LOGO-L> Erasing LABELed text

[Yehuda Katz <yehuka@beitberl.ac.il>]

Hello,

Sometimes it's desirable to erase a LABELed text from the screen, e.g.,
when creating the digital part in an analogical-digital watch. CS is no
help here, as it erases EVERYTHING on the screen, while we want
SELECTIVE erasure. 

Erasing LABELed text is done by re-LABELing the text *precisely* on the
already LABELed text, but this time in the background color. This can be
accomplished in UCBLogo by:
	SETPC BG 
	LABEL [your text]

and in MSWLogo by saying:
	SETPC SCREENCOLOR
	LABEL [your text]

After it you need, of course, restore the original pen color, for the
rest of your program.

As an illustration, I wrote the following program:

===================================================
to main ; blink
cs ht
draw
blink[Welcome to Logo]
end

to draw
setpc[0 0 0]
repeat 6[repeat 6[fd 50 rt 60] rt 60]
end

to blink :txt
pu setpos[-85 -110] pd seth 90     ; LABEL location
setpc [255 0 0]   label :txt wait 30 ; red pencolor
setpc screencolor label :txt wait 30 ; screen color
blink :txt
end
===================================================

Run it: That's an endless loop; to stop it hit Ctrl+Q or Ctrl+Break (in
UCBLogo), or push HALT in MSWLogo.

The "trick" for erasing LABELed text is undocumented, maybe because the
designers found it self evident.

Incidently, George might want to add a digital watch upon his analogical
demo CLOCK.LGO.

Enjoy it,

[[Yehuda]]


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