Moved to Chat because I have nothing lacy to say.
Discussion was of a picture in Lace Express in which the
relief was reversed, so that lace looked like quilting.
[EMAIL PROTECTED] wrote:
But it does beg the question... why does it work that
way? I don't have an answer -
First, it doesn't *beg* the question, it *raises* it.
To "beg the question" is to assume your conclusion,
as in this old joke:
"I saw an angel!"
"How did you know he was an angel?"
"He told me."
"How do you know he wasn't lying?"
"Angels don't lie."
--------------------
The reversal of relief happens because humans have a
hard-wired tendency to assume that light comes from above.
It takes a very strong signal to overcome this tendency,
and even when we can see the light source, things lit from
below look very strange. (Hence the Halloween trick of
holding a light under one's chin in order to look scary.)
When the light in a photograph comes from the top of the
page, a ridge will be light on the side nearer the top of
the page and shadowed on the side nearer the bottom. A
groove will be the other way around.
Our optical system finds reversed relief much easier to
believe than light that comes from below, particularly when
the reversed image makes perfect sense: fine quilting
instead of fine lace in the picture that started this
thread, veins lying on the terrain instead of river valleys
in the case of satellite photographs -- reverse-relief
satellite photographs are very common, because the
convention is to place maps with north at the top, and in
this hemisphere, light nearly always comes from the south.
If the picture makes better sense with the dents dented and
the bumps bumped than the other way around, it may be
possible to freeze it in the correct relief by turning the
page upside down, fixing the correct appearance in your
mind, then turning the page back the right way. But for me,
it snaps back into reversed relief the first time my
attention flickers.
When a picture is lit from the side, the relief may flicker
back and forth, particularly if you don't know whether the
round spots are dents or rivets. Finding a feature that you
are familiar with may make the rest of the picture snap into
focus.
Optical illusions happen because the human optical system
puts out more data than it takes in. The image falling on
the retina is flat; you have to use various tricks to get a
three-dimensional picture of the world out of it -- and
sometimes the simplifying assumptions are wrong.
--
Joy Beeson
http://joybeeson.home.comcast.net/
http://roughsewing.home.comcast.net/
http://n3f.home.comcast.net/ -- Writers' Exchange
west of Fort Wayne, Indiana, U.S.A.
where it's a sunny late-spring day.
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