Images
Unit 5 Summary - part 2
Pixels:
A digital picture is composed of thousands/millions of tiny points, called pixels.
On a computer screen, each pixel is a point of light.
When it's printed, each pixel is a dot of ink.
Bitmap:
information about images pixels.
Grid of points:
A bitmap represents an image as a grid of points.
Monochrome:
only uses white and one more colour.
To store an image, the computer uses binary: 0 means white, 1 means colour.
Every pixel has a colour, it's represented as a binary (code number).
High colour depth:
the image has a lot of colours.
example: 24-bit colour
(each colour uses 3 bytes = 16 million different colour codes).
Characteristics:
-
the colour code number is long, or made of several numbers added together.
-
the colour codes take up more space.
-
there are lots of different colour codes.
Low colour depth:
the image has only few colours.
example: 256-colour
(each colour code uses only 1 byte = 256 different colour codes).
Characteristics:
-
the colour code number is short.
-
the colour codes don't take up much space.
-
there are only a few colour codes.
Remember binary lesson?
In 8 bit binary, there are these columns:
If you put 1 in each column, it results in 255.
There's also 00000000 binary number, so altogether, 8 bits can have 256 colour variations.
Image size and file size are different
Image size:
This image above is 8 pixels wide & 8 pixels high, so the image size is:
8*8 = 64 (it means this image has 64 pixels).
File size:
If the image above is stored using 256-colour, each pixel uses 8 bits.
So the file size is:
64 bytes
or
64*8 = 512 bits
If the image above is stored using 24-bit colour, each pixel uses 3 bytes or 24 bits.
So the file size is:
64*3 = 192 bytes
or
64*24 = 1536 bits.
High resolution
means the image has very tiny pixels, that's why it has more pixels, so the image is sharp and clear.
The more pixels an image has, the bigger the file size.
Pixelation
If the pixel size is large (low resolution), the image looks pixalated/bitmapped/blurred/grainy.
The less pixels an image has, the smaller the file size.
RGB:
24-bit colour has 3 bytes for each pixel, each byte can store from 0-255 colours (0 is also included, so it has 256 colours).
- Red (the 1st byte)
255 is maximum red.
- Green (the 2nd byte)
255 is maximum green.
- Blue (the 3rd byte)
255 is maximum blue.
If all values are 0, the colour is black.
If all values are 1 (255), the colour is white.
Combining all possible colours is:
256*256*256 = 16,777,216 possible colours.
Oxford AQA IGCSE 2019
04.0.
Figure 3 shows an 8 × 8 pixel bitmap image. The image uses box four colours.
Figure 3
04.1.
What is the minimum number of bits that could be used as the colour depth for the image in Figure 3? [1 mark]
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Answer:
2 (scroll up to see previous explanation why it's 2 bits.)
04.2.
Calculate the minimum file size, in bits, that will be required to store the image in Figure 3.
Show your working. [2 marks]
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Final answer ____________________________
Answer:
width*height*colour depth = bits
8 * 8 * 2 = 128 bits.
Final answer: 128.
Oxford AQA IGCSE Specimen paper
03.
Figure 1 shows a black and white icon that is stored as a bitmap image.
In the bitmap, a white pixel is represented by the value 0 and a black pixel by the value 1.
Figure 1
03.1.
Calculate the minimum amount of memory, in bytes, that will be required to store the image in Figure 1. [2 marks]
Final Answer:
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03.2.
The image in Figure 1 is changed so that the eyes are coloured blue and the mouth is coloured red.
Calculate the minimum amount of memory, in bytes, that will be required to store the new image. [1 mark]
Final Answer:
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