Problem Scans

Hi

I scanned some images in grayscale for a PowerPoint presentation and 
some of the scans came out with a diagonal type of hashing.  I am not 
sure if this is what is called "moiré" or if moiré is something else.

Is there a way to get rid of this scan artifact in PaintShopPro 9?

Also, what is the recommended resolution (ppi) for images that will be 
used in a PowerPoint presentation will they will fill up the slide.

Thanks.

Jeff

Jeff wrote:
> 
> Hi
> 
> I scanned some images in grayscale for a PowerPoint presentation and
> some of the scans came out with a diagonal type of hashing.  I am not
> sure if this is what is called "moiré" or if moiré is something
else.

One way to determine if this is moiré or not is to post a
crop from the image for us to see instead of going for a
verbal description. If you didn't scan a photo or inkjet
print but instead some kind of printed matter then it is
probably moiré.

> Is there a way to get rid of this scan artifact in PaintShopPro 9?

Well, yes, there is a way but it is much more satisfactory
to get rid of it by scanning properly. To do that it is
helpful to understand what you have scanned. I think it is
unlikely to be a photo. More likely it is something from a
newspaper, magazine or art book. These typically contain
halftone images made of dots of ink - cyan, magenta, yellow
and black for color images or just black for greyscale.
However, the three mentioned media have these dots printed
at different spacings (called a line screen). Knowing this
spacing helps you choose the optimum scanning conditions.
To tell you what these are and what to do with them you first
have to tell me exactly what it is you are scanning.

Moiré arises when the ink dot spacing in the image is too
small for the scanner resolution you are using, and the
inadequate sampling causes aliasing artifacts. To properly
resolve the ink dots you need to scan with a resolution that
is two to three times the dot spacing. After you have done
this, you will have a very large image. At this point you
apply just enough Gaussian Blur to blur the dots together but
not so much as to blur any image detail. Finally, you resize
the image down to the size in pixels that you need. Now
you won't have any moiré and you will have all the detail
you need. You can even sharpen the image at this point and
the moiré won't reappear.

For a newspaper there are around 85 dots per inch so you scan
at a resolution of about 250 ppi. For a magazine there are
about 150 dots per inch so you scan at a resolution of around
400 ppi. For a very high quality art book there might be as
many as 250 dots of ink per inch meaning that you should scan
at about 600 or 700 ppi. After that you can apply the procedure
described above. To learn more about how to scan things
properly take a look at Wayne Fulton's http://www.scantips.com

Another solution is to enable moiré suppression in your scanner
software. You many have to search for the option under Advanced
or Manual settings, or whatever it's called in your software.
Because the moiré suppression built into the scanner software
has access to the full resolution raw scanner data it can do
a much better job than after the fact fixing in PSP using a
smaller image.

If you don't want to use either of the easy approaches I've
outlined you can apply the Moiré Pattern Removal filter, though
it may blur your image to some degree. Another thing to try
is the Digital Camera Noise Removal filter. As the PDF file at
the bottom of this link http://campratty.com/questions.html
explains, this filter is not designed for removing moiré and
often produces imperfect results. However, there are instances
when the results can be very good. I suggest you review the
file to learn more.

> Also, what is the recommended resolution (ppi) for images that will be
> used in a PowerPoint presentation will they will fill up the slide.

I haven't a clue. I try to avoid PowerPoint entirely. However,
the premise of your question is probably wrong. The important
factor that determines the size of the image in PowerPoint is
the number of pixels in the image. The resolution is just a
conversion factor from pixels to inches. You can change it at
any time to anything you like in PSP without any effect at
all on the pixels of which your image is made. If PowerPoint
actually cares about the resolution you can set the resolution
as you please. However, if you don't have enough pixels in
your image to represent the detail it contains it will never
look good in PowerPoint no matter what resolution you set.