RE: filmscanners: Vuescan grain removal idea

["Shough, Dean" <dean.shough@lmco.com>]

> Maybe Ed or someone else has a better idea about how the Vuescan grain 
> removal option could be expanded and in a practical sense work even
> better.  
> If you do then please give us your input.
>

I don't know about VueScan, but I did read through US Patent 5,673,336 which
seems to be the basis for ROC and  probably lead to the idea for GEM.  One
of the inventors is Albert Edgar, now with Applied Science Fiction.  The
basic idea in the patent is that the grain in each layer of color film is
independent of the other layers.  Thus the grain noise in each color layer
is independent of the grain noise in the other layers.  This agrees with
your observation that the grain noise shows up a multi colored noise.

A more technical excerpt of the patent follows:

" ... the present invention corrects for the interaction for the dye layers
in the film and the scanner spectral sensitivity as well as correcting for
the changes in the dye layers in the film with aging.

"The invention is based on the observation that as each dye layer is
deposited separately in the film, one would expect that the "noise" from the
grain boundaries in one layer to be independent from the "noise" due to the
grain boundaries in other layers.  If there is correlation in the noise
between color scans, it is because the scanner is measuring the grain
boundaries in more than one layer in the color scan.

"To measure the noise, the image and noise must be separated...

"The infrared scan is used to detect imperfections in the film medium
itself.  As discussed in ... US Pat No, 5,266,805 ...  Unfortunately, in the
infrared scan, there can be cross talk from the red sensitive, cyan forming
layer which would be identified as defects.  The present invention can be
used to correct for the red crosstalk in the infrared scan.

"FIG. 5 shows the process by which the correlated noise is separated from
the scanned image.

"Once the invention has removed the effects (sic) to dye color changes and
retrieved the pure separate color images, the effects of aging from the pure
separate color images can be removed using the changes common to black and
white images in particular contrast stretch between black and white points
in the images.  It should be emphasized that applying this contrast stretch
works well only after the invention has separated the color records.

"There is virtually nothing above about 40 line pairs per millimeter spatial
frequency recorded with today's lenses and film from real world images.
This cutoff corresponds to a 2000 by 3000 pixel scan of 35 millimeter film.
Conversely, the grain noise begins with  flat spectrum and is attenuated
only at very high frequencies by grain size and dye diffusion as discussed
above, which have an effect above 100 line pairs per millimeter.

"A practical solution first isolates frequencies around 40 line pairs to
eliminate those parts of the image in which the energy seen at these high
frequencies is predominately from grain noise, and prunes out or emphasizes
those where the high frequencies also contains image detail.  For example, a
sky, a blurred background, ...  Because the noise is a constant across the
image, a region that contains more high frequencies than elsewhere in the
image is more active because o image detail...