1. Describe how a graphic would be stored in memory using a bit-mapped graphics package.

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1 HIGHER COMPUTING COMPUTER SYSTEMS DATA REPRESENTATION GRAPHICS SUCCESS CRITERIA I can describe the bit map method of graphic representation using examples of colour or greyscale bit maps. I can describe the relationship of bit depth to the number of colours represented per pixel, using examples up to and including 24 bit depth (true colour) I can calculate the storage requirements for bit mapped images, given the number of pixels used in the image and the number of bits used to represent colours or shades of grey for each pixel. I can describe the relationship between the bit depth and file size of a bit mapped graphic. I can explain the need for data compression using the storage of bit-map graphic files, as examples. I can describe the vector graphics method of graphic representation. I can describe the relative advantages and disadvantages of bit mapped and vector graphics. 1. Describe how a graphic would be stored in memory using a bit-mapped graphics package. The image is represented on screen by a two dimensional grid of pixels, where there is a one-to-one mapping between pixels and bits. 2. Ian is aware that the quality of a bit-mapped image depends on its bit depth and resolution. What is meant by the terms bit depth resolution? The number of bits used to store the value of each pixel, used to represent colours or shades of grey. The number of pixels in a given area OR the number of dots per inch. 3. How many different colours can be defined by bit depths of 8 bits 16 bits (iii) 24 bits? 2 8 = 256 colours 2 16 = colours (iii) 2 24 = colours 4. A scanner is set to a resolution of 1200dpi using 24 bit colour depth and the photographs are 6 inches by 8 inches. Calculate the uncompressed size of the file. Express your answer in appropriate units. Show all working. Size of file = total number of pixels * bit depth Number of pixels = (6 x 8 x 1200 x 1200) Bit Depth = 24 = * 24 = bits / 8 = Bytes / 1024 = KB / 1024 = MB 5. Ian scans a photograph. The image is 800 by 600 pixels and uses 65,536 colours. Calculate the file size of the scanned image. Show all working and express your answer in appropriate units. 1 of 6

2 Number of pixels = 800 x 600 = Bit Depth = colours requires 16 bits per pixel = x 16 = bits / 8 = Bytes / 1024 = KB 6. An image is 4 X 6 inches has a resolution of 800 dpi and each pixel can display different colours. Calculate the storage requirements for this image. Express your answer using suitable units. Show ALL working. = (4 x 6 x 800 x 800) x 16 bits = bits / 8 = Bytes / 1024 = KB / 1024 = 29.3 MB 7. John uses his digital camera to take photographs. It has a 512 Megabyte memory card. His camera uses 16,777,216 colours and is set to a resolution of 3000 X 2000 pixels. Calculate the file size of a single image. Your answer should be in appropriate units. Show all working. What is the maximum number of images of this size that can be stored on John s memory card? = (3000 x 2000) x 24 bits = 144,000,000 bits = 18,000,000 Bytes / 1024 = KB / 1024 = 17.2 MB No of images is 512 MB/17.2 MB = so 29 images can be stored. 8. An image created is 3 X 2 inches at 2048 dpi and has a file size of exactly 72MB. What is the colour depth of the image? Size of file = total number of pixels X bit depth Size of file = 72 MB = bits Total number of pixels = 2 x 3 x 2048 x 2048 = bits = = x bit depth = / = bit depth Bit depth = 24 bits 9. A 4 inch by 6 inch photograph with a resolution of 600 dpi and using 16-bit colour depth is stored. Calculate the file size of the photograph. State your answer using appropriate units. Show all your working pixels bits (= ) = Mb 2 of 6

3 10. Winnie is going to include a photograph of her garden on her website. The image has a resolution of 640 X 800 pixels. The size of the file is 500 Kilobytes. The file is stored in GIF format. How many colours does GIF allow? If the resolution of the image is doubled, what effect will this have on the file size? Explain your answer. 256 colours 640 X 800 = bits = Bytes = 62.5 KB 1280 X 1600 = bits = Bytes = 250 KB New file size is 4 X the original. As resolution is a linear measure there are four times the pixels in a given area. 11. What happens if you increase the bit depth of a file? It increases the file size. The number of colours or shades of grey that can be represented per pixel would increase. 12. John changes the setting of his digital camera to reduce the bit-depth. Describe one effect this will have. He will be able to store more images. The file size of each image will be reduced. The number of available colours (per pixel) is reduced. 13. Explain the need for compression when storing graphics. Compression reduces the file size which reduces the storage space required or download time of the graphic. 14. John s photographs are stored as JPEGs. State two reasons why digital images are stored as JPEGs. JPEG files are compressed and therefore reduce file size. JPEG is a common file format and used by many software applications. JPEG supports a bit depth of 24 bits, representing millions of colours which results in more realistic images. 15. The graphic on the right was created using a vector package. (iii) (iii) Describe how graphics are stored using a vector package. Describe in detail how this graphic would be stored using a vector package. State two operations that could be carried out on the object. Each object is stored as a list of attributes (a mathematical description) Square: start x, start y, line length, line width, line colour, fill colour The object could be - filled with a colour or pattern - line pattern or colour could be altered - line width could be changed - it could be scaled - it could be rotated 3 of 6

