Explain image quality in terms of stroke, details, colour
accuracy, dynamic range of the image.
In This Chapter, you’ll learn on:
Describe image quality.
Compare the quality of images acquired from
Improve image quality using appropriate software
Image quality is a characteristic of an image that
measures the perceived image degradation
(typically, compared to an ideal or perfect image).
Imaging systems may introduce some amounts of
distortion or artifacts in the signal, so the quality
assessment is an important problem.
Factors that affect image quality
Sharpness determines the amount of detail an
image can convey.
Noise is a random variation of image density, visible
as grain in film and pixel level variations in digital
Dynamic range (or exposure range) is the range of
light levels a camera can capture, usually
measured in f-stops, EV (exposure value), or zones
(all factors of two in exposure). It is closely related to
noise: high noise implies low dynamic range.
Tone reproduction is the relationship between
scene luminance and the reproduced image
Factors that affect image quality
Contrast, also known as gamma, is the slope of
the tone reproduction curve in a log-log space.
High contrast usually involves loss of dynamic
range — loss of detail, or clipping, in highlights
Color accuracy is an important but ambiguous
image quality factor. Many viewers prefer
enhanced color saturation; the most accurate
color isn't necessarily the most pleasing.
Distortion is an aberration that causes straight
lines to curve near the edges of images.
At full resolution, this image has clearly
visible compression artifacts
Improve image quality using appropriate software filters
We can say that these effects or filters have a positive
effect on image quality. But there is no definite way to
improve an image quality 100% by just using software. It
depends fully on the source of the capturing device.
For example, to scale up a 72 dpi resolution photo to
200%, despite the fact that you can achieve this by
increasing the resolution in Photoshop or GIMP, you are
not adding new detail into the photo. In effect, all you
are doing is increasing the size of the pixels which rapidly
decreases the quality as you scale higher.
But Images can often be improved or transformed
by using graphics editing software such as
Photoshop. Such applications or programs usually
have photo-enhancing tools and allows you to
execute semi-automatic image enhancement with
only the click of a button.
Some examples of such filters or tools are
o filter the background
Blurring of the background to make the object more stand out
Add a filter to the background to bring focus to the
robot in the foreground.
Frequency Modulation Screen
Frequency Modulation Screening, also known as
Stochastic screening, is a halftone process based on
pseudo-random distribution of halftone dots to change
the density of dots according to the desired gray level.
FM screening keeps the dots the same size and varies
the frequency, or number, of dots and the location of
those dots to simulate the original image. This illustration
shows enlarged simulations of FM screening.
FM Screening Sample
You can see from these illustrations that, regardless
of the type of screen employed, the eye perceives
a shade of gray depending on the percentage of
In FM screening, the concepts of screen angle and
frequency no longer apply. Because the dots are
randomly placed, there is no direction (the screen
angle used in AM screening) to the dots. The
variable spacing of the dots means there is no fixed
spacing, and therefore there is no screen
Who Can Benefit from FM Screening…
Applications for FM screening span the industry: newspapers,
magazines, commercial printers. The key to successful use of
any FM screening technology is summed up in one word:
control. The user who successfully controls the production
environment can realize the considerable benefits of FM
Magazines with in-house image setting operations can use FM
screening to produce high-quality color. Once the dot gain
calibration has been performed in collaboration with the
printer for the press, paper, and ink, the magazine can use
these settings for every production run.
Newspapers can also use FM screening to good
advantage. Newspaper industry tests with
frequency modulated screening show higher
quality color and image detail than with
conventional screening, due to the relative
coarseness of the conventional screen used with
Newspapers can output pages more quickly at
lower resolutions -- high enough to satisfy
typographic quality requirements -- and the color
images will be of much higher quality than if
conventional halftone screening were used.
Define the term Image Resolution
Image resolution refers to the spacing of pixels in the
images and is measured in pixels per inch(ppi).
Resolution is the number of pixels or dots per inch
(ppi/dpi). Typical resolution values vary between 72 ppi
(pixels per inch) for a computer screen up to 2400 ppi for
very high quality professional printing. The higher the
resolution, the more detail is stored for the graphic. This
means that a higher resolution image is higher quality,
but it is also larger in file size.
The screening of four colors are no longer made with four
different angles as with the traditional screen therefore it
eliminates screening moiré.
FM screening does not create rosette patterns.
