Pixel Perfect
How a computer stores a picture: pixels in a bitmap grid, colour depth and resolution, how to calculate a bitmap file size from width, height and colour depth, and why more colours or more pixels make a bigger file.
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How a picture becomes numbers
A computer stores a picture as a grid of tiny coloured squares called pixels, and each pixel is just a binary code for its colour. That means an image can be measured and its file size worked out exactly. This module works through pixels, colour depth and resolution, how to calculate a bitmap file size, and why sharper, more colourful images take more room.
Image words to know
Learn these before you size up a picture.
More against less
Turning the settings up improves the picture but costs storage.
Work it in full, end in the right unit
In the exam, work a bitmap file size in full and finish in the right unit. First find how many pixels there are by multiplying the two side lengths together, then multiply that by the colour depth to get the total bits. If the answer is wanted in bytes, divide by 8 at the end. Remember that more bits per pixel means more colours, and more pixels means more detail, but both make the file bigger.
Match the term
- pixel
- bitmap
- colour depth
- resolution
- a single dot of colour
- a grid of pixels
- the bits stored for each pixel
- how many pixels the image has
Match the fact
- 1 bit per pixel
- 2 bits per pixel
- more colour depth
- higher resolution
- gives 2 colours
- gives 4 colours
- more colours but a bigger file
- more detail but a bigger file
How many pixels are there?
How do you work out the number of pixels in a bitmap image?
- Multiply the width by the height
- Add the width and the height
- Divide the width by the height
- Count the number of colours
True about bitmaps
Select the TWO true statements about bitmap images.
- Colour depth is the number of bits used for each pixel
- More colour depth gives more colours but a bigger file
- A single pixel is a whole image on its own
- Lowering the resolution makes the file bigger
Work out the file size
A bitmap is 10 pixels wide and 8 pixels tall, and uses 2 bits for each pixel. Multiply the width, the height and the colour depth together, that is 10 times 8 times 2, to find the file size in bits. What is the answer?
Order the calculation
Put the steps of finding a bitmap file size in order, earliest first.
- Multiply the width by the height to get the pixels
- Multiply the pixels by the colour depth to get the bits
- Divide by 8 to change the bits into bytes
- State the answer in the unit that was asked for
Complete the rules
A single dot of colour in an image is a _____. An image made of a grid of these is a _____. The number of bits used for each pixel is the colour _____. The number of pixels in the image is its _____.
Spot what enlarges a file
Tap the TWO things that make a bitmap file bigger.
- using a higher colour depth
- using a higher resolution
- using fewer pixels
- using fewer colours
Make the call
Read each case and choose the best answer.
- An image is 20 pixels wide and 10 pixels tall. How do you find the number of pixels?
- A photo is saved using more bits for each pixel. What happens?
- You have a file size in bits but the question asks for the answer in bytes. What do you do?
Explain bitmap images
Explain how a bitmap image is stored and how to calculate its file size.
- Explain what pixels, colour depth and resolution are
- Explain how to calculate a bitmap file size in bits and then bytes
- Explain how colour depth and resolution affect size and quality