Transcript HTML5&CSS_CH5_REV

VISUAL QUICKSTART GUIDE
HTML and CSS
Chapter 5: Images
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8th Edition
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Objectives
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Create images in GIF, PNG, and JPEG formats.
Add visual interest with transparency.
Find, purchase, and create images for a webpage.
Insert images on a webpage.
Offer alternate text when images don't appear.
Specify image size.
Find an image’s size with a browser or Photoshop.
Scale images with the browser or image editor.
Add icons to a website.
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Images
• There are six factors to keep in mind when you
create images for the web:
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Format
Download speed
Color
Size (dimensions)
Transparency
Animation
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Format and Download Speed
• Three most widely used formats on the web are GIF,
PNG, and JPEG.
• JPEG format is good for color photographs.
• JPEG is a lossy format, which means some of the
image’s original information will be lost when the
image is saved.
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PNG and GIF
• PNG and GIF formats compress areas of continuous
colors or repetitive patterns better than JPEG.
• PNG and GIF formats are good for saving files that:
– Have large amounts of solid colors and patterns and need
transparency.
• PNG has a better compression algorithm for smaller
file sizes and superior transparency support (alpha
transparency).
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Choosing Image Formats
Save full-color photos as
JPEG or PNG-24
Zip and save logos in PNG8 format
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The WebP Image Format
• WebP is Google’s image format, named.
– WebP supports both lossy and lossless compression and
can result in significant file-size savings over JPEG and
PNG. It also features alpha transparency, like PNG.
• WebP is still evolving, and browser support is limited
at the time of this writing: Chrome, Opera 12+, and
some Android browsers are the only ones with full
support, and it’s uncertain if other browsers will adopt
it.
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Color
• GIF and PNG-8 images have a limited color palette.
– Only 256 colors.
• Fine for logos and icons.
• Save photographs and complex illustrations in JPEG
or PNG-24 format.
– Accommodate more colors in a single image.
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Size (Dimensions)
• Digital images are measured in pixels.
– Keep images to within a few hundred pixels wide.
• Dimensions vary depending on the image’s purpose.
– Icons will be smaller, logos a little bigger, and photos, bigger
still. Maximum width of a site is typically 960 pixels wide.
• Images typically print smaller than they appear on a
display.
– Printers print more dots per inch (dpi) than most displays
have pixels per inch (ppi).
– Screen resolution plays into this, too.
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Comparing Image Sizes
2048 pixels wide and 1536
pixels tall
400 pixels wide and 300
pixels high
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Transparency
• Transparency can give an image a non-rectangular
outline, blending it in with a background color or
texture behind the image.
– PNG and GIF allow transparency; JPEG does not.
• Index transparency: Pixel can be transparent or not.
– Both GIF and PNG support index transparency.
• Alpha transparency: Controls the degree of a pixel’s
transparency.
– Only PNG supports alpha transparency.
• Images with complex transparent backgrounds look better as
PNGs because edges appear smooth, not jagged.
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PNG-8 and PNG-32
• PNG-8 supports both index and alpha
transparencies, but you’ll need a program like
Fireworks to save an alpha PNG-8.
– Photoshop doesn’t support alpha transparency for PNG-8,
but it does support it for PNG-32.
– One reason most transparent PNGs on the web are PNG32.
• Use PNG-8 or PNG-32 for transparent images. The
latter is required if your image has more than 256
colors.
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Photoshop, PNG-24, and PNG32
• PNG-24 and PNG-32 are essentially the same except
the latter supports alpha transparency.
• Photoshop refers to both PNG-24 and PNG-32 as
PNG-24, so it’s common to think PNG-24 has alpha
transparency. In truth, when you select the
Transparency option for a PNG-24 in Photoshop, it
creates a PNG-32 behind the scenes.
• The term “PNG-24” is often used when