Re: [cgmo-webcgm] Drawing model

[Benoit Bezaire <benoit@itedo.com>]

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

This e-mail and any attachments are confidential and may be protected
by legal privilege. If you are not the intended recipient, be aware
that any disclosure, copying, distribution or use of this e-mail or
any attachment is prohibited. If you have received this e-mail in
error, please notify us immediately by returning it to the sender and
delete this copy from your system. Thank you for your cooperation. 


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.