# Rounded Corners for images?

Dar Scott dsc at swcp.com
Tue Oct 10 04:16:33 EDT 2006

```On Oct 9, 2006, at 10:32 PM, David Bovill wrote:

> Yes - but it seems a bit fragile. Or  that is it took a lot of
> tinkering to
> set it right, and then I tried it on an existing group which
> continas other
> objects as well as the graphic and image - and try as I can i can"t
> get the
> same effect.

This will depend on the layers.  And remember to set the ink of the
group to something other than srcCopy.

> Is there some documentation on this or can you explain why / what
> grouped
> inks work?

If a group or card has a blend = 0 and an ink that is not srcCopy,
then it has an alpha channel, and then acts as though adding
parentheses in image math.

Here is a math oriented explanation:

Suppose "*" is the binary infix ink operator and it performs the kind
of ink is that associated with the first parameter.  Suppose it is
written like this a*b where a is a src control (or image from a
group) and b is a dst image.  Suppose "card" and "stack" means the
color/pattern of the card and stack, and "black" means a black
image.  A group or card may have opaque false and in that case a
transparent trivial image is used in its place.  Suppose "*" is right
associative, that is a*b*c = a*(b*c).  Suppose objects of layers 3 to
1 are a, b and c.  Then the resulting image like this:

(a*b*c*card*stack)*black      if the card does not have an
alpha channel
((a*b*c*card)*stack)*black    if the card has an alpha channel

Now suppose controls a and b are grouped and the group has an alpha
channel.  The result would be this:

( (a*b*group)*c*card*stack ) * black       if the card does not
have an alpha channel
(( (a*b*group)*c*card)*stack ) * black     if the card does
have an alpha channel

In this case, (a*b*group) is calculated separately to form an image
that is processed with the rest.

The ink operator for the stack is fixed.  If the card does not have
an alpha channel, a control with a transparent section can burn a
hole all the way to black.  Otherwise, it burns a hole only to the
stack background.

Now for something with less math.  It is like gluing pictures and
cards onto a collage.  You work from back to front using the right
kind of glue for each layer.  The group means that you glue some
things together, put them in a clear envelope and then using the
right kind of glue for the group glue that to your collage.

Those of the first batch of the new inks are called "structural" in
the property inspector and are the well-established Porter-Duff image
composition operators.  You can find info on these online.
Revolution Porter-Duff inks are a variant (bug) similar to the
variant (bug) used by Java2D which makes them less useful and can
give surprising results.  I have seen these called clipped Porter-
Duff operators (bugs).  This SVG article points out this variant
(bug) and describes it:

http://www.svgopen.org/2005/papers/abstractsvgopen/index.html#S10

This same article also is a good intro to the P-D operators.

The next lot of inks is similar to the blends available in
Photoshop.  Those are described lots of places.

I hope this helps.  I have trouble explaining things.

Dar

--
**************************************
Dar Scott
Dar Scott Consulting  and  Dar's Lab
8637 Horacio Place NE
Albuquerque, NM 87111

Lab, office, home:  +1 505 299 9497
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http://www.swcp.com/dsc
dsc at swcp.com

Computer programming
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