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future
Joined: 14 May 2004
Posts: 330
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| Interpolation and casting problem. |
 Posted: Fri Jun 18, 2004 7:46 pm |
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My project needs the use of an interpolation routine..
PIC 18F452.
| Code: | #include <18f452.h>
#opt 10
#zero_ram
#use rs232(baud=38400,parity=n,xmit=pin_c6,rcv=pin_c7,bits=8)
#fuses wdt,wdt128,h4,noprotect,nooscsen,nobrownout,borv20,noput,stvren,\
nodebug,nolvp,nowrt,nowrtd,nowrtb,nowrtc,nocpd,nocpb,noebtr,noebtrb
int8 linear_interp(char x1, char x2, char y1, char y2, char x) {
int16 n;
int8 x21, dx, r;
if (x<=x1) return y1; // below or equal lower bound
if (x>=x2) return y2; // above or equal upper bound
dx=x-x1; //
x21=x2-x1; // (y2-y1)*(x-x1)
// return = Y1+______________
n=y2-y1; //
n*=dx; // x2-x1
r=n/x21; //
return y1+r;
} //function call=linear_interp(22,225,0,100,101); |
If I split the operation n=(y2-y1)*dx; to the one in the formula, I get the correct results.
I Think it is a cast problem, but n=((int16 *)y2-(int16 *)y1)*(int16 *)dx gives the same wrong results.
Does anyone have a hint? |
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Neutone
Joined: 08 Sep 2003
Posts: 839
Location: Houston
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| Posted: Fri Jun 18, 2004 10:03 pm |
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If you can, pre-calculate m and b because division takes a lot of time.
Y=(m*X)+b
m=(Y2-Y1)/(X2-X1)
b=Y1-(m*X1)
Yn=(m*Xn)+b
Yn=((Y2-Y1)/(X2-X1)*Xn)+Y1-((Y2-Y1)/(X2-X1)*X1)
Yn=((Y2-Y1)/(X2-X1)*Xn)-((Y2-Y1)/(X2-X1)*X1)+Y1
Yn=((Xn-X1)*(Y2-Y1)/(X2-X1))+Y1
You can expect better casting results if you put everything on a line by it's self. It forces the compiler to perform operations in an explisit maner.
int8 linear_interp(char x1, char x2, char y1, char y2, char x) {
int8 rise,run;
int16 Y;
if (x<=x1) return y1;
if (x>=x2) return y2;
rise=y2-y1;
run=x2-x1;
Y=x-x1;
Y*=rise;
Y/=run;
return Y+Y1;
} |
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