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From: hughes@ux.acs.umn.edu (Steve Hughes)
Newsgroups: comp.sys.mac.hypercard
Subject: Need XCMD to dim screen (like a screen saver)
Message-ID: <3848@ux.acs.umn.edu>
Date: 2 May 91 07:26:22 GMT
Article-I.D.: ux.3848
Sender: news@ux.acs.umn.edu
Organization: University of Minnesota, Academic Computing Services
Lines: 102


"All I want out of life is a simple way to turn the screen off 
instantly, then bring it back on instantly and without flicker."

IUm using HyperCard 2.0 and the picture XCMD to present 256 level 
grey scale photographs as visual stimuli in a series of right-
hemisphere testing neuropsychology experiments, and brother, I got 
trouble. 

In the good ol' days of early April, the Ph.D. candidate whose 
dissertation is riding on this stuff was really happy with how 
things were looking--just loved to see those grey scale photos 
flash onto the computer screen (they appear in pairs, one after 
the other). 

Here is how I did it:

    put "PictureA" into pic1
    put "PictureB" into pic2
    picture pic2,file,rect,false
    picture pic1,file,rect,false
    set the visible of window pic1 to true
    wait for 30 ticks
    set the visible of window pic1 to false
    wait for 30 ticks
    set the visible of window pic2 to true
     wait for 30 ticks
    set the visible of window pic2 to false

My problem: One week before subjects are to be run, she decides 
that the pictures do not appear smoothly enough, too much flicker 
when we set visible to true. The Ph.D. candidate screams, "THE 
PRESENTATION OF STIMULI MUST BE INSTANTANIOUS! HOW COULD WE HAVE 
MISSED THIS? IT'S A DISASTER!" Hmmm...seems all the prior 
literature is based on instant presentation of stimuli (using a 
fast iris called a "tachistascope" mounted on the front of a slide 
projector lens--instant open, instant closed).

Here's what happens: when I set visible to true, the picture sort 
of "rolls" into visibility--presentation is not in sync with the 
vertical retrace of the monitor (or it can't do it fast enough to 
make it in one blanking period). The pictures sure appear fast on 
a IIci, but not without some noticeable flicker. Trouble. 

Facing total ruin, I consider a new career cleaning out public 
lavatories.

 Then: Inspiration! 
I don't need to make the picture XCMD faster--I can create exactly 
the effect I want if I can shut off the whole screen while I set 
the visible of the picture to true, then turn the screen back on 
after I've made the picture visible. I'd do it like this:

    screenOff  <--- XCMD to dim screen to black instantly
    set the visible of window pic1 to true
    screenOn  <--- XCMD to turn screen to white instantly
    wait for 30 ticks
    screenOff
    set the visible of window pic1 to false
    wait for 30 ticks
    set the visible of window pic2 to true
    screenOn 
    wait for 30 ticks
    screenOff
    (etc.)

Can this be done? I think so, screen savers do it all the time. 
I've got AfterDark set to the "Face Away" module with 0 second dim 
time and 0% intensity. "Blip!" screen off. "Blip!" screen on. 
Pictures (and, of course, everything else) appear and disappear 
INSTANTLY. just like I want them to. Different settings make it 
fade slower or to a less dark final brightness. So it looks like I 
CAN turn the screen off and on, instantly and with out flicker.

My other problem:
I have no idea how this is done! Looks like there must be some 
(God willing, fairly easy) way to adjust the screen brightness 
with software (note the Mac Classic's Brightness cdev).
 
Maybe a hardware location? 

Maybe a toolbox call? 

Well? 

Does anybody know anything about this?

-Steve

P.S. Changing the odd and confusing bit depth parameter of the 
picture XCMD does not speed it up (like it should?).


"I'm at the end of the road, my back's against the wall, I'm 
outta' ammo, sweet God Almighty, does _somebody_ know the answer?"

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
	Steve Hughes							
	Pediatric Neuropsychology					
	University of Minnesota						
	hughes@ux.acs.umn.edu						
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++