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Subject: Re: [cgmo-webcgm] Drawing model

I already got feedback about this (from Dieter). I forgot to mention
that the equations use pre-multiplied values (sorry!). I'll add that
and also add some examples shortly.

 Benoit   mailto:benoit@itedo.com

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Wednesday, April 19, 2006, 9:43:13 AM, you 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.

> Thoughts?

> ---

> 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:

> 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

> 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 primitives are 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.

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