt, PDL -> PCL, whatever.  By the stage of the translation 
to the PDL, XSLFO has done its work.  If someone wants to define a PDL 
in XML, fair enough; but he has fallen off the end of the spec, and is 
creating something new.  At least that's the way I see it.  And if you 
want an already well-defined PDL, with, I assume, a number of converters 
already in place, why not Postscript?  Maybe even PDF.

>
>OTOH, for structure-based renderers like RTF and MIF, I think the only 
>parts of FOP that are reusable are the first two stages: parsing and 
>property resolution (apart from infrastructure like logging, config, 
>etc. which might come from Avalon in the future?).
>
>Last week we had a talk about using XML to communicate between FOP 
>pipeline components, but for me this currently would only make sense 
>between the "property resolver" and "structure renderers" components.
>
>The deal is being able to reuse FOP's property resolution (p.res) in 
>different contexts. I think the following usage scenarios could greatly 
>benefit from reusing FOP's front-end (parser + p.res).
>
>In the following I call "pr.fo" an XSL-FO document where all properties 
>(fonts, sizes, etc.) are explicitely written, for example when 
>inherited from parents:
>
>a) XSL-FO to RTF conversion:
>FOP parser -> FOP p.res -> pr.fo -> jfor converter
>
>b) XSL-FO to MIF conversion
>FOP parser -> FOP p.res -> pr.fo -> yet-to-write XML-to-MIF converter 
>
>c) automated testing of first FOP stages
>FOP parser -> FOP p.res -> pr.fo -> XML testing tool
>
>Keeping in mind that RTF and MIF are formats where the page layout is 
>left to the client software to compute (word processor or FrameMaker), 
>keeping these converters independent of FOP instead of integrating them 
>has several advantages:
>
>b) Helps keep FOP focused on its main task: generating great PDF from 
>XSL-FO documents
>
>c) If FOP is ever rewritten in another language for performance, these 
>converters, being much less compute-intensive, can stay in java and 
>keep the same interface to the FOP components that they use
>
>d) assuming I want to write a MIF converter, basing it on XSL-FO input 
>instead of on a FOP API allows me to easily include MIF-specific 
>constructs for applications where XSL-FO conformance is not needed but 
>precise control of the generated MIF is (often a requirement for MIF 
>when producing "half-finished" documents that are typographically 
>reviewed before printing).
>
>In conclusion, I think an interface based on XML documents (possibly 
>this "pr.fo" discussed above) is the best choice to use between the FOP 
>"property resolution" stage and the "structure renderers" like RTF and 
>MIF renderers.
>
The big problem is in defining the p.res step.  How far do you need to 
go with this?  If you require all of the relative lengths resolved, 
e.g., you'll have to wait until the layout is done.  The properties are 
only finalised as the area tree is being constructed.  It's one of the 
things that makes this all so frustrating.

>OTOH I agree that using XML between the layout and rendering stages 
>doesn't make sense at this stage of FOP development.
>
Ok.

>
>Due to many other commitments, I don't have time right now (sorry, 
>I know you're getting used to hear this) to implement this "pr.fo" 
>interface, but if we agree on its usefulness I'll put this high on my 
>list and hopefully give it a shot in the next few weeks...
>

Peter


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