1. http://en.wikipedia.org/wiki/Raster_graphics

2. Graphics An image is made up of tiny dots called pixels (“picture elements”) The resolution determines the quality of the picture. The smaller the pixels the finer the detail that can be displayed on the screen. 300 dpi 25 dpi

3. Graphics Packages Graphics packages can be classed as either: Bitmapped / Raster graphics Vector graphics http://en.wikipedia.org/wiki/Vector_graphics http://en.wikipedia.org/wiki/Raster_graphics

4. Bitmapped Graphics The image is saved in a two dimensional array using binary numbers to represent the colors in the pixels. Each pixel is represented by the same number of bits. 001111 1 1 1 1 1111 0 0 00 0 0 0 0 0 0 0 0 0 0 00 1 1 1 1 0 0 0 0 0 0 0 0 0 00 0 0 011 0000 00 00 1100 11 one bit In this example, two colors have been used (black/white) and each pixel is represented by one bit (“monochrome”) 0 = white, 1 = black

5. Bitmapped Graphics two bits In this example, four colors are used in the graphic and each pixel is represented using two bits. 00 = white, 01 = blue, 10 = red, 11 = black 00 10 00 10 00 1000 01 00 11 00 11 00 11 00

6. Bitmapped Graphics How many colors could be represented using 3 bits ?

7. Bitmapped Graphics How many colors could be represented using 3 bits ? 000 001 010 011 100 101 110 111 8 colors ( 2 3 ) could be represented using 3 bits ( 3 bpp )

8. Bit Depth The number of bits used to represent the color of the pixels is called the bit depth. ColorsBits or Bpp 21 42 83 164 325 646 1287 2568 6553616 16777216 24 (True Color)

9. Color Depth http://en.wikipedia.org/wiki/Color_depth

10. Resolution http://en.wikipedia.org/wiki/Display_resolution

11. Refresh Rate http://en.wikipedia.org/wiki/Refresh_rate

12. Bitmap Storage 8 Pixels x 8 pixels x 1 bit = 64 bits in theory BITMAPHEAD BITMAPINFOHEADER RGBQUAD array COLOR-INDEX array Operating System file system info  Something like 752 bits (or 94 bytes)

14. Indexed to a palette

19. Exercise 1: Storage Requirements An image, 5 inches by 7 inches is stored at 600 dots per inch (dpi) using 65536 colors. How much memory would be required to store this image?  7 inches   5 inches 

20. Storage Requirements (examples) Pixels used to store image = ( 5 x 600 dpi ) x ( 7 x 600 dpi ) = 12,600,000 Image size5 in x 7 in DPI600 Colors65536

21. Storage Requirements (examples) Pixels used to store image = ( 5 x 600 ) x ( 7 x 600 ) = 12,600,000 65,536 colors = 16 bits = 2 bytes Image size5 in x 7 in DPI600 Colors65536

22. Storage Requirements (examples) Pixels used to store image = ( 5 x 600 ) x ( 7 x 600 ) = 12,600,000 65,536 colors = 16 bits = 2 bytes Amount of memory = 12,600,000 x 2 bytes = 25,200,000 bytes = 25,200,000 / 1024 / 1024 = 24.03 Mb Image size5 in x 7 in DPI600 Colors65536 there are 1024 bytes in a kilobyte there are 1024 kilobytes in a megabyte

23. Exercise 2: Storage Requirements A 10 inch by 8 inch photograph is stored at 1200 dpi using 256 colors. How much memory would be required to store this image?  10 inches   8 inches 

24. Storage Requirements (examples) Image size10 in x 8 in DPI1200 Colors256 Pixels used to store image = ( 10 x 1200 ) x ( 8 x 1200 ) = 115200000

25. Storage Requirements (examples) Image size10 in x 8 in DPI1200 Colors256 Pixels used to store image = ( 10 x 1200 ) x ( 8 x 1200 ) = 115200000 256 colors = 8 bits = 1 byte

26. Storage Requirements (examples) Image size10 in x 8 in DPI1200 Colors256 Pixels used to store image = ( 10 x 1200 ) x ( 8 x 1200 ) = 115200000 256 colours = 8 bits = 1 byte Amount of memory = 115200000 x 1 byte = 115200000 bytes = 115200000 / 1024 / 1024 = 109.86 Mb

27. Advantages of bitmapped graphics The file area is fixed The A4 page in the previous slide will need 110 Mb of memory irrespective of the size of the graphic on the page. The graphic can be edited to pixel level

28. Disadvantages of bitmapped graphics Enlarging the image causes a loss in quality An A4 page using 256 colors at 1200 dpi will need 110 Mb of memory An A4 page using 65,536 colors at 1200 dpi will need 220 Mb of memory Compression can be used to decrease the size of these files when they are stored on disc The file is printed at the same resolution as it is shown on the screen The file size is very large A graphic created on the screen using 300 dpi cannot be printed at a higher resolution (e.g., 600 dpi)

29. Data compression Reduces the size of bit-mapped files to make them faster to download and take up less disk space / memory There are a variety of different techniques used to compress files Lossless Compression None of the original data is lost Methods are found to store the information in a more efficient way e.g. Instead of 250 white pixels, you store 250 times white pixel (pointing to pixel by address) Example - TIFF Lossy Compression This method involves sacrificing some of the data that our eyes will probably not notice e.g. Instead of using millions of colors we can just use hundreds. Therefore we only use 16 bits rather than 24 bits to represent the color depth Example - JPEG