Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!newstop!exodus!peregrine!falk
From: falk@peregrine.Sun.COM (Ed Falk)
Newsgroups: comp.graphics
Subject: Re: Fast Image Scaling
Message-ID: <1526@exodus.Eng.Sun.COM>
Date: 19 Oct 90 01:30:54 GMT
References: <1990Oct18.014905.1846@lia>
Sender: news@exodus.Eng.Sun.COM
Organization: Sun Microsystems, Mt. View, Ca.
Lines: 58

In article <1990Oct18.014905.1846@lia> jgro@lia.com (Jeremy Grodberg) writes:
>
>Does anyone have any references on raster image scaling that is *FAST*,
>even if it is slightly non-optimal.  I need to do arbitrary scaling of
>1-bit-per-pixel images as fast as possible, and am not too concerned with
>optimum averaging or stuffing of the reduced or expanded pixels.  What I
>am concerned with is scaling a 2MB source image in 1-2 seconds on a 
>SPARCstation.  Note that I can't take advantage of simple power-of-2
>optimizations.

What you obviously want to do is drop rows and columns out of the
image.  The "simple power-of-2" method you refer to is the most
obvious one, simply drop alternate rows and columns:

	for(i=0; i<out_height; ++i)
	  for(j=0; j<out_width; ++j)
	    output[i][j] = input[i*2][j*2]

for other scale factors, you want to make a sort of Bresenham walk
through the input.


	int	iw,ih ;		/* size of input image */
	int	ow,oh ;		/* size of output image */
	int	ix,iy ;		/* indices into input image */
	int	ox,oy ;		/* indices into output image */
	int	cx,cy ;		/* counters */


	cx = -iw/2 ;
	cy = -ih/2 ;
	iy = ix = 0 ;

	for(oy=0; oy<oh; ++oy)
	{
	  for(ox=0; ox<ow; ++ox)
	  {
	    output[oy][ox] = input[iy][ix] ;
	    cx += iw ;
	    while( cx > 0 )
	    {
	      ++ix ;
	      cx -= ow ;
	    }
	  }
	  cy += ih ;
	  while( cy > 0 )
	  {
	    ++iy ;
	    cy -= oh ;
	  }
	}

This should work for scaling in either direction.  I haven't tried it.

		-ed falk, sun microsystems
		 sun!falk, falk@sun.com
		 card-carrying ACLU member.