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Re: opacity clarification

From: Jon Ferraiolo <jferraio@Adobe.COM>
Date: Mon, 09 Aug 1999 11:35:02 -0700
Message-Id: <199908091831.LAA03300@mail-345.corp.Adobe.COM>
To: Craig Brown <cmb@research.canon.com.au>
Cc: www-svg@w3.org

At 10:51 AM 8/1/99 +1000, Craig Brown wrote:
>I might have missed it but is blended opacity possible?

I'm not sure what you mean by blended opacity. SVG offers masks, which
provide lots of blending flexibility.

>In the past - I have implemented the opacity as part of the
>color (with a default of opaque) and not a seperate attribute.
>Was this considered?

There is more flexibility if you disassociate opacity from color. For
example, you can more easily animate an object going from totally
transparent to opaque if the opacity value is separate. Also, SVG offers
advanced compositing features such as masks. Object opacity can be thought
of as a mask with the same opacity value at each pixel. (See the
description of the rendering model in chapter 4, particularly section 4.7.)
To make object opacity consistent with the advanced compositing features
such as masks, it needed to be separated from color.

>Also - I don't see the point of a opacity for the fill, the
>stroke and a third for the entire object.  Why was this done?

The most important property will be object opacity (the 'opacity'
property). Fill opacity and stroke opacity offer some additional
flexibility (e.g., a semi-transparent fill covered by an opaque stroke),
which is desirable in some cases. There are existing authoring products
will the ability to set fill opacity and stroke opacity and we wanted to
accommodate these authoring products.

Maybe there were other reasons, but that is all that comes to mind now.

Jon Ferraiolo
SVG editor
Adobe Systems Incorporated
Received on Monday, 9 August 1999 14:32:01 UTC

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