← Image Representation (Colour by Numbers)
Flashcards
Image Representation (Colour by Numbers)
Generated from the original open resource by CS Unplugged. Built only from the resource — nothing invented. Free, no login.
Image Representation (Colour by Numbers) — Flashcards
| Front | Back |
|---|---|
| What is a pixel? | A "picture element" — one of the small dots that a computer screen is divided into to make up an image. |
| How does a computer store a black and white picture? | It stores which pixels are black and which are white, using numbers. |
| What does the first number in a coded picture line represent? | The number of white pixels at the start of that line. |
| What does it mean if a line's code begins with a 0? | It means the first pixel in that line is black (since the first number always represents white pixels, a 0 shows there are no white pixels before the black ones start). |
| What is run-length coding? | A method of compressing images by recording the length ("run") of consecutive pixels of the same colour, instead of storing every single pixel. |
| Why do computers compress images? | To save storage space and reduce the time needed to transmit pictures (e.g., over fax or the internet). |
| What does a fax machine do? | It scans a black and white page into pixels (about 1000 × 2000), sends them via modem to another fax machine, which prints the pixels onto paper. |
| Why do fax images compress well? | Because they often have large blocks of white (like margins) or black (like lines), which repeat and can be run-length coded efficiently. |
| By how much are fax images generally compressed? | To about a seventh of their original size. |
| By how much can photographs/pictures be compressed? | Often to a tenth or even a hundredth of their original size, using different compression techniques. |
| What would happen without image compression? | Images would take much longer to transmit and require much more storage space, making things like faxes or web photos impractical. |
| In coloured run-length coding, what do the two numbers in each pair represent? | The first number gives the length of the run of pixels; the second number specifies the colour code (e.g., 0 = black, 1 = red, 2 = green). |
| Why is there usually a limit to the length of a pixel run in coding? | Because the run length is represented as a binary number, which can only hold values up to a certain size. |
| How can you code a run of twelve black pixels if you can only use numbers up to seven? | Code seven black pixels, then a run of zero white pixels, then a run of five black pixels. |
| What situations require computers to store pictures? | Drawing programs, games with graphics, and multi-media systems. |
| What skills does the Colour by Numbers activity develop? | Counting and graphing. |
| What tool is used to demonstrate image coding in this activity? | An OHP (overhead projector) transparency showing a magnified letter "a" made of pixels, with its numeric code. |
| What is the benefit of drawing on tracing paper over the grid? | It allows the final image to be viewed clearly without the grid lines showing through. |
Original licensed under CC BY-NC-SA 4.0. This teaching material is provided free by OER.ai.