Bitmapped graphics

Overview

Recall questions on pixels, resolution and colour depth, how many colours a given depth allows, and calculating the storage an image needs.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Explain how a bitmap represents an image.

   Model answer

   The image is divided into a rectangular grid of small squares called pixels. Each pixel is given a single colour, and that colour is stored as a binary number. The file holds those numbers in order, row by row, so the image is reconstructed by colouring the pixels in the grid according to the stored values. No shapes or lines are recorded, only the colour of every individual pixel.

2. 2

   What is meant by the size of an image in pixels?

   Model answer

   It is the width of the image in pixels multiplied by its height in pixels, so an image 800 pixels wide and 600 pixels high has a size of 480 000 pixels. Note that the size of an image in this sense is sometimes loosely called its resolution, which is why questions state the width and height explicitly.

3. 3

   What is meant by the resolution of a bitmapped image?

   Model answer

   Resolution measures how tightly the pixels are packed rather than how many there are: it counts the pixels — each one a single dot of the picture — falling within one inch of the printed or displayed image. A 600 by 600 image reproduced at 300 pixels to the inch covers two inches square; the same file at 100 covers six inches square and the individual dots start to show. So the pixel dimensions tell you how much detail is stored, while the resolution tells you how far that detail is spread out when the image is reproduced.

4. 4

   What is meant by colour depth?

   Model answer

   Colour depth is the number of bits stored for each pixel. It determines how many different colours a pixel can be given: a colour depth of n bits allows 2ⁿ different colours.

5. 5

   An image is 1200 pixels wide and 900 pixels high, at 300 dots per inch, with a colour depth of 24 bits. State its size in pixels, its resolution, its colour depth and its printed dimensions.

   Model answer

   Its size in pixels is 1200 × 900 = 1 080 000 pixels. Its resolution is 300 dots per inch. Its colour depth is 24 bits per pixel, allowing 2²⁴ colours. Printed at 300 dots per inch it measures 1200 ÷ 300 = 4 inches wide by 900 ÷ 300 = 3 inches high.

6. 6

   How many colours are available at colour depths of 1 bit, 8 bits and 24 bits?

   Model answer

   1 bit gives 2¹ = 2 colours, normally black and white. 8 bits give 2⁸ = 256 colours. 24 bits give 2²⁴ = 16 777 216 colours, usually 8 bits each for red, green and blue.

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