#1587 image put errors on {} color

[Jeffrey Hobbs]

image put dies when passed "" as a color, although this
can be used in various other places in Tk to mean
an "empty" color or transparent color. image put should
do the same, so you can pass in large color arrays that
don't specifically have to set a color for all pixels.

[Donal K. Fellows]

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Q1: Does this require a TIP to fix?
Q2: Should the empty string indicate that a pixel is to be
ignored or that a pixel is to be made transparent?

[Donal K. Fellows]

Patch

[Jeffrey Hobbs]

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1. It requires a TIP if you view it as not a bug.
2. This may justify it more as TIP-worthy.

[Donal K. Fellows]

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I've been thinking about this a bit more, and I feel that
the empty string ought to represent a transparent pixel, and
not a pixel that should be ignored when writing into the
image. The idea is that you are putting a blank into the image.

OTOH, I might do a TIP for all this anyway so that I can add
a syntax for specifying non-trivial transparency at the same
time.

[Eric Boudaillier]

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Donal,

Could you look at the last part of FR 557090 before TIP'ing,
dealing with "xcolor@alpha" syntax ?
Then, setting transparent pixel could be done with @0, and ""
could denote a "do nothing" behaviour.

[Donal K. Fellows]

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It's *extremely* difficult to get both @0 and "" supported
at the same time with different behaviour as you suggest.
The problem is that setting a pixel to be transparent
requires one kind of compositing rule (set) and ignoring a
pixel requires another (overlay).

[Eric Boudaillier]

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You're right, I missed that.
So, let "" raise an error and @xx deal with alpha ;)

[Donal K. Fellows]

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Wrong answer. Make the empty string work and make @xx throw
an error instead. Which is as far as I could see myself
going without a TIP (since arguably the empty string is
already interpreted in Tk as transparent and alpha support
is currently abysmal.)

[Donal K. Fellows]

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I'm going to do this in TIP#166 http://purl.org/tcl/tip/166
as I want more complete control than just "{} ==
transparent" anyway.

[Eric Boudaillier]

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Thank you Donal.