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# Images, file sizes JS Objects Literals Boolean IF/IF ELSE CASCADED IFs - PowerPoint PPT Presentation

CS110: Computer Science and the Internet. Images, file sizes JS Objects Literals Boolean IF/IF ELSE CASCADED IFs. HW 4 Office hours (leaving Thursday) Really well on the quiz Project Design document. http:// cs.wellesley.edu /~cs110/ scott /L07-color-and-images/color-and- images.shtml.

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### Images, file sizesJS ObjectsLiteralsBooleanIF/IF ELSECASCADED IFs

• HW 4

• Office hours (leaving Thursday)

• Really well on the quiz

• Project Design document

http://cs.wellesley.edu/~cs110/scott/L07-color-and-images/color-and-images.shtml

# 94 00 D3

RGB

blockquote {

color: #9400D3;

background-color: #E6E6FA;

}

44 pixels wide

Digital Images

Images are often represented in a computer as a 2-dimensional (2D) array of pixels

RGB color for each pixel is stored in 24 bits (3 bytes)

How much memory is needed to store a 300x500 pixel image?

• Problem: large image files can take a long time to download!

• Example: On a 300 kbps (300k bits per second) cable modem, a 450 kB image takes about 12 seconds to download

• Solutions:

• Downsampleimages to make them smaller (fewer pixels)

• Compress the images, using “lossless” compression (no information is lost) or “lossy” compression (less important information is lost) techniques

image

image contents

color palette

• How large is a file that stores a 300x500 pixel indexed color image with 4 colors, and its color palette?

• 300x500 pixels, with 2 bits per pixel (why?)

• 300 x 500 x 2 = 300,000 bits

• (2) 4 colors, with 24 bits per color

• 4 x 24 = 96 bits

•  ~37.5kB (compared to the 450kB uncompressed file!)

2 bits per pixel

24 bits per color

• How large is a file that stores a 300x500 pixel indexed color image with 16 colors, and its color palette?

• What is the number of bits used to store the image pixels?

• (2) What is the number of bits used to store the color palette?

• Total file size (in bytes):

“bits per pixel” is also called the “bit depth”

bit depth must be large enough to store all the colors

The GIF file format is an indexed color format that allows 256 colors (out of a possible 16 million colors!)

• Computing the file size for a GIF image, in bytes:

• number of bytes to store the image pixels:

• (width * height * bit-depth) / 8

• (2) number of bytes to store the color palette: num_colors * 3

• (3) total number of bytes*:

• (width * height * bit-depth) / 8 + 3 * num_colors

• (4) divide by 1,000 or 1,000,000 to convert to kilobytes or megabytes

• Recap on Date Objects (lecture notes from last time, Firebug)

• Multiple Date Objects

• Object literals

• Using object literals with jQuery

IF FlowChart

• if ( your_name = “Stella”) {

alert (“Hi ” + your_name + “ !”);

}

• if ( number % 2 == 1) {

} else {

}

• How do we grade you?

• When would we use if/else in our websites?

• Try in firebug

• Not

• &&

• ||

• Always use parenthesis when using many conditional operators

• Logical operators can be used with more than two choices but those choices must be true or false statements, or?

• do we need to memorize this code?

• Scott’s notes