Bitmap Image Formats

Transcription

1 LECTURE 5 Bitmap Image Formats CS 5513 Multimedia Systems Spring 2009 Imran Ihsan Principal Design Consultant OPUSVII Faculty of Engineering & Applied Sciences

2 1. Image Formats To store an image, the image is represented in a two dimensional matrix of pixels. Information about each pixel has to be stored. Additional information may be associated to the image as a whole, such as height and width, depth. The most popular image storing formats include PostScript, GIF (Graphics Interchange Format), JPEG, PNG, TIFF (Tagged Image File Format), BMP (Bitmap), etc. Slide: 2

3 2. GIF GIF stands for Graphic Interchange Format GIF87a first format of GIF used on the Web was called, representing its year and version. savesimagesat8pits per pixel (256 colors) by using a color loop up table (color palette). lossless file compression format (i.e. all image information retained). GIF89a format updated in 1989 to include transparency, interlacing and animation. GIF89a Transparency Transparency allows for the specification of one of the colors in the palette to be ignored while processing the image for your display device. Without Transparency With Transparency Slide: 3

4 General file format of a GIF87 standard file Screen Descriptor comprises a set of attributes that belong to every image in the file. Slide: 4

5 Color Map is set up in a very simple fashion as in Fig. However, the actual length of the table equals 2 (pixel+1) as given in the Screen Descriptor. Slide: 5

6 Each image in the file has its own Image Descriptor Slide: 6

7 3. GIF89a Interlacing Interlacing lines are stored in an unusual order: the first pass has pixel rows 1, 9, 17, etc (every eighth row); the second pass has rows 5, 13, 21, etc (every remaining fourth row); the third pass has rows 3, 7, 11, 15, etc (every remaining odd row); and the last pass has rows 2, 4, 6, etc (all the even numbered rows). allows the user to get a good idea what is coming up, so they don't have to wait until the whole image is downloaded images don't really load any faster, they just seem to. Pass 1 Pass 2 Pass 3 Pass 4 Slide: 7

8 Ifthe interlace bitissetinthelocalimagedescriptor,thentherowsoftheimage are displayed in a four pass sequence Slide: 8

9 4. GIF89a Animation Animation GIF89a specification added a few enhancements to the GIF file header which allows browsers to display multiple GIF images in a timed and/or looped sequence. Slide: 9

10 5. JPEG, JPG JPEG stands for Joint Photographic Experts Group. Saves image in 24 bit color, images can have 16,777,216 colors Best for photo quality images No animation No transparency Lossy image compression format Progressive JPEG Similar to interlaced GIF Image is transmitted and displayed in a sequence of overlays, with each overlay becoming progressively higher in quality. Slide: 10

11 6. JPEG Compression Quality : 100% Size: 54,124 bytes Quality : 90% Size: 21,868 bytes Quality : 50% Size: 9,096 bytes Quality : 10% Size: 3,519 bytes Slide: 11

12 7. JPEG Quality vs Size The chart below shows the relationships between image quality and the compression ratio. A small amount of compression has a negligible effect on the image quality, yet a substantial effect on file size. Slide: 12

13 8. JPEG Compression Requirements on JPEG implementations JPEG Image Preparation Blocks, Minimum Coded Units (MCU) JPEG Image Processing Discrete Cosine Transformation (DCT) JPEG Quantization Quantization Tables JPEG Entropy Encoding Run length Coding/Huffman Encoding Slide: 13

14 9. Variants of Image Compression Four different modes Lossy Sequential DCT based mode Baseline process that must be supported by every JPEG implementation. Expanded Lossy DCT based mode enhancements to baseline process Lossless mode low compression ratio allows perfect reconstruction of original image Hierarchical mode accommodates images of different resolutions Slide: 14

15 10. JPEG2000 Designed to overcome the limitations of the original JPEG standard and provide high quality images at low bit rates. Advantages Over JPEG Better image quality at the same file size Good image quality even at very high compression ratios, over 80:1 Large Images: JPEG is restricted to 64kx64k images. JPEG2000 will handle image sizes up to (2 32 1) Support both lossless and lossy compression Handle up to 256 channels of information Improved error resilience Compound document can include non image data as part of the file Slide: 15

