Session Objectives#9
COULD explain the representation of an image as a series of pixels represented in
binary
SHOULD describe the effect of colour depth and resolution on the size of an image
file.
MUST identify the data needed for a computer to display an image correctly
(metadata)
Create an app that will display any given image at the centre of the
screen.
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Starter
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Representing Data: Images
What are the differences between these 2 images:
This is called a bitmap
image which is made of
a number of coloured
dots called pixels
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This is a vector graphic
which has program of a
instructions to draw the
image therefore keeping
its proportions.
Representing Data: Images
Images are stored in computers as binary data. The data is
interpreted by the processor and displayed according to the data
it is given. Bitmap images consist of a number of boxes of
colour, called pixels (picture elements).
Pixel – The smallest element of an image. The dots make the
image on screen.
The dimensions of the image above are 8 x 5 pixels. If this is a
1 bit image (1 bit per pixel or 1bpp) it would have 2 values per
pixel, 0 for white and 1 for black.
TASK: Draw the image from the following binary data: 00000100
00000110 11111100 01001000 01001000.
EXT: Give 2 technical explanations of how this picture could be
improved?
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Representing Data: Images
The above image contains 40 pixels.
If this was stored in an 8 bit file
then it would account for 5 bytes of data
(8 x 5) / 8 = 5 bytes
TASK 2: If each pixel was expanded to hold 2 bits, then 2 shades
of grey could be used i.e 00 is white, 01 is light grey, 10 is
dark grey and 11 is black.
a) How large would the image file size be now?
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Representing Data: Images
b)
A good quality monochrome picture would use 8 bits to
represent each pixel (8 bpp or one byte), therefore 256
different shades can be achieved. How large would the file size
now be? Show your working.
c)
If the picture was in colour, each pixel would need a red,
green and blue value, so each pixel would need a minimum of 3
bytes. What would the minimum memory requirement be for storing
the file in colour? Show your working
d)
If the resolution of the image was increased (the number
of pixels used to create the image) to a size of 1600 x 1000,
and keeping the same colour depth, what would the minimum file
size now be?
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Recap
Key Terms:
Resolution: The concentration of pixels used to create an image
(usually measured in dots or pixels per inch, dpi or ppi)
Colour depth (or bit depth): The number of bits used for each
pixel or dot. The more bits the more colours that can be
represented.
Metadata: Information about the image which the computer needs
to recreate the image correctly from a binary data file. Must
contain height and width in pixels, and colour depth in bpp
(bits per pixel)
Height &
Width in
pixels
(8 x 5)
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Colour depth:
bits per pixel,
bpp (2 bpp)