Moire can occur when
scanning from printed material. The problem arises when there is a
clash between the regular pattern of the dots on the printed page of a
magazine or book, say, and the regular raster pattern of the scanner.
This conflict can cause a new pattern - moirÈ - to be superimposed over
the image, usually with unsightly results.
Question from Mark about Moire when Scanning -
I'm experiencing a problem whereby each time I
think I've hit on the solution, the goalposts seem to move by
themselves. I'm hoping you can help, although I'll have to ask you to
bear with me while I explain the problem.moire
Essentially it's this. I've scanned in a
photograph from a book, specifically a photograph of a Vermeer
painting. The original photograph was quite large - slightly wider than
A4. I've scanned it at 600 ppi and cropped it using PaintShop Pro 8 to
exactly A4 dimensions. So far, all is well : until I try printing the
result on a sheet of A4-sized Kodak glossy photographic card. The
problem is that the resultant picture has a really horrible
cross-hatching effect across most - though, puzzlingly, not all - of
the image. ( I think this is referred to as moire, but I'm not sure if
I'm using the term correctly or not ). At any rate : I've tried just
about every permutation of settings in the printer driver software to
no effect. Upon researching the subject on the internet, it appeared to
be the case that the problem may have been caused by the fact that the
original image had been scanned at too high a resolution for the
printer to faithfully reproduce. So, I tried changing the resolution of
the image using PaintShop Pro 8. I experimented with various
resolutions, from 200 ppi to 500 ppi. The bizarre thing - or to be more
accurate, the first bizarre thing - was that the best result seemed to
be from the 200 ppi image. The 300 ppi image was better than the
original 600 ppi, but not as good as the 200; the 500 ppi image seemed
to produce around the same result as the 200.moire
At this point I should explain two things.
Firstly, each of the above permutations, including the original,
appeared exactly the same ( i.e. perfect to the naked eye ) when viewed
on the monitor; the cross-hatching only occurred in the actual print.
Secondly, it was quickly apparent that conducting this process of
experimentation using an actual A4 sheet of photographic card each time
was going to be an extremely expensive learning curve. For this reason,
for the purposes of the experiment, I printed each test picture at
one-ninth of A4 size. Whether this in itself would have made a
difference, I have no way of knowing without wasting sheet upon sheet
of A4 photographic card.moire
Of course, the problem was made no easier by the
fact that the Print Preview window is always going to display a poorer
image than the one that actually gets printed. For that reason, it was
impossible to tell without actually printing each image whether or not
the problem had been solved completely, partially or not at all.
Eventually, after many hours of trial and error ( mostly error, if I'm
honest ) I settled on the 500 ppi version of the picture and printed
this to an A4 sheet. The result was markedly better than the first
attempt using the original, 600 ppi version - but still not perfect.
I've yet to risk wasting a third full sheet of A4 card on another
experiment, particularly since there seems to be no sure way to predict
what the resultant print will look like without actually printing it.
What really puzzled me was that it seemed odd that the 600 ppi
resolution should be creating such a huge problem, so to prove a point
I printed a photograph that had been scanned in at 1200 ppi ( though
the original was much smaller, admittedly - approximately postcard size
). Confusingly, this picture printed perfectly without adjusting the
resolution in any way, and upon further experimentation, so did three
others of about the same size that had also been scanned in at 1200
I understand that due to the nature of the way in
which colours are produced, a picture will never look the same on the
monitor as it does on a sheet of photographic card. I understand that
the print preview will never be exactly the same as the final printed
picture ( although I have to say that it seems to me that this rather
defeats the object of having a 'preview' at all - but still ... ). I
understand more or less why a picture that has been scanned at, say,
1200 ppi, will not be reproduced in print exactly<font
color="#e5e5e5">moire</font> pixel-by-pixel the
way it appears on the screen, given the difference between ppi and dpi.
I consider myself averagely intelligent, but I have to admit freely
that I really don't understand why it should be the case that a given
picture will print better at 200 ppi than it does at 600 ppi, but worse
at 300 ppi than it does at 200 ppi, but better at 500 ppi than it does
at either 300 ppi or 600 ppi. I also don't understand why a picture
that was scanned at 600 ppi should cause this amount of difficulty when
other pictures that were scanned at 1200 ppi print perfectly first time
without adjustment or experimentation.
If you can enlighten me in any way before I'm left
with no choice other than to risk wasting further ink and photographic
card, I would very much appreciate it. Really all I want is to be able
to know before I print a picture, regardless of whether it's A4-sized
or postcard-sized or any other size, that the cross-hatching thing
won't happen and that the print will be faithful to the picture on the
screen ( taking into account an allowance for the factors mentioned in
the previous paragraph ). To me, this doesn't seem like a lot to ask!
I look forward to hearing from you at your
earliest opportunity. Thank you in advance for indulging me.
Phew - you have clearly hit some problems. I must
confess that I'm not an expert at scanning although I have had
excellent results when scanning from printed material then printing it
out. I use a flatbed scanner and am assuming you are using the same.
The first thing to do is calculate the scanning
resolution based upon the size of the original and the size at which
you want to print it using the following formula - [ input resolution
in ppi = (size of output in cm x output resolution in ppi) / size of
original in cm ]
So for example, if the longest side of the
original is 32 cm and it is to be printed on A4 and you usually work at
300ppi then the formula would give an input resolution of ir = (29.7 x
300) / 32 = 278 ppi. I suggest you try this and do a test print on
cheaper paper such as Photo Quality Ink Jet Paper first.
There are several ways of dealing with moire.
slight change in the input resolution can make all the difference.
Also, you could try aligning the page slightly differently on the
scanner's platen - just a small change of angle may do the trick. Some
scanner software offers filters designed to remove, more or less
successfully, such moire patterns, but this is often at the cost of
sharp image detail. Also, bear in mind that sharpening filters can
bring out moirÈ pattern.
I hope this helps.
Please let me know how you get on.
P.S. I learned quite a lot about scanning from Tom
Ang's Digital Photographer's Handbook.