Computer Science 1033A/B Lecture Notes - Lecture 4: Paintshop Pro, Image File Formats, Lossless Compression

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COMP SCI – WEEK 4
Indexed Colour
indexed colour can look pretty good even though we will only ever have at most 8-bit colour (or
256 shades of colour)
Bitmapped images vs. Vector graphics
Bitmapped graphic image
image consists of pixels in a grid
icons are an example of a bitmapped image
icons usually 32 pixels by 32 pixels
when bitmapped images are enlarged (re-sample), the computer adds new pixels and
guesses on the colour to colour the new pixels (called interpolation) based on surrounding
pixels
this icon is now 245 pixels by 245 pixels
bitmapped images edges become jagged
also called rostergraphics image
bitmapped images are resolution dependent
bitmapped image on a 640 by 480 screen (lower resolution) appear larger than on a 1280
by 1024 screen (higher resolution)
bit mapped images that are enlarged:
have larger file size than original
become distorted
all images from scanners and digital camera are bitmapped
common bitmapped file formats:
BMP
GIF
JPEG, JPG
PNG
PICT (Macintosh)
PCX
TIFF
PSD (Adobe Photoshop)
popular bitmap editing tool/software packages:
microsoft paint
adobe photoshop
corel photo-paint
corel paint shop pro
the gimp
Vector graphics
vector image is made up of individual, scalable objects
objects are defined by mathematical equations
objects consist of lines, curves, and shapes
no distortion as image is enlarged
as image is enlarged, it still has crisp clean lines
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most browsers don't display vector graphics without a plug in
only can be used with drawings, not photographs
usually vector graphic images have a smaller file size than the same image stored as a
bitmap
great for logos because:
can be scaled down for business card
can be scaled up for a trade show poster
note: the text in PDF files are vector based, but not the images in a PDF file
common vector file formats include:
AI (adobe illustrator)
CDR (corelDRAW)
CMX (corel exchange)
CGM (computer graphics metafile)
DXF autoCAD
WMF (windows metafile)
EPS (encapsulated postscript)
popular vector drawing programs software packages are:
adobe illustrator
corelDRAW
xara xtreme
serif drawplus
Image size
for printing in a flyer or magazine
file size doesn't matter
most important issue is the quality of the image!
Image will be printed with at least 300 dpi
for display in a web page
file size is very important
bigger files take longer to download
quality is important too, but want to try to get best quality with the smallest file size
Length of time for uncompressed image to download in a web page
if the client is using high speed connect at 1500 kilobits per second?
33 seconds
if the client is on a modem that is 56 kilobits per second?
14 minutes
on a really old modem that is 28.8 kilobits per second?
28 minutes
Need to make web pages download as fast as possible
we want then to be the “appropriate size” for out page, in terms of proportion
want the image to look good (“high quality”)
NEED to make our image files smaller, but our image to still look good!
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