Multimedia Systems- bitmap and vector images-part 5.pptx

describing the difference between vector and bitmap images

1. Multimedia Systems Lecture 2

2. Digital Representation of Images All images are represented digitally as pixels . An image is defined by image width, height, and pixel depth . The image width gives the number of pixels that span the image horizontally and the image height gives the number of lines in the image. Each pixel is further represented by a number of bits , which is commonly called the pixel depth . The pixel depth is the same for all pixels of a given image.

3. Digital Representation of Images The number of bits used per pixel in an image depends on the color space representation ( gray or color ) and is typically separated into channels. The total number of bits per pixel is, thus, the sum of the number of bits used in each channel . For instance, in grayscale images, the gray-level value is encoded on 8 bits for each pixel .( use a single value per pixel that it is called intensity or brightness.) In color images, each R, G, B channel may be represented by 8 bits each , or 24 bits for a pixel. The size of the image can, thus, vary depending on the representations used.

4. Digital Representation of Images For example , a color image has a width of 640 and height of 480. If the R, G, B color channels are represented by 8 bits each : The size of color image 640 x 480 x 3 x 8 = 7.37 Mbits (921.6 Kbytes). If this were a gray image, its size would be: 640 x 480 x 8= 2.45 Mbits (307.2 Kbytes).

5. Digital Representation of Images Sometimes, an additional channel, called the alpha channel (or channel, or matte), is also used. In such cases, gray-level images have two channels ( one gray channel and one alpha channe l) whereas color images have four channels ( one for R, one for G, one for B, and one for alpha ). The alpha channel is used to composite the pixels of the foreground with the background image , producing the final image on the far right.

6. Digital Representation of Images The alpha channel typically has the same bit depth as all the other channels, for example , 8 bits for the alpha channel, resulting in each pixel having 32 bits ( 8 bits for R, G, B and 8 bits for alpha ). The 8-bit alpha channel value for each pixel associates the degree of importance of each pixel in a compositing operation . A value of 0 indicates that the pixel will not be composited , a high value of 255 (for 8-bit alpha values) indicates that the pixel will be entirely composited , while an intermediary value indicates a percentage used in the composition.

7. Digital Representation of Images Images that are captured using digital devices, such as cameras and scanners, are normally colored . However, when these color or continuous tone images are printed, printing technologies often prefer to print halftone images where the number of colors used is minimized to lower printing costs. The halftone printing process creates ranges of grays or colors by using variable-sized dots. The resolution of halftone images is measured in terms of the frequency of the halftone dots, typically in dots per inch, instead of pixels. The dots control how much ink is deposited at a specific location while printing on paper.