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Index › Programming Questions & Help › Syntax Questions › sinus discontinuity - update
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sinus discontinuity - update (Read 870 times)
pljvp
sinus discontinuity - update
Nov 3rd, 2006, 10:48am  
Hi,

I'm coverting a sinus to a basic binary pattern. This works fine, only for a large wavelength.
Drawing a smaller sinus results in a strange pattern. I would expect greenpixel-whitepixel-greenpixel-whitepixel..., but that's not what's happening...

How come the sinus isn't continuous for a wavelength of 2 pixels?

http://www.wereldopener.nl/sinus/index.html
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« Last Edit: Nov 3rd, 2006, 7:07pm by pljvp »  
 
movax
Re: sinus discontinuity - update
Reply #1 - Nov 4th, 2006, 12:33am  
I beleive the answer is rounding.  Once you get to the limit of screen resolution the results are kinda unpredictable.  If your wavelength is exactly 2 pixels, it will show black,white,... but if your wavelength is even 1.9999 pixels, the results could be something else.

If you had an infinite resolution display, it would always look perfect.
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pljvp
Re: sinus discontinuity - update
Reply #2 - Nov 4th, 2006, 2:43pm  
Thank you for replying...

Your answer triggered a thought: could it be a sampling problem as well?
If I sample just before or just after Pi/2 instead of right on target, it would also explain this interference.
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WebDext
Re: sinus discontinuity - update
Reply #3 - Nov 7th, 2006, 10:00am  
Looks like a float to int conversion truncation error to me, perhaps the way your rendering your patterns? I'd have to see the code to be sure.

Jack
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WebDext
Re: sinus discontinuity - update
Reply #4 - Nov 7th, 2006, 10:19am  
Yep just checked out your programs source, just change maxline and mySlider to floating point values and you'll get the output you expect, the problem is the remainder is being dropped off in the devide your doing of the two ints.

Another option is to cast the values but that is slower.

Jack
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pljvp
Re: sinus discontinuity - update
Reply #5 - Nov 8th, 2006, 12:25pm  
Jack, thanks for crawling through my code.

I'm wondering whether the integers (slider & maxline) are the only problem.

I've scaled up the amplitude and added the following to illustrate

   line(pixnum, (myheight/2)+7*myheight, pixnum, (myheight/2)+7*myheight - (myheight/3)*(sin(pixnum*(TWO_PI/(myheight/12)))));

This function doesn't use the 2 integers. The corresponding sinus however isn't a continuous pattern either.

I also tried your suggestion, but changing the slider & maxline to float requires some additional tweaking of the scrollbarcode. I believe it's wise to look at the basic ;) problem with the sinus, before tweaking the scrollbar to match my wishes...

Any idea?



//pieter
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« Last Edit: Nov 10th, 2006, 3:33pm by pljvp »  
 
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