L3 how do images work

How do images work?

Year 11 Computing

Learning Outcomes

By the end of this session you will be able to;• Explain how images are represented as a

series of pixels• Explain the need for metadata to be

included within a file• Discuss how colour depth and resolution

effect file size

Using Binary Patterns

• Binary patterns are used to store graphics. In a bit-mapped system the image is divided up into a grid.

• Each cell in the grid is called a PIXEL (picture element).

• A pixel is the smallest

addressable area or smallest

solid block of colour of an image.

Resolution

• The resolution of an image or VDU screen is expressed as the number of pixels by row by the number of pixels per column.

• An image with a resolution of 1024 by 798 pixels has 1024 x 798 pixels (817,152 pixels).

Resolution of an Image

• The resolution of an image must not be confused with the resolution of the screen.

• The colour of each pixel is stored as a binary pattern in memory.

• In a monochrome screen (two colours) just 1 bit will be needed for each pixel: – 0 for white – 1 for black

Resolution of an Image

• How many colours can there be in the following bit-codes?– 2 bit-code– 3 bit-code– 8 bit-code– 16 bit-code

Metadata

• Metadata is data about data. Images from digital cameras are stored with a considerable amount of metadata that records information such as the size of the image, the camera used, the camera settings and the date.

Image File Size

• Resolution is commonly expressed in dots per inch or dpi. When you buy a scanner or printer, you need to know its best resolutions so that you can tell if it will produce the quality you require. Bitmapped graphics do not enlarge well. When they are enlarged, the pixels get enlarged too and you can see them, which makes the image poor quality or pixelated.

• Photographs have to be bitmapped because the composition is unpredictable. They are often compressed in order to make them take up less file space. JPEG image files are compressed bitmaps.

Estimating Image Size

If 12-bit colour is used and the image size is 800 x 600 pixels, what is the minimum file size for the image?

Step 1 – get you resolution

Step 2 – get the bits per pixel

Step 3 – calculate the total bits

Step 4 – covert to bytes

Step 5 – Convert to kilobytes

Estimating Image Size

If 12-bit colour is used and the image size is 800 x 600 pixels, what is the minimum file size for the image?• Number of pixels = 800 x 600 • Number of bits per pixel = 12 • Total number of bits = 12 x 800 x 600 = 5760000 • Number of bytes = 5760000 ÷ 8 = 720,000 • Number of kilobytes = 720,000 ÷ 1000 = 720

Your Task

• Visit topic 7 of the GCSE Computing section and complete the knowledge quiz.

• You may need to read through the ‘Metadata’ file first to help with one of the questions.

• Try the Computing Podcasts course to give you some more help.