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Re: [css-shapes] shape-image-threshold should be clearer about >= vs >

From: Alan Stearns <stearns@adobe.com>
Date: Wed, 11 Sep 2013 08:43:52 -0700
To: "L. David Baron" <dbaron@dbaron.org>, "www-style@w3.org" <www-style@w3.org>
Message-ID: <CE565952.3E445%stearns@adobe.com>
On 9/11/13 8:22 AM, "L. David Baron" <dbaron@dbaron.org> wrote:

>http://dev.w3.org/csswg/css-shapes/#shape-image-threshold-property
>says:
>  # <alphavalue>
>  #   A <number> value used to set the threshold used for extracting
>  #   a shape from an image. Any values outside the range 0.0 (fully
>  #   transparent) to 1.0 (fully opaque) will be clamped to this
>  #   range. 
>
>which isn't clear about whether you're looking for pixels that are
>greater than ( > ) the alphavalue or greater than or equal ( >= ) to
>the alphavalue.
>
>There is an example earlier:
>  # A value of 0.5 means that the shape will enclose all the pixels
>  # that are more than 50% opaque.
>which suggests that it's greater than ( > ).

It is greater than ( > ). This is defined in the first paragraph of
section 3.3. I'll add it to the <alphavalue> definition to make sure
there's no possible misinterpretation.

>
>I tend to think it's actually preferable for it to be a >=, because
>then 0.0 is useful as a default value that has the current behavior
>(no image shape threshold) since all image pixels have an opacity
>that are >= 0.

The initial value as 0.0 is useful because that will construct a shape
that encloses all of the pixels that are not fully transparent, which is
the basic use case for a shape from alpha data. A shape constructed from
all of the image pixels isn't any different than a rectangle enclosing the
image's content area, so it would not be a useful default.

>
>I think the alternative ( > ) doesn't seem as useful because:
> * using >, the 1.0 value doesn't seem useful (no shape, always)

Either 1.0 or 0.0 is going to be strange, depending on whether we pick >
or >=. I chose to have 1.0 be strange (no shape). The other way forces an
author to choose some amount of precision in a 0.0001 value to enclose all
of the not-fully-transparent pixels.

> * the Initial value doesn't describe the current behavior, so
>   another value would be needed

The intended behavior is for the 0.0 initial value to enclose all of the
pixels that are not fully transparent. I want that to be the behavior of
the initial value, and I don't think picking an arbitrary amount of
decimals for a 0.0001 initial value is a good idea.

Thanks,

Alan
Received on Wednesday, 11 September 2013 15:44:20 UTC

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