16 ORIGINAL IMAGE JPEG2000 (1:20) JPEG (1:20) Slide: 16

17 Compression Ratio => 345:1 (top row), 195:1 (bottom row) Original Image (600x480x24) JPEG (3 KB, 2441 bytes) JPEG2000 (3 KB, 2483 bytes) BMP (844 KB) JPEG (5 KB, 4370 bytes) JPEG2000 (5 KB, 4261 bytes) Slide: 17

18 11. PNG Stands for Portable Network Graphic. Designed to replace GIF. Features: Lossless compression Support for up to 48 bit color information Varying levels of transparency Gamma correction Better compression, percent smaller files than GIF Interlacing Patent free compression algorithm Slide: 18

19 12. PNG Alpha Values Slide: 19

20 13. Possible PNG image pixel types true color with alpha PNG image has 32 bit: 8 bit for red, green, blue and alpha. Slide: 20

21 14. JPEG vs GIF Is JPEG always better than GIF? JPEG is good for photo image compression. For cartoon and B/W images, GIF always performs much better. 24 bit BMP JPEG GIF 363 KB 23 KB 6 KB Slide: 21

22 15. Which image should be used? Small images, like icons and buttons: GIF or PNG Line art, grayscale (black and white), cartoons: GIF or PNG 24 bit color depth lossless Image: PNG, JPG2000 Scanned images and photographs: JPEG Large images or images with a lot of detail: JPEG Slide: 22

23 16. Other Formats TIFF Tagged Image File Format (TIFF), stores many different types of images (e.g., monochrome, grayscale, 8 bit & 24 bit RGB,etc.) > tagged Developed in the 1980 s TIFF is a lossless format It does not provide any major advantages over JPEG and is not as user controllable It appears to be declining in popularity EXIF EXIF (Exchange Image File) is an image format for digital cameras: Compressed EXIF files use the baseline JPEG format. A variety of tags are available to facilitate higher quality printing, since information about the camera and picture taking conditions (flash, exposure, white balance, etc.) can be stored. The EXIF standard also includes specification of file format for audio that accompanies digital images. Slide: 23

24 PS and 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. Postscript includes text as well as vector/structured graphics. GL bit mapped images can be included in output files. Encapsulated Postscript files add some additional information for inclusion of Postscript files in another document. Postscript page description language itself does not provide compression; in fact, Postscript files are just stored as ASCII. 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. Slide: 24

25 17. Some Other JPEG Formats Microsoft Windows: BMP Major system standard graphics file format for Microsoft Windows, used in Microsoft Paint and other programs Many sub variants within the BMP standard Microsoft Windows: WMF (WindowsMetafile) Native vector file format for the Windows operating environment: Features 1. Consist of a collection of GDI (Graphics Device Interface) function calls, also native to the Windows environment. e.g., PlayMetaFile() function to render 2. Ostensibly device independent and size unlimited Macintosh: PAINT and PICT: PAINT was originally used in the MacPaint program, initially only for 1 bit monochrome images PICT format is used in MacDraw (a vector based drawing program) for storing structured graphics. Unix X windows: PPM (Portable Pixmap) Supports 24 bit color bitmaps, and can be manipulated using many public domain graphic editors, e.g., xv. Adobe Photoshop: PSD Much more... (check save as in PhotoShop or ACDSee) Slide: 25

26 #2. Assignment Find each of the following kind of image and store them in GIF, JPEG and PNG. Formulate a table that shows the file formats used and their relative sizes. Images are of type Circuit Diagram Architectural Drawing Botanical Drawing PIE Chart Finger Print A Map of the World Brain Scans Illustration for a children s alphabet book A reproduction of Mona Lisa A simple cartoon Character A photograph of yourself Floor plan of a museum Slide: 26