Drawing model (take 2)

[Benoit Bezaire <benoit@itedo.com>]

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.
 
Questions:
 - Is this too much?
 - Is this not enough?
 - What should we say (if anything) about Escape 45 and RGBA/sRGBA?
 - What should we say (if anything) about transparent rasters?
 - What should we say (if anything) about transparent cell arrays?

---

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:benoit@itedo.com

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