SofaKing said:
When I custom set my preview resolution at 200, very low quality image. At
201 however, a DRAMATIC difference. Why?
I don't know the Epson 3200 but this explanation (with different
numbers) should apply to any scanner.
When you scan at the native resolution of the scanner divided by an
integer the image is created by dropping samples (pixels if you prefer
to think of it that way). So if the native resolution is 3200 and you
scan at 200, that is full resolution divided by 16. The software
(VueScan) instructs the scanner to take only every 16th sample in
horizontal and vertical direction so you keep only 1 sample (pixel) in
256. (256 = 16 squared). That accentuates the noise (both random and
grain aliasing)in the image by a similar factor. The result is a very
poor quality image.
When you scan at a resolution which is not an integral division of the
native resolution (e.g. 201 does not divide into 3200 a whole number
of times), the image cannot be created by dropping samples. So VueScan
scans the film at full resolution (3200) and then downsamples the
result by software interpolation. i.e. it looks at all the samples
(pixels) in the high resolution (3200) image and creates the low
resolution (201) image by forming each final pixel from an average of
many pixels. At 201, each pixel is (roughly) the average of 256
original samples. (The exact averaging mechanism depends on the
software interpolation method that the software uses, so that
bilinear, bicubic and others will produce slightly different results,
but the principle is the same. I believe VueScan uses something like
bilinear interpolation). This averaging process results in a huge
reduction of image noise and produces a high quality result.
You should also notice that scanning at a whole number division of
full resolution (200) is much faster than at non integral divisions
(201) because so many fewer samples are taken and the motor can
advance the scanning device many steps before stopping to take a
sample. These integral divisions of full resolution are the ones that
appear in VueScan's drop down menu of scanning resolutions.
(PS I post using Google so it is possible that someone else will post
a similar explanation before this reaches the News Group!)
