kevin@jtsv16.UUCP (kevin) (05/09/88)
I am trying to implement an image cacher program for use with NeWS.
The images will be looked up in a database by a "C" program, and
shipped to the NeWS Server via a socket. At the moment, I am using
the Sun rasterfile format to store the images, which are scanned at
150, 200, 300 and 400 dpi.
I believe the NeWS implementation of sockets is quite slow compared
to using 2 "C" programs. Therefore I want to reduce the number of
bits coming in to NeWS, and since my screen resolution is only
something like 83 dpi (I am using a monochrome Sun 3/60), scaling
the image down before sending it down the socket seems the way to go.
I have worked out some algorithms for doing the scaling, but they
are either too slow, or not able to handle scaling by fractional
amounts. I have not been able to find any literature on the
subject, either in the Sun manuals, or elsewhere. I am sure there
must be something, because in PostScript you can just say
"x y scale".
Does anyone have algorithms to do this, or can you point me in
the right direction for material on the subject? Any help you
can give will be greatly appreciated.
Thanks,
--
Kevin Brighton
JTS Computer Systems Ltd. Toronto
(416) 665-8910
UUCP: {geac | utzoo | utgpu}!syntron!jtsv16!kevin |
suncan!jtsv16!kevingreg@gergle.UUCP (05/12/88)
>I have worked out some algorithms for doing the scaling, but they >are either too slow, or not able to handle scaling by fractional >amounts. I have not been able to find any literature on the >subject, either in the Sun manuals, or elsewhere. I am sure there >must be something, because in PostScript you can just say >"x y scale". The quick and dirty way is to skip pixels or use pixels more than once while building a destination image from the source image. This works for any ratio of sizes. An error variable is kept based on the ratio of the src & dst sizes. This technique works well in software and in hardware. By well I mean my personal judgement of the speed of the operation versus the quality of the resulting image. The Sun NeWS port uses this technique in software, and the Parallax NeWS port has this function in hardware and in software. The only type of image it will look terrible on are CAD images. -greg.
holland%hplwdh@HPLABS.HP.COM (Bill Holland) (05/13/88)
> >I have worked out some algorithms for doing the scaling, but they > >are either too slow, or not able to handle scaling by fractional > >amounts. I have not been able to find any literature on the > >subject, either in the Sun manuals, or elsewhere. I am sure there > >must be something, because in PostScript you can just say > >"x y scale". > >The quick and dirty way is to skip pixels or use pixels more than once >while building a destination image from the source image. This works >for any ratio of sizes. > >An error variable is kept based on the ratio of the src & dst sizes. > >This technique works well in software and in hardware. By well I mean >my personal judgement of the speed of the operation versus the quality >of the resulting image. The Sun NeWS port uses this technique in software, >and the Parallax NeWS port has this function in hardware and in software. > >The only type of image it will look terrible on are CAD images. Literature does exist on the subject. The general problem is changing the sample rate of a sampled function to a new rate. The theoretically correct method is to reconstruct the original sampled function at the new sample points. That produces the desired scaled (re-sampled) output. A good textbook on the subject is "Multirate Digital Signal Processing", by Ronald E. Crochiere and Lawrence R. Rabiner (Prentice-Hall, 1983). This book contains a readable description of the problems involved, followed by a theoretical treatment of their solutions, and some practical examples. However, that book does not deal with the specific problem of scaling two-dimensional images. Various papers in such journals as the ACM Siggraph Proceedings have dealt with this in years past. Ampex and Lucasfilm (among others) have their own implementations of image scaling algorithms, and have found extensions of those algorithms to provide image translation and rotation as well. In general, such algorithms produce good looking output when dealing with gray scale images (images with intensity quantized to multiple bits), that don't have a lot of jaggies. That's just saying that the output can be only as good as the original output: if the original image is quantized to one bit intensity levels, or contains aliasing errors such as jaggies, the defects in the original image are likely to be worse in the scaled output image. This problem only gets worse if the output image must be quantized to one bit. There the best bet is to regenerate the image, if that's possible. - Bill