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Don
Bergland
bergland@uvic.ca
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We
have been examining a variety of ideas concerning the content
and structure of visual images and how images are used in communication.
As our technical abilities increase, we need to start understanding
the structure of the visual image in its digital format.
The Pixel Grid
There are only two types of digital 2D images: the “Vector”
image and the “Bitmap” image. The more common type
(and the type we are working with in Photoshop) is called a
“bitmap” image. All jpegs are “bitmaps.”
Basically, this means that the image itself contains small bits
in a kind of map. This can be explained in the following way.
Every bitmap file comes in a rectangular form (either square,
portrait, or landscape). This rectangle is always divided into
a grid (called the raster grid). |
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Each
of the cells in the grid is called a “pixel” (abbreviated
- px). Pixels are tiny color dots (rectangles) that make up
what you see on your screen. The image uses the number of pixels
on this grid to classify its dimensions. An image that is 800
px X 600 px has 800 pixels along the width of the image and
600 along its width.
Note - By multiplying these two numbers together, you get the
“megapixel (MP)” size of the image, So, our 800
px X 600 px image contains a total of 480,000 pixels. We count
back 6 decimal places, and this becomes a .4 MP image. This
is how digital cameras are showcased. For example, a 2.4 MP
digital camera is capable of taking a 2.4 MP image (i.e., 2000
px X 1200 px).
Resolution
Resolution means nothing more than how many pixels are contained
in an inch. We have been working with a resolution of 72 ppi.
This means 72 pixels per inch. The higher the number, the more
pixels per inch (and hence, the more detailed the image). The
more pixels in an image, the higher the resolution (and thus
the quality). For web-based materials, we use a resolution of
72 ppi. If we want to print out our materials, we need a higher
resolution. For our purposes, a resolution of 150 is good. |
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Let’s
do a small activity which helps us understand pixels in an image
a little better and also introduces a few more Photoshop tools
for you to use. For this activity, you need the course CD-ROM. |
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| a. |
Insert
the CD-ROM in the computer.. |
| b. |
Launch
Photoshop. When the screen appears asking if you want
to “Start from Scratch,” just click on the
CLOSE button. |
| c. |
Go
to the TOP MENU BAR - FILE - OPEN. A window will appear.
Locate the CD-ROM. |
| d. |
Open
the folder called IMAGES. Inside this, you will find another
folder called Corel - RF. These are a collection of Royalty-Free
images by Corel. Open this folder. |
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Find
the first image called, “p0100252.jpg.” It
is a photograph of the side of a building. |
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| f. |
Remember,
this image is a jpeg, which means it is a bitmap image.
It is composed of a grid of pixels. |
| g. |
To
find out how many pixels it is composed of, go to the
TOP MENU BAR - IMAGE - RESIZE - IMAGE SIZE. A window will
appear giving us the pixel dimensions of this image.
- 900 pixels wide
- 595 pixels high
- 72 pixels per inch
That means that if we were able to actually see the grid,
it would have 900 pixels along the top and 595 pixels
down the side. |
| h. |
Close
the Image Size window by pressing OK. |
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We
want to be able to ZOOM into this image (which means magnifying
it) so that we can see the pixels in it. |
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Look
at the TOOLBAR and find the ZOOM tool. This is the small
magnifying glass right under the MOVE tool. |
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Click
on the Zoom tool to select it. Note that when you click
on the tool, the top Option bar changes. On the top Option
bar, |
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You
should click on the + icon. This will allow you to ZOOM
IN. |
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Now
move the cursor over to the document window and click
once. Note what happens. You ZOOM IN one increment. Look
at the top of the document window, and you can watch the
% of zoom change. Click once more. Now keep clicking until
you can’t click anymore (The magnifying glass is
white in its interior. This should be at about 1600%. |
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Now
you can see individual pixels. You can identify them by
their rectangular appearance and the fact that a pixel
can only contain one color. It is the smallest unit of
the image and can contain only one piece of information. |
| l. |
You
are only looking at a small section of the original image.
Your document window is looking at this small section,
but most of the image is now not showing. You can drag
the rest of the image around by using the HAND tool. Go
to the TOOLBAR. Underneath the ZOOM tool is the hand tool. |
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Click
on this to select it. Now move the hand to the document
window, hold down, and drag. You are dragging the unseen
parts of the image into view under the document window.
Explore this function while you enjoy the tour of pixels. |
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When
you want to return to the full image, you can click on
the “-“ icon in the option bar and then click
multiple times to ZOOM OUT, or you can double-click the
HAND on the TOOLBAR. This will immediately return the
document window to a percentage that fits on your screen. |
| n. |
Now
close the image without saving. Open some more photographs
from the Corel - RF folder and try going through the following
procedure with them:
1. Open the image
2. Check the Image Size to see the Pixels (how many) and
the Resolution
3. Zoom into the image to get to 1600% and examine the
pixels.
4. Zoom out.
5. Close the image. |
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| Click
on IMAGE
categories at the top left side of the page to access further
knowledge and skills. |
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