Multimedia Graphics and Image Data Representations

Graphics/Image Data Types

• Proliferation of file formats in multimedia.

1-bit Images

• Each pixel is a single bit (0 or 1), called binary image.
• Also called 1-bit monochrome image (no color).
• Example: "Lena" image.

8-bit Gray-level Images

• Each pixel has a gray-value between 0 and 255 (single byte).
• Bitmap: 2D array of pixel values.
• Image resolution: Number of pixels (higher is better).
  • High resolution: $1,600 \times 1,200$.
  • Low resolution: $640 \times 480$.
• Frame buffer: Hardware to store bitmap (video card).
• 8-bit image as bit-planes: Each plane is a 1-bit representation at increasing levels.

Multimedia Presentation

• Pixel stored as a byte (0 to 255).
• $640 \times 480$ grayscale image: 300 kB.
• Dithering: Trades intensity resolution for spatial resolution on 1-bit printers.

Dithering

• Dithering: Calculates dot patterns for printing on 1-bit printers.
• Replaces pixel value by larger pattern (e.g., $2 \times 2$ or $4 \times 4$).
• Example dither matrix: $\begin{bmatrix} 0 & 2 \ 3 & 1 \end{bmatrix}$
• Ordered dither: Turns on printer bit if intensity is greater than matrix element at pixel position.
• Algorithm:
  • Iterate through image pixels, compare intensity to dither matrix.
  • $O(x, y) = 1$ if I(x, y) > D(i, j), else $O(x, y) = 0$

Image Data Types

• Common data types: 24-bit color and 8-bit color.
• Formats can be platform-specific or cross-platform.
• Image formats use compression (lossless or lossy).

24-bit Color Images

• Each pixel is three bytes (RGB).
• $256 \times 256 \times 256 = 16,777,216$ possible colors.
• $640 \times 480$ 24-bit image: 921.6 kB.
• Some images use 32-bit (extra byte for alpha value/transparency).

8-bit Color Images

• Uses a lookup table to store color information.
• Bytes are indices into a table with 3-byte (color) values.
• $640 \times 480$ 8-bit color image: 300 kB.

Color Look-up Tables (LUTs)

• Stores index/code value for each pixel.

How to devise a color look-up table

• Divide the RGB cube into equal slices.
• Scale R and G to 3-bit range (0..7), B to 2-bit range (0..3).
• Median-cut algorithm: Sort R values, label with "0" or "1" based on median.

Popular File Formats

• 8-bit GIF.
• JPEG.

GIF

• Limited to 8-bit (256) color images.
• Supports interlacing (4-pass display).
• Two flavors: GIF87a (original), GIF89a (animation support).

JPEG

• Most important standard for image compression.
• Exploits limitations of human vision.
• User can set quality level/compression ratio.

PNG

• Portable Network Graphics; supersedes GIF.
• Up to 48 bits of color.
• Gamma-correction, alpha-channel.
• Progressive display (2D fashion, 7 passes through $8 \times 8$ blocks).

TIFF

• Tagged Image File Format.
• Supports additional information via “tags”.
• Can store 1-bit, grayscale, 8-bit, 24-bit RGB images.
• Originally lossless, now supports JPEG compression.

EXIF

• Exchange Image File: for digital cameras.
• Uses JPEG format.
• Tags for camera/picture-taking conditions.

Graphics Animation Files

• Stores graphics animations (series of drawings).
• Less demanding than video files.
• Examples: FLC, FLI, GL, animated GIF89.

PS and PDF

• Postscript: Language for typesetting (vector-based).
• Includes text and vector graphics.
• PDF: Portable Document Format (includes LZW compression).

Some Other JPEG Formats

• WMF (Windows Metafile): Vector format for Windows.
• BMP: Major system standard for Windows.
• PAINT and PICT (Macintosh).
• PPM : graphics format for the X Window system