[guide-user] Blinking

Bill J. Gray Jan 11, 2000

Quite a few people now have seen the odd blinking in Charon (blink
images are loaded upside down). I tracked it down eventually; it
turned out to be a side effect of a recent improvement, one that causes
Charon to usually load an image north up. In some cases, subsequent
images confused it enough to produce just the errors reported.

Fixing this was not nearly as straightforward as you might expect,
for reasons I won't bore you with. I eventually decided that the time
had come to implement blinking 'the right way', as described in my
previous e-mail: by loading blink images without attempting to distort
them, and with Charon figuring out that "in blinking these two images,
we have to offset by (say) 3.2 pixels in x and 1.8 pixels in y to get
them to match."

Getting _that_ to work, in turn, required a slight change in the
IMAGES.DAT file (an extra line to store plate constants, stored in
a way that will cause Guide to ignore that line, so it's backward-
compatible.) This means that your existing images in IMAGES.DAT won't
blink. You'll have to re-load them into Charon, and _then_ you can
blink them.

The result of this is that there is a Charon update, stored at

http://www.projectpluto.com/charon2.zip

I've not replaced the 'usual' update, charon.zip, because the above
is the result of a long day or so of work. I'll wait until a few people
tell me they tried charon2.zip before moving it to the 'usual' place and
announcing it on the Web page.

On the positive side, the code that handles blinking actually makes
some sense now (as will be apparent when I update the source code on the
Web site.) Future improvements should be far less frightening for me to
consider, and those using my source code are less likely to wonder what
madman came up with the algorithms for blinking images.

-- Bill