Re: Comments on CSS, Level 1, 23-Nov-95

lilley (
Tue, 5 Dec 1995 11:59:39 +0000 (GMT)

H&kon said:
> writes:
> > > (12) bg-light-source - move to level 2 (please don't propose new and
> > > exciting but unimplemented properties for level 1!)
> >
> > I agree that the name is misleading; it does not specify a light source.
> > It specifies a way of interpolating between two colours. Unlike Glenn I
> > think this should (suitably renamed) be in CSS level 1, precisely because
> > it allows those mid blue to dark blue transitions (for example) that are so
> > popular for presentations and which canniot be done in a window-size
> > independent manner in current UAs.
> Yup. Do you have a better name/metaphor?

bg-blend-direction ?

Also it seems you are overloading this to represent two types of blend:

a) linear
b) radial

Problem is, with linear blends you only need one directional specifier as
the other direction is 180 degrees away. With radial blends, you really
need at least a "centre" direction as well, and people will soon want
to place the start point at say 20% from the left margin and 50% down...

I suggest that this property be left in CSS1 (for reasons given in my
previous posting) but limited to linear blends.

> Also, the use of N(orth)
> S(outh) etc to specify a "light-source" (ooops, there is is again) was
> a first attempt.

That part seemed fairly clear.

> Suggestions welcome,

OK, slip this rough draft on for size. I have started off by saying
what it does ;-) then defined how it works. The example (and default)
was chosenn to replicate a widely used background effect and also
to show how the colours blend together (linear interpolation in a
cartesian colour space such as RGB, LAB, or whatever).


Value: N | NW | W | SW | S | SE | E | NE
Initial: S
Example: bg-blend-direction: NW

This property is used to blend two background colors. It specifies the
direction where 'bg2' is the background color at the edge or corner of
the window (UA display area). bg1 is the background color at the
opposite edge or corner.

The values are shorthands for north, north-west, west etc where N is the
top of the window (UA display area). These directions are absolute and
do not depend on the primary writing direction.

The initial value is S, so if bg1 is dark blue and bg2 is light blue,
the window (UA display area) will be dark blue at the top, smoothly
blending through mid blue to light blue at the bottom.

Specifying bg-blend-direction: NW would give light blue in the top-left
corner smoothly blending through mid blue to dark blue in the bottom
right corner.

If only one background color is specified, the blend direction is ignored.

Note: bg-style says:

This property describes how the background image should be laid out. By
default, the background image is repeated in both x and y direction, and
the image is scrolled along with the other content. A 'fixed'
background is fixed with regard to the canvas.

Does this apply to background color as well? If not, what is the
behaviour if the UA cannot display all the document at once? Does it

a) blend the colors over the whole document and then display a part of
that blend, or

b) blend the colors over the current window and then scroll the content
over that fixed background?

Option a requires the total virtual window length to be known before
the background can be computed, giving slow initial display but fast
scrolling. Option b means the background can be computed right away
but the text must be re-composed against the background for scrolling.
I favour b.

Chris Lilley, Technical Author and JISC representative to W3C 
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