Chapter 3 Graphics and Image Data R i Representations 3.1 Graphics/Image Data Types 3.2 Popular File Formats 3 3 Further Exploration 3.3 Further Exploration 1 3.1 Graphics/Image Data Types y The number of file formats used in multimedia i lif continues to proliferate. y Table 3.1 shows a list of some file formats used in the popular product Adobe Director Adobe Director . → Link to details on Director file formats. 2 1-bit Images bit Images y Pixels , or pels : picture elements y Each pixel is stored as a single bit (0 or 1), l f d t bi i so also referred to as binary image . y Such an image is also called a 1-bit monochrome image since it contains no color . y Fig. 3.1 shows a 1-bit monochrome image ( ll d "L " b l i di i i (called "Lena" by multimedia scientists — this is a standard image used to illustrate many algorithms). 3 Fig. 3.1: Monochrome 1-bit Lena image. 4
8-bit Gray bit Gray-level Images level Images y Each pixel has a gray-value between 0 and 255 255. Each pixel is represented by a single byte; e.g., a dark pixel might have a value of 10 and a , and a bright one might be 230 . y Bitmap : The two-dimensional array of pixel values that represents the graphics/image data. y Image resolution refers to the number of pixels in a digital image (higher resolution always yields better quality). ◦ Fairly high resolution for such an image might be 1, 600 × 1, 200 , whereas lower resolution might be 640 × 480 480 . 5 y Frame buffer: Hardware used to store bitmap. ◦ Video card (actually a graphics card) is used for this purpose. ◦ The resolution of the video card does not have to match the desired resolution of the image, but if not enough video card memory is available then the data has to be shifted around in RAM for display has to be shifted around in RAM for display. y 8-bit image can be thought of as a set of 1-bit bit- planes , where each plane consists of a 1-bit representation of the image at higher and higher levels of "elevation": a bit is turned on if the image pixel has a nonzero value that is at or above that bit level. y Fig. 3.2 displays the concept of bit-planes graphically. 6 Fig 3 2: Bit ‐ planes for 8 ‐ bit grayscale image 7 Fig. 3.2: Bit planes for 8 bit grayscale image. Multimedia Presentation y Each pixel is usually stored as a byte (a value between 0 to 255), so a 640 × 480 grayscale image requires 300 kB of storage (640 × 480 = 307, 200). y Fig. 3.3 shows the Lena image again, but this time in grayscale. y 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 . 8
Fig. 3.3: Grayscale image of Lena. 9 Dithering y 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 l f i ti 1 bit i t values, for printing on a 1-bit printer .
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- Fig, Portable Network Graphics, Graphics Interchange Format, Tagged Image File Format