One additional issue:
Where are the two implementations for each of these features that we will
need?
We don't have one.
> -----Original Message-----
> From: Lofton Henderson [mailto:]
> Sent: Thursday, April 20, 2006 5:34 PM
> To: Benoit Bezaire;
> Subject: Re: [cgmo-webcgm] Drawing model (take 2)
>
> [...2 of 2...]
>
> Thanks to Benoit for a thorough job on this!
>
> My opinions are embedded at Benoit's questions. Can we hear
> from others as well, especially *implementors*.
>
> At 11:57 AM 4/19/2006 -0400, Benoit Bezaire wrote:
> >Here is an attempt to address the Drawing model comment:
> "Color space
> >for compositing not described, is alpha linear or not (maybe
> described
> >later?) What is the influence of grnode on compositing - grouping
> >complexifies compositing, neds to be carefully described."
> >
> >This could be a replacement or in addition to 2.2.2 Drawing model.
> >
> >Modifications:
> > - color values are premultiplied.
> > - addition of three simple examples.
> > - typos in last paragraph.
>
> I'm basically repeating what I said on the call...
>
> This stuff has been sitting underspecified and unimplemented
> for 6 years. My basic instinct is: the less we say, the
> better. Or rather, maybe it isn't "better', but IMO there is
> less the chances of getting dragged into a complex issue and
> having to agree on a bunch of new specifications and write
> them into 2.0 (probably would be errata [clarifications] on
> 1.0 as well).
>
> >
> >Questions:
> > - Is this too much?
>
> I wish it could be shorter, but on the other hand I don't see
> what to cut. Examples? Could the 10 lines of equations be
> consolidated somehow? Are those fairly conventional and
> uncontroversial (except for my previous question about possible bug)?
>
> > - Is this not enough?
>
> I think it's enough -- just answers his questions, plus a little more.
>
> > - What should we say (if anything) about Escape 45 and RGBA/sRGBA?
>
> I'm somewhat in favor of saying nothing until/unless asked.
>
> Alternatively, we could say that it was the intention of the
> original authors that you use one or the other, but not both
> together, therefore using both together would be prohibited
> (and that would also be a 1.0 erratum). It sounded yesterday
> like everyone agreed about that.
>
> The only problem, why this isn't my first choice, is ... once
> we go there, we start to potentially open lots of doors and
> end up having to specify how all of the bits work together
> (or don't). Which is something of a waste, as no one has
> implemented Esc45, or RGBA.
>
> > - What should we say (if anything) about transparent rasters?
>
> You mean transparency within the PNG data, as opposed to
> Transparent Cell Colour of CGM. (I don't know how the PNG
> transparency is defined. Is it an Alpha channel?)
>
> Like before, my preference is saying nothing until/unless asked.
>
> > - What should we say (if anything) about transparent cell arrays?
>
> (And it gets uglier, when combined with potential for Esc.45
> and RGBA in the cell colours.)
>
> Like before, my preference is saying nothing until/unless asked.
>
> I could be convinced that it's NOT a good idea for us to try
> to remain mute on these questions. I would listen to a
> specific solution. That solution could be as simple as
> something like, "These functionalities [there are something
> like a half-dozen?] were designed to be used individually,
> and not in combination." Then what? Choice: "...their use
> in combination is prohibited in this version of WebCGM"; or,
> "the effect of their use in combination is undefined in this
> version of WebCGM, and therefore it is strongly discouraged
> (may be prohibited in future version)."
>
> The problem again ... as soon as we say something simple, we
> need to look at the 6x6 matrix of combinations and see if the
> statement makes sense. Then we start arguing about simple
> exceptions (combinations), ...etc...
>
> Enough for now. Other thoughts?
>
> -Lofton.
>
>
> >Implementations of WebCGM are expected to behave as though they
> >implement a drawing model corresponding to the one described
> below. A
> >real implementation is not required to implement the model
> in this way,
> >but the result on any device supported by the implementation shall
> >match that described by this model.
> >
> >WebCGM uses a painters model of rendering. Colors are applied in
> >successive operations to the output device. When an area overlaps a
> >previously colored area the new color partially or
> completely obscures
> >the old. When the color is not completely opaque the result on the
> >output device is defined by the following (mathematical) rules for
> >compositing (all color values use premultiplied alpha):
> >
> >Pr, Pg, Pb - Primitive color value
> >Pa - Primitive alpha value
> >Cr, Cg, Cb - Canvas color value (before blending)
> >Ca - Canvas alpha value (before blending)
> >Cr', Cg', Cb' - Canvas color value (after blending)
> >Ca' - Canvas alpha value (after blending)
> >Ca' = 1 - (1 - Pa) * (1 - Ca)
> >Cr' = (1 - Pa) * Cr + Pr
> >Cg' = (1 - Pa) * Cg + Pg
> >Cb' = (1 - Pa) * Cb + Pb
> >
> >Example #1:
> >Primitive is semi transparent red: (Pa, Pr, Pg, Pb) = (0.5,0.5,0,0)
> >Canvas is opaque black: (Ca, Cr, Cg, Cb) = (1,0,0,0) Ca' = 1 - (1 -
> >0.5) * (1 - 1) = 1 Cr' = (1 - 0.5) * 0 + 0.5 = 0.5 Cg' = (1
> - 0.5) * 0
> >+ 0 = 0 Cb' = (1 - 0.5) * 0 + 0 = 0 The result is an opaque dark red
> >(1,0.5,0,0).
> >
> >Example #2:
> >Primitive is opaque red: (Pa, Pr, Pg, Pb) = (1,1,0,0) Canvas is semi
> >transparent black: (Ca, Cr, Cg, Cb) = (0.5,0,0,0) Ca' = 1 -
> (1 - 1) *
> >(1 - 0.5) = 1 Cr' = (1 - 1) * 0 + 1 = 1 Cg' = (1 - 1) * 0 +
> 0 = 0 Cb' =
> >(1 - 1) * 0 + 0 = 0 The result is an opaque red (1,1,0,0).
> >
> >Example #3:
> >Primitive is semi transparent red: (Pa, Pr, Pg, Pb) = (0.5,0.5,0,0)
> >Canvas is semi transparent black: (Ca, Cr, Cg, Cb) =
> (0.5,0,0,0) Ca' =
> >1 - (1 - 0.5) * (1 - 0.5) = 0.75 Cr' = (1 - 0.5) * 0 + 0.5 =
> 0.5 Cg' =
> >(1 - 0.5) * 0 + 0 = 0 Cb' = (1 - 0.5) * 0 + 0 = 0 The result is a
> >transparent dark red (0.75,0.5,0,0).
> >
> >Alpha compositing is performed in the current COLOUR MODEL
> (see T.16.19).
> >
> >Primitives in a WebCGM document have an implicit drawing order, with
> >the first primitives in the WebCGM document getting drawn first.
> >Subsequent primitives are drawn on top of previously drawn
> primitives.
> >
> >Primitives which have a value for Escape 45 other than fully opaque,
> >have the effect of producing a temporary separate canvas
> initialized to
> >transparent black onto which the primitive is drawn. The
> canvas is then
> >composited into the background, taking into account the Escape
> >45 value. The presence of APS in the primitive list has no effect on
> >the rendering. No temporary canvas are created. It is
> identical to the
> >case of no APS.
> >
> >--
> > Benoit mailto:
> >
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