1. Year 8 Unit 2 Bitmap Graphics

2. Today’s Aims 1
Start to understand the terms bitmap, pixel and resolution 2
Start to grasp how colour depth sets the number of colours in an image 3
See links between how we use binary to handle numbers and colours

3. Bitmaps
You know computers handle different kinds of images – but here’s what makes bitmaps special…
Vector 3D
Bitmap
Shapes & fills
Wireframes
Pixels
e.g. Clip art
e.g. Movies
e.g. Photos

4. Pixels
Are the tiny coloured squares that make-up bitmap images such as photos…
Each pixel is a single colour
So each colour needs its own binary code
And there are a LOT of pixels in a photo…

5. Resolution
So how many pixels in this tiny sample?
15 pixels
10 pixels
15 x 10 = ? pixels
150

6. Resolution 2,345 x 3,519 = ? pixels 8,252,055 million 8 megapixels 8
How many pixels in the whole photo?
2,345 pixels
2,345 x 3,519 = ? pixels
8,252,055
million 8
3,519 pixels
megapixels 8

7. 2. Bitmap Basics Images such as photos are called bitmaps.
x/9/18
Images such as photos are called bitmaps.
They are made from a grid of tiny squares called pixels.
Each pixel’s colour is stored as a number in binary.

8. Colour Depth Imagine a computer that works with just 1 bit
How many colours could it use?
White 1
Black 1
1 bit has 2 values
so can handle 2 colours

9. Each pixel’s colour is stored as a number in binary.1 1
This example uses just one bit that’s either 0 or 1. So you can only have 2 colours eg black & white.
To store more colours, you need more bits This is called colour depth:

10. Colour Depth Now imagine the computer can use 2 bits
How many colours could it now use?
Black
White 1
Red
Blue 01 00 10 11 1 1 1 1
2 bits have 4 values
so can handle 4 colours

11. Spot anything familiar?
Colour Depth
Now imagine the computer can use 2 bits
How many colours could it now use?
Black
White 1
Red
Blue 01 00 10 11 1 1 1 1 1 1 1 1
Spot anything familiar?
2 bits have 4 values

12. Spot anything familiar?1 1 1 1
It’s that same binary number pattern!
Spot anything familiar?

13. Colour Depth Next, what if the computer uses 3 bits
How many colours could it cope with now? 4s 2s 1s =0 1 =1 1 =2 1 1 =3 1 =4 1 1 =5 1 1 =6 1 1 1 =7
3 bits have 8 values
so can handle 8 colours

14. Colour Depth Next, what if the computer uses 3 bits
How many colours could it cope with now?
Black
White
Red
Blue 1
Orange
Green
Grey
Yellow
111
001
110
101
010
100 1 1 1 1
110
011
000 1 1 1 1 1 1 1 1
3 bits have 8 values
so can handle 8 colours

15. Spot anything else familiar?
Colour Depth
Do you recognise a number pattern here? 1
bit = 2
colours x2
bits = 2 4
colours x2 =
bits
colours 3 8 x2 =
colours 4
bits 16 x2 =
colours x2 5
bits 32 =
colours x2 6
bits 64 =
Spot anything else familiar? 7
bits
128
colours x2
bits 8 =
1 byte
256
colours

16. Spot anything else familiar?
Colour Depth
Do you recognise a pattern here? 1
bit = 2
colours x2 2
bits = 4
colours x2 =
colours 3
bits 8 x2 =
colours 4
bits 16 x2 =
colours x2 5
bits 32 =
colours x2 6
bits 64 =
colours
Spot anything else familiar? 7
bits
128 x2
bits =
colours
1 byte 8
256

17. Spot anything else familiar?x2 x2 x2 x2 x2 x2
128 64 32 16 8 4 2
128s
64s
32s
16s 8s 4s 2s 1s 1 1 1 = ? 42
It’s that pattern of place values we used with binary!
Spot anything else familiar?

18. Colour Depth So this number pattern is all about doubling – again!1
bit = 2
colours
bits = 2 4
colours =
bits
colours 3 8 = 4
bits
colours 16 =
colours 5
bits 32 =
colours 6
bits 64 = 7
bits
128
colours
bits
1 byte 8 =
256
colours
And here’s another way to show that same pattern…

19. Can you see the pattern here?
1 bit
2 bits
3 bits
4 bits
5 bits x2 x2 x2 x2 2
values 4
values 8
values 16
values 32
values
Can you see the pattern here?

20. 1 bit
2 bits
3 bits
4 bits
5 bits
6 bits
7bits
8 bits
8 bits = 1 byte x2 x2 x2 x2 x2 x2 x2 2
values 4
values 8
values 16
values 32
values 64
values ?
values ?
values
128
values ?
values
256
values

21. To store more colours, you need more bits. This is called colour depth:
1 bit = 2 colours x2
2 bits =
4 colours x2
3 bits =
8 colours x2
4 bits = 16 colours
5 bits = 32 colours
6 bits = 64 colours
7 bits = 128 colours
8 bits = 256 colours

22. Today’s Aims 1
Start to understand the terms bitmap, pixel and resolution 2
Start to grasp how colour depth sets the number of colours in an image 3
See links between how we use binary to handle numbers and colours