Digital BW, The Print

Clayton Jones wrote:



> I hope these will give some ideas.  I'd be interested in whatever you
> find out.

Clayton, thanks for the thorough reply. It gave me several ideas about 
how to attack this issue, and I'm pleased to report I've solved the problem.

Making a long story short, after several blind alleys I got even more 
suspicious of dither artifacts when earlier today I reprinted an image 
that I had successfully printed a couple weeks ago. This print came out 
fine, which suggested to me that nothing was physically wrong with the 
printer, and that it wasn't a mysterious software change that had led to 
problems. But I did notice that this print's source file was around 500 dpi.

So I printed some test strips. The ones at 360 dpi and 720 dpi looked 
fine. The one at 320 dpi was banded in all but the darkest zones - or 
maybe it is more correct to say the banding was variably visible, being 
most visible in tones from just below middle gray almost to white.

Anyway, that seemed pretty diagnostic to me.

I resampled my troublesome image using the s spline from 320 dpi to 720 
dpi. I printed it out and it looks great.

I went back to the original 320 dpi file, and resampled it to 360 dpi. 
Printed it, and I can't tell the difference from the 720 dpi version.

I reprinted the original 320 dpi file without changing it, and it is 
banded all to hell again.

I'm concluding that this printing system appears to be sensitive to the 
source file's linear resolution.

I'm not sure in what way, because I haven't done enough tests - and I 
probably won't bother, I prefer to just raise the flag of victory and 
enjoy my ability to print. But I wonder whether 320 is a magic number 
where you get a poor print with this system, or whether the dither 
algorithm works best at multiples of 80 dpi and hence there are a lot of 
sub-optimal magic numbers, or what. I also don't know whether this is 
idiosyncratic of my system specifically, or whether it is an attribute 
of all C86's used this way. Anyway, I throw this out for those who might 
encounter the problem in the future.

Paul Roark, thanks to you also for the help you gave me off-list.

--
Jeff Medkeff
Eagle River, Alaska