1 Perception, Illusion and VR HNRS 299, Spring 2008 Lecture 14 Introduction to Computer Graphics

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3 Storage of Graphic Images Graphics are stored as 1's and 0's on the disk or in the computer's random access memory (RAM). The numbers form a code that a web browser or graphics application interprets to display the graphic on the monitor. Computer displays are composed of many dots, known as picture elements (pixels). The numeric code in an image file specifies the color of each pixel in the image.

4 Counting in Base 10 In the decimal number system (base 10), we use 10 digits We count until we run out of digits, and then add a new place with value place value = 10 = 10 1 place value = 1 = 10 0 We continue to count, adding 1 to the 10's place every 10th number. When we run out of digits for the 10's place, we add a new place with value 10 2 (or 100) place value = 100 = 10 2

5 Counting in Binary With binary numbers, we have only 2 digits to work with (0 and 1), so we add places more frequently. Each new place has a value that is a power of two. DecimalBinary Note: Each 1 or 0 is called a binary digit or bit.

6 Base 10 vs. Base 2 In base 10, each place represents a power of 10: 4173 = 4 x x x x 10 0 In base 2, each place represents a power of 2: = 1 x x x x x 2 0 = 22 Practice: Convert from binary to decimal.

7 Resolution The number of pixels on the screen determines the resolution of the image. More pixels per inch implies better resolution and better image quality. Older PC's had resolutions of 800 pixels x 600 pixels. Older Macs had resolutions of 640 pixels x 480 pixels. Newer PC's and Macs have much higher resolutions (e.g x 768).

8 Resolution affects Image Size The more pixels there are per inch (ppi) the smaller each pixel is. Image sizes are specified in terms of pixels. So the same image will appear smaller on a monitor with a higher resolution. Increasing resolution decreases the image size.

9 Bit Depth The number of colors each pixel can assume contributes to image quality. The bit depth of an image is the number of bits used to specify color for that image. The bit depth determines how many colors can be shown. Black and white monitors have 2 colors. Only one bit is needed to specify color (bit depth = 1). A bit depth of n allows representation of 2 n colors.

10 Changing Bit Depth Some images don't need a large bit depth: Bit depth 8 Bit depth 4 Bit depth 1 Smaller bit depths help to keep the file size (in kb or mb) small. This makes it faster to download the image file.

11 Bit Depth and Image Quality For more complex images, higher bit depth leads to better resolution: Bit depth = 32 (16.7 million colors)

12 Indexed Color With indexed color, the computer stores a table of colors (256 colors for bit depth of 8). Each pixel is given a value between 0 and 255, which corresponds to one of the colors in the table.

13 RGB Uses Hexadecimal Numbers The RGB system uses hexadecimal (base 16) numbers. To count in base 16, we use the letters A - F to stand for decimal equivalents of Base 10Base 16Base 10Base FF A 11B 12C 13D 14E 15F = 256

14 Converting from Binary to Hexadecimal Every 4 bits represents 1 hexadecimal digit. To convert from binary to Hexadecimal, group the bits into sets of four (from right to left) and find the corresponding hexadecimal digit for each set of 4 bits B ? ?

15 RGB (Red, Green, Blue) Color In the RGB color system, each pixel is given a specified amount of red, green and blue (between 0 and 255). The color value is written in Hexadecimal, using 2 digits for each color: #FF0000Red #00FF00Green #0000FFBlue #FFFFFF? #000000? #880066Purple

16 Image File Compression Images on the web are stored as separate files that must be downloaded from the server to the client to be viewed. If the image file size is large, it will take a long time to download. Therefore, it is important to try to keep the file size as small as possible without losing much image quality. Ways to reduce file size: 1. Reduce the size of the image (in pixels) 2. Reduce the resolution of the image. 3. Reduce the bit depth of the image 4. Use image file compression (GIF or JPEG)

17 GIF files The Graphics Interchange Format (GIF) is excellent for compressing images with large areas of uniform color. It is "lossless", meaning that the original image can be regenerated with no loss of information. GIF supports transparency in images. GIF supports animations (animated GIF's) This format is limited to 256 colors.

18 JPEG compression The JPEG (Joint Photographic Experts Group) compression method works well for complex images, such as photographs. JPEG supports millions of colors (up to a bit depth of 24). JPEG is "lossy", meaning that the original image cannot be regenerated exactly from the original. Some information is lost in the conversion to JPEG.

19 Computer Graphics in VR In virtual reality applications, the graphics are often computed as the program runs. The color value for each pixel is stored in computer memory (RAM) Values for each pixel on the monitor are stored in the frame buffer. If we are using 24 bit color, each pixel needs 24 bits. There are 8 bits in a byte, so each pixel needs 3 bytes. If the screen is 600 x 800 resolution, how many pixels are there? How many bytes are needed to store color values for the entire screen?

20 Computing the Pixel Values Computer graphics are computed in the following stages: 1)Build 3D models of objects in the world. 2)Specify Lighting and Material properties. 3)Transform the modeled objects to desired positions. 4)Project the 3D model onto the 2D image plane 5)Rasterize the image (convert it to pixels).