Chapter Three Graphical Image and Data Representation

1. Chapter Three Graphical Image and Data Representation Dr. Abbas Fadhil Mohammed Ali AL-Jubooriabbas.aljuboori@uokerbala.edu.iqabbaszain2003@yahoo.com2013

2. Graphics: Terminology Pixels picture elements in digital images Image Resolution number of pixels in a digital image (Higher resolution always yields better quality.) -- Most common Aspect ratio: 3:4 (lines:columns) Bit-Map a representation for the graphic/image data in the same manner as they are stored in video memory. Bits/pixel also contributes to the quality of the image.

3. Monochrome (1-bit) image Simplest type of image - Each pixel consist of only ON / OFF information - Called 1-bit monochrome (since no color) image - Suitable for simple graphics & text JBIG (Joint Bi-level Image experts Group ), A compression standard for binary image

4. Monochrome (1-bit) image

5. Monochrome (1-bit) image

6. Gray (8-bit) image • Gray levels between 0 to 255 (black to white) • Image resolution refers the number of pixels in an image. -- The higher resolution, the more pixels in the image. -- Higher resolution allows for more detail and subtle color transitions in an image. Image resolution refers the Shown is 256x256 byte image

7. Gray (8-bit) image

8. Gray (8-bit) image

9. Bits per pixel

10. Color Image 24- bit color image = Each pixel is represented by 3 bytes, RGB = Each R, G, B are in the range 0-255 = 256 x 256 x 256 possible colors = If space is a concern, reasonably accurate color image can be obtained by quantizing the color information 8- bit color image = Carefully chosen 256 colors represent the image = We get information can be received from the color histogram

11. Color Image

12. Color Image = For 640 x 480 image represented with 24 bits requires 921.6 kbytes , How ? 8 bit requires 300 kbytes = The 8-bit color image stores only the index of the color, the file header will contain the mapping information. = The table where the color information for all the 256 indices is called color lookup table (LUT)

13. Color Image

14. Dithering • When an image is printed, the basic strategy of dithering is used, which trades intensity resolution for spatial resolution to provide ability to print multi-level images on 2-level (1-bit) printers. • Dithering is used to calculate patterns of dots such that values from 0 to 255 correspond to patterns that are more and more filled at darker pixel values, for printing on a 1-bit printer.

15. Dithering • The main strategy is to replace a pixel value by a larger pattern, say 2×2 or 4×4 binary matrix, such that the resulting black and white dot pattern approximates shading. • For example, if we use a 2× 2 dither matrix • We can first re-map image values in 0..255 into the new range 0..4 by (integer) dividing by 256/5. Then, e.g., if the pixel value is 0 we

16. Dithering print nothing, in a 2× 2 area of printer output. But if the pixel value is 4 we print all four dots. • The rule is: If the intensity is > the dither matrix entry then print an on dot at that entry location. Replace each pixel by an n× n matrix of dots.

17. Dithering

18. Dithering

19. Dithering

20. Popular File Formats Many graphical/imaging applications create their own file format particular to the systems they are executed upon. The following are the most popular System Dependent Formats: Microsoft Windows: BMP • http://www.cica.indiana.edu/graphics/image_specs/bmp.format.txt • A system standard graphics file format for Microsoft Windows • Used in PC Paintbrush and other programs • It is capable of storing 24-bit bitmap images Bitmap File Header Format Ch 2 Byte “BM” FileSize Long RS Long OfsPos Long HeadSiz Long width Long Height Long Plan Integer BitPlan Integer Comp Long DataSize Long C1 Long C2 Long R1 Long R2 Long

21. Popular File Formats The following brief format descriptions are the most commonly used formats. Known as Standard System Independent Formats 1. GIF standard: (We examine GIF standard because it is so simple! yet contains many common elements.) Limited to 8-bit (256) color images only, which, while producing acceptable color images, is best suited for images with few distinctive colors (e.g., graphics or drawing). 2. JPEG: currently the most important common file format.

