Transcript Document

Book Scanning
&
Digital Image Production
The VRC Guide to Imaging
By Kate Stepp
Why doesn’t the scanned image look
exactly like the print image?
Why does the image need software to
manipulate it?
Print Dot Matrix Versus Pixel Maps
and
CMYK Versus RGB
Above is a CMYK print process matrix (left), and an RGB overlay color pixel map
(right). Print matrix images rely on the blending of overlapped spot (circular) colors
and the white spaces of the paper. This is what is used to create color images in
books and magazines. Pixel maps rely on overlapped individually colored pixels
(square) with no spaces. This is what is used to create color images in computers.
Book Images are Created with CMYK dot matrix overlays
Scans of a Printed Image must be Transformed
from a Dot Matrix into a Pixel Map
When scanned, we use a descreen filter to turn these overlapping color dots into individually
colored pixels – which blurs the image but creates a uniform color map with no white spots or
visible matrix pattern.
Here are close-ups of both Dot Matrix and Pixel Maps.
Detail --->
Detail --->
Scans of Printed Images
must be transformed from
CMYK to RGB.
With the CMYK process, to print any
color image, Cyan, Magenta, Yellow and
Black inks are overlapped to achieve the
full color spectrum.
Color monitors for desktop
microcomputers are based on cathode
ray tubes (CRTs) or back-lighted flatscreen technologies. Because monitors
transmit light, displays use the red-greenblue (RGB) additive color model.
Technical
Note:
What resolution do we use? - Part II
There are two types of resolution: Display resolution and Document/Printing
resolution.
DISPLAY RESOLUTION
The display resolution of
projectors/computers is simply the
physical number of columns and rows
of pixels creating the display. Note that
the use of the word resolution here is
misleading. The term "display
resolution" is usually used to mean
pixel dimensions (e.g., 1024 X 768),
which does not tell you anything about
the resolution of the display on which
the image is actually formed (dpi/ppi).
Tulane’s Smartroom Projectors display a maximum display
resolution of 1024 Pixels Wide X 768 Pixels High.
How many pixels per
inch is determined by
the physical space of
the display.
There are only 1024 X 768
pixels - whether they take up
10 X 7 inches of physical
space
or
10 feet X 7 feet of physical
space.
Technical Note:
The smartroom digital projectors have a display resolution of:
XGA (1024 x 768) native
SXGA (1365 x 1024 widescreen or 1280 x 1024 regular) maximum
Overall, the scaling engines have gotten to be very accurate with video these
days. Quite often a scaled video image looks just about as clear and crisp as it
would if displayed in its own native format. However, this is not as true of
computer data signals (digital images). So let's focus on data projection. It is
common for a native XGA resolution projector (1024x768) to have, say, SXGA
(1280x1024) listed as its maximum resolution. All that means is that you can feed
a 1280x1024 computer signal into the projector. However, when the projector
compresses this signal into its native 1024x768 display, the picture will be
somewhat fuzzier (sometimes a lot fuzzier) than it would be if the signal was
native XGA to begin with.
There are projectors being built with SXGA (1365 X 1024) and SXGA plus
(1400x1050) native resolutions. More on that to come…
72 ppi became known as “Web
Standard” resolution. This actually
refers to the physical space/size of
monitors, not document or printing
resolution.
Most screens are configured at 768 X 1024
pixels. At 72 Pixels Per Inch this equals a
visible space of 10.66 Inches High X 14.22
Inches Wide, about the physical size of most
computer monitors.
Thus the true but often times misleading
saying “You can’t see more than 72 ppi on a
computer.”
What is dpi or ppi?
DPI stands for Dots Per Inch.
PPI stands for Pixel Per Inch.
It has nothing to do with how your images will display on
computers or through a digital projector.
Combined with the document size, it has everything to do
with how your image prints.
The Dpi/Ppi of digital images is stored information for
printing on paper. It tells the printer how to process the
image, how many dots per inch. As digital images are
created with pixels (squares) not dots, it helps translate
pixel information into dot information for the printer.
Thus we have stored Dpi/Ppi resolutions in digital
images. The higher the Dpi/Ppi is, the better quality of
print you get.
Again - Dpi/Ppi has nothing to do with how your
images will display on computers or through a digital
projector. That is based entirely on pixel dimensions.
What resolutions do we use?
The VRC uses these settings for images:
1000 Pixels Wide (Display Resolution)
750 Pixels High (Display Resolution)
10 Inches Wide (Document Resolution)
7.5 Inches High (Document Resolution)
100 Pixels Per Inch (Document Resolution)
Why don’t we use larger display resolutions?
We are balancing out display resolutions, printing quality (dpi/ppi) and printing
sizes as well as the built in size defaults of Powerpoint and Keynote (7.5 X 10
inch presentation screens).
1000 X 750 is 97.65% of 1024 X 768
Pixel Dimensions Matter for Projection.
If your image has MORE pixels than the maximum your
computer or projector can display - then your image will
not look good.
If your image has LESS pixels than the maximum your
computer or projector can display and you try to enlarge
your image to full screen - then your image will not look
good.
Scaling digital images is in no way similar to scaling slides or film (which
you can scale simply by adjusting the lens throw on a film projector).
For Example:
Say your maximum screen display is 6 Pixels, and your image is 12 Pixels. You
can not “shrink” your pixels down to squeeze in the smaller pixel area. You can
only discard pixels.
Those 6 discarded pixels still exist, however, in the image data. The data is not
changed (your image is still 12 pixels wide) but only 6 of your pixels will be displayed.
This often leads to unwanted visual affects: blurring, pixelation, color changes, etc.
An example of
50% scaling.
Reducing an image
by 50% throws
away approx 75%
of the information.
This is important to remember when doing side by side
comparisons - you are shrinking the image (in general) and
may want to also present the image full screen to examine
details and have better clarity.