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Subject: RE: A CGM Question: handling transparency

Hi Lofton,
-----Original Message-----
From: Lofton Henderson [mailto:lofton@rockynet.com]
Sent: Thursday, August 28, 2003 11:59 AM
To: Benoit Bezaire; DULUC Franck
Cc: cgmopen members mailing list
Subject: RE: A CGM Question: handling transparency


At 11:42 AM 8/28/03 -0400, Benoit Bezaire wrote:
Would this do it from an SVG perspective? 

I was thinking of something similar an SVG solution, and in fact one could simulate that SVG solution in WebCGM.  (Your solution:  put the whole picture in a nested 'svg' within a 'defs', then 'use' it over rectangles of different colours).
[Benoit Bezaire] There are several ways of achieving this in SVG... What I did is one approach; you could also do <svg> with <svg>; or use the <image> tag to external <svg> documents etc...  Point being that, what I did, is only one approach, there are others. 

The interesting question here arises about a difference between CGM and SVG.  The CGM standard says that every "Picture" (that which occurs between BegPic and EndPic) is completely defined, including a background of a solid color.  The background color has a default, at least, and can be explicitly set.  So in CGM, the solution has to be something like (in WebCGM) making the background fully (alpha) transparent.

SVG 1.0 ... what is the background?  I.e., if you just have a simple file, <svg>...blah...blah...</svg>, what is the "...blah...blah..." drawn on or over?  You are much more involved with this than I, and I'm drawing a blank on the answer.  Is it well defined in the SVG 1.0 standard, or undefined (i.e., "nothing", transparent), or viewer-dependent?
[Benoit Bezaire] Yes, it is well defined.  A canvas in SVG 1.0/1.0 is always initialized to transparent black.  You can find more info at:
In your example, it is clear that the 'svg' in the 'defs' is being drawn with "undefined/transparent".


-----Original Message-----
From: DULUC Franck [mailto:franck.duluc@airbus.com]
Sent: Thursday, August 28, 2003 4:56 AM
To: 'cgmopen members mailing list'
Subject: RE: A CGM Question: handling transparency

I do not remmember, is this something that is in the DOM CGMOpen is developping.
-----Message d'origine-----
De : DULUC Franck
Envoyé : jeudi 28 août 2003 10:13
À : 'cgmopen mailing list'
Objet : RE: A CGM Question: handling transparency

Dear all,
    Thanks for all the replies I get.
    With the lights brought by your answers, I precise I was actually talking about background transparency, as I am looking at displaying the same graphics on black or white background.
    From the answers I get I understood that this is only available if the viewer/interpreter allows it when rendering; and this is not envisionned by CGM standard..
    For information, it appears also that SVG has not envisionned these features. It implements only the alpha transparency (equivalent to the Escape 45).
Any comments
-----Message d'origine-----
De : DULUC Franck
Envoyé : mercredi 27 août 2003 12:10
À : 'cgmopen mailing list'
Objet : A CGM Question: handling transparency

Dear all,

        I would like to ask to you (users, vendros, CGM experts, ...) some help on understanding the transparency management proposed by CGM
        I I have well understood the standard:
                - The TRANSPARENCY primitive is used to decide wether the Colour specified by AUXILIARY COLOUR primitives with the following behavior:
                        - if TRANSPARENCY is set to off, all primitives (marker, filled area, text, pattern, ...) that needs to, use the AUXILIARY COLOUR for rendering for instance dash line.
                        - if TRANSPARENCY is set to on, all the things to be rendered in the AUXILIARY COLOUR are then rendered as TRANSPARENT (nothing drawn)
        Then, It seems to me that I have the mean to display some of my graphics with transparent (no colour) parts. But I have not found the way to have the background of my graphics rendered as transparent.
        Any help, comment, suggestion will be appreciated.

        Last but not least, what is the tool support level of such features?

Best Regards,

Franck DULUC
SDND - Digital Data Definition and Research
B.P. D0611
316, route de Bayonne
Phone: (33)
Fax: (33)

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