22. Popular File Formats GIF • One of the most important format because of its historical connection to the WWW and HTML markup language as the first image type recognized by net browsers. • Graphics Interchange Format (GIF) devised by the UNISYS Corp. and Compuserve, initially for transmitting graphical images over phone lines via modems • Uses the Lempel-Ziv Welch algorithm (a form of Huffman Coding), modified slightly for image scan line packets (line grouping of pixels) • Limited to only 8-bit (256) color images, suitable for images with few distinctive colors (e.g., graphics drawing) • GIF89a supports simple animation (Graphics Control Extension has control over delay time, transparent index, etc. Software such as Coral Draw will allow access and editing of GIF images.) • GIF standard supports interlacing — successive display of pixels in widely-paced

23. Popular File Formats GIF • Animation via a Graphics Control Extension block in the data, provides simple control over delay time, a transparency index, etc. • For the standard specification, the general file format of a showed in the next slides. • Screen Descriptor comprises a set of attributes that belong to every image in the file. According to the GIF87 standard, it is defined as in next slide.

24. GIF

25. GIF

26. GIF

27. GIF

28. GIF

29. JPEG

30. By using Dithering Technique • Can lower download time FILE Formats - .GIF options Dithering: • dithering increases the apparent number of colors in the output image. • Changes the colors of pixels in a neighborhood so that the average color in each neighborhood approximates the original RGB color. 8 colors – no dither - 8 colors – dither - Onion.jpg

31. By using Dithering Technique • Can lower download time .jpg  .gif FILE Formats - .GIF • You can reduce the number of colors in a GIF and choose options to control the way colors dither in the application or in a browser.

32. Interlacing Applies to .GIF files Interlacing describes --- How images are downloaded to your screen Interlacing: • Is a process by which the image is drawn in a series of passes rather than all at the same time (file size bigger)

33. Transparency Applies to Transparency Transparent image • Is one whose background allows the page background to display through so the image appears to have no visible border around it. • Image will appear to float right against the page background. • Currently the only formats that support transparency are the GIF and PNG formats.

34. Transparency Image Optimization - cont

35. Image Optimization - cont By using Dithering Technique • Can lower download time

36. JPEG JPEG: The most important current standard for image compression. The human vision system has some specific limitations and JPEG takes advantage of these to achieve high rates of compression. JPEG allows the user to set a desired level of quality, or compression ratio (input divided by output).

37. JPEG As an example, Fig. 3.17 shows our forestfire. image, with a quality factor Q=10%. This image is a mere 1.5% of the original size. In comparison, a JPEG image with Q=75% yields an image size 5.6% of the original, whereas a GIF version of this image compresses down to 23.0% of uncompressed image size.

38. GIF versus JPEG

39. PNG FILE Formats - .PNG (Portable Network Graphics) • Developed as a replacement for GIF due to legal issues resulting from GIFs use of the patented LZW compression scheme • PNG-8 format uses 8-bit color (Like the GIF format, PNG-8 efficiently compresses solid areas of color while preserving sharp detail) • PNG-24 format supports 24-bit color (preserves a large range of color and subtle variations in brightness and hue) • Benefits: • Smaller or same size as GIFs and still lossless (5%-25% smaller) • PNG is better than JPG when dealing with images that contain text or line art, see :http://en.wikipedia.org/wiki/Portable_Network_Graphics#Comparison_with_JPEG • Supports opacity and transluacency • Disadvantage: • All browsers do not support PNG-8 • No animation

40. PNG PNG format: standing for Portable Network Graphics→ meant to supersede the GIF standard, and extends it in important ways. Special features of PNG files include: • Support for up to 48 bits of color information — a large increase. 2. Files may contain gamma-correction information for correct display of color images, as well as alpha-channel information for such uses as control of transparency. 3. The display progressively displays pixels in a 2-dimensional fashion by showing a few pixels at a time over seven passes through each 8 × 8 block of an image.

41. PS & PDF Postscript is an important language for typesetting, and many high-end printers have a Postscript interpreter built into them. Postscript is a vector-based picture language, rather than pixel-based: page element definitions are essentially in terms of vectors.

42. PS & PDF 1- Postscript includes text as well as vector/structured graphics. 2- GL (graphic language) bit-mapped images can be included in output files. 3- Encapsulated Postscript files add some additional information for inclusion of Postscript files in another document. 4- Postscript page description language itself does not provide compression; in fact, Postscript files are just stored as ASCII.

43. PS & PDF - Another text + figures language has begun to supersede or at least parallel Postscript: Adobe Systems Inc. includes LZW compression in its Portable Document Format (PDF) file format. – PDF files that do not include images have about the same compression ratio, 2:1 or 3:1, as do files compressed with other LZW-based compression tools.