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researchv10 Dan Cross

.ds ZZ DEVELOPMENT PACKAGE
.TH MULDIV 3L "630 MTG"
.XE "muldiv()"
.SH NAME
muldiv \- calculate (a*b)/c accurately
.SH SYNOPSIS
.ft B
#include <dmd.h>
.br
short muldiv (a, b, c)
.br
int a, b, c;
.SH DESCRIPTION
The
.I muldiv
function
is a macro that returns the 16-bit result
.IR (a*b)/c .
.I (a*b)
is calculated to 32 bits to minimize the precision lost.
The
.I muldiv
function
is convenient for calculating transformations.
.SH EXAMPLE
The following subroutine implements the transform(3R) function. It
converts a point from window coordinates to screen coordinates:
.sp
.nf
.ft CM

#include <dmd.h>

Point
transform(p)
Point p;
{
	Point Do, Dc, ret;

	Do = Drect.origin;
	Dc = Drect.corner;

	ret.x = muldiv(p.x, Dc.x-Do.x, XMAX) + Do.x;
	ret.y = muldiv(p.y, Dc.y-Do.y, YMAX) + Do.y;

	return(ret);
}
.ft 1
.fi
.P
The following subroutine does the opposite of the transform(3R)
function. It converts a point from screen coordinates to window 
coordinates.
.nf
.ft CM

#include <dmd.h>

Point
untransform(p)
Point p;
{
	Point Do, Dc, ret;

	Do = Drect.origin;
	Dc = Drect.corner;

	ret.x = muldiv(p.x-Do.x, XMAX, Dc.x-Do.x);
	ret.y = muldiv(p.y-Do.y, YMAX, Dc.y-Do.y);

	return(ret);
}
.fi
.ft R
.SH SEE ALSO
globals(3R), ptarith(3R), transform(3R/3L).

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