Halftone dot sizes can be as fine as 10 micrometres, which
gives at the product a quality comparable to that of
photographic prints.
The effects of misregistration is not completely eliminated, but
the effect is certainly less apparent than in the traditional
screening, this feature is very favorable for printing on rotary
machines where the misregistration is very common due to
effects such as web growth.
The use of FM screening allowed Archant, a UK regional
publisher, to switch to fonts with "tiny holes"; such an "eco-font"
permitted a reduction in ink without turning fine text grainy
The small dots used in FM screening require special care
and cleanliness, especially when plates are made from
Measure the Resolution of an image
The first thing you need to be aware of when dealing
with digital images is the resolution of the image. Simply
put, an images resolution is determined by how many
pixels of information exist per inch within the image at
100 percent of the size it will be printed. Your digital
images are very similar to a mosaic made with colored
tiles. Pixels are the tiles with which the image is built. The
more tiles you can use per inch, the more refined the
detail can be within the image.
Here's an illustration of the differences you might expect to see in
image quality between a 72 pixel per inch image and a 300 pixel
per inch image. The image on the left shows a detailed look at
what 72 pixels per inch looks like up close.
72 pixels per inch is considered low resolution although it is
perfectly fine for a website or other on-screen display, it is not
really good enough for a high-resolution image setter which will
reproduce your artwork to be printed on a printing press. The 300
pixel per inch image has over three times as much detail.
72 dpi close up 72 dpi imag 300 dpi image 300 dpi
close up (Smaller in size as
compared to an
300 dpi image)
When working with images destined for a printing press you
should be using a general rule of thumb of 2 to 1.
The digital image should posses twice as many pixels per
inch as the line screen in which your artwork will be
printed at. So Cal Graphics likes to print images at 175
line screen or 150 line screen. So, using the 2 to 1 rule,
your images should be scanned at 350 or 300 pixels per
inch for maximum quality.
When in doubt, you should be safe saving your images
at 100% of the size they will print, CMYK (or grayscale if
you are printing 1 color), 300 pixels per inch for photos
and 600 pixels per inch for hard-edged line art, and in TIF
Choosing the Resolution
Choosing the resolution of a new picture depends on
several factors. If you are creating graphics for the web
and most other non-printing applications, you can
choose whatever you prefer because the image pixels
are mapped 1:1 to the screen pixels. In these cases you
will enter the image size in pixels and not a real world
unit. I suggest you use the default resolution of 72 ppi so
you get halfway meaningful numbers in information
For printing, the most important factor is the hardware
resolution of the target device. It makes no sense to
choose a resolution that exceeds the capabilities of the
device. While it is usually no problem for most printers to
scale the image down, the larger size can significantly
slow down processing and printing without any benefits.
For black and white pictures you will usually match the
image resolution to the printer resolution to get the best
quality possible. Since no scaling is involved and the
color information does not take a lot of space (1 bit per
pixel), there are few reasons why you should go lower.
Grayscale or color pictures take some more
consideration. You should keep in mind that almost all
printing methods put a pattern of points of the four base
printing colors (cyan, magenta, yellow and black) on
the paper to fake a large range of colors. Especially with
colorful scanned pictures, half or even a quarter of the
hardware resolution will result in the best quality possible.
Going higher will result in exactly the same picture and
therefore waste precious resources. For example if you
have one of the common ink jet printers with 300 dpi
resolution in color mode, an image with 150 ppi will
usually result in the best quality.
Identify the steps to modify image resolution based on
The most common setting for computer monitors is 72
dpi; therefore, images intended for web-based or other
on-screen projects should have a resolution of 72 dpi.
This lower resolution will reduce file sizes and download
times. The normal resolution for low-end printing is 150
dpi, and for high-end, professional, quality photographs
the resolution should be 300 dpi or higher.
To change the resolution of a graphic, pull down the Image
menu and select Image Size. The following dialog box will
Change the resolution in the Resolution box (in blue above).
Keep these facts in mind:
If you have a 72-dpi graphic and you change it to a higher
resolution, you will not increase the quality of the graphic. The
information was originally stored at 72 dpi, and the computer
cannot “make up” more information to improve the graphic
If you have a 300-dpi graphic and you change it to a lower
resolution (say, 72 dpi), you will decrease the quality of the
graphic. This is because you will lose the extra pixels of
information. If you need to reduce the resolution of a graphic,
save the original, high-resolution image in case you need it in