4 16. This image below could be created by a vector graphics package. Describe one of the objects in the image in terms of the vectors that would be used to recreate the image on the screen with a vector graphics package. STOP rectangle start x, start y, line length, line breadth, line width, line colour, fill colour circle centre x, centre y, radius, line colour, fill colour text font type, font size, start x, start y. 17. Describe one advantage and one disadvantage of using a bit-mapped package and a vector graphics package. Bit-mapped: Advantage Disadvantage A graphic can be captured by a digitizer such as a digital camera or scanner. An image can be edited a pixel level. As the graphic resolution is fixed in size when created, it will take the same amount of storage space no matter how complex you make the graphic. Bit-mapped graphics are best suited to photographs. A pixel can be difficult to edit without affecting the rest of the graphic. You cannot isolate an individual object in a graphic and edit it. When shapes that overlap are separated they leave blank areas. File storage sizes are greater compared to vector graphics. Graphics are resolution dependent. Because the resolution is fixed when the bitmap is created it cannot take advantage of high resolution output devices. If you scale a graphic upwards it will become pixilated. If you scale it downwards it will become dense. Vector: Advantages Individual attributes of objects in a graphic can be easily edited such as line colour, line width or resized, without affecting the rest of the image. They are more storage efficient than bit-map graphics for simple graphics and could have smaller file size. Objects can be layered. If overlapping objects are separated their shapes will remain the same. Graphics are resolution independent. They will be displayed at the highest resolution of an output device. If you scale a graphic upwards or downwards it will not lose its original quality and still be output clearly and accurately. Vector graphics are best suited to logos, line drawings, cartoons, diagrams and simple illustrations. 4 of 6

5 Disadvantage The file size increases rapidly the more complicated the object layers of the graphic. It is not possible to edit individual pixels. They are not suited to photographic images. 18. Students use vector graphics to create 2D diagrams. Describe two advantages of using vector graphics over bitmapped graphics. They are more storage efficient than bit-map graphics for simple graphics and could have smaller file size. Graphics are resolution independent. They will be displayed at the highest resolution of an output device. Objects to be layered. If you scale a graphic upwards or downwards it will not lose its original quality and still be output clearly and accurately. Individual attributes of objects in a graphic can be easily edited such as line colour, line width or resized, without affecting the rest of the image. 19. Is the image bit-mapped or vector? Justify your answer. The image is bit-mapped: It is made up of grid of individual pixels. Is not made up of a collection of individual objects. It is pixilated. It is a photograph taken with a digital camera. 20. A scanner is used to capture photographs. When the images are printed, lines are discovered across them. The lines are where the original photographs had been folded over. Explain why a bit-mapped package would be used to remove the lines. The image can be edited at pixel level. Each pixel in the image will have to be edited to match surrounding pixels. There are no individual objects to edit. 21. Dicken has to create an advertising image for a new radio show called Big Sister. The creator likes the idea of an ear as the logo. The logo can be created using vector or bit-mapped graphics. State one reason for using bitmapped graphics to create this logo. Individual pixels can be edited. Can apply special effect e.g. blur 5 of 6

6 22. Peter runs a photography store. He develops and prints graphics in various formats. He decides to offer a red eye removal service on all digital images. Would he use bit-mapped graphic or vector graphic software? Explain your answer. Bitmapped as it allows individual pixels to be edited All digital photographs are bitmapped. 23. If this is the graphic to be created, explain why it would take up less memory if created using vector rather than bit-mapped graphics. The graphic is not stored as grid of pixels as in bit-mapped graphics Only the descriptions of two simple objects and their attributes need to be stored. 24. If you increase the resolution at which a vector graphic object is displayed, what effect will it have on the graphic? It displays clearly and accurately. 25. Two versions of the image below have been created. One in a bit-mapped graphic package and the other in a vector graphic package. If a circle is added to the centre of the graphic, what is the effect on the file size in each case? Bit-mapped graphic no change Vector graphic increase in the file size 6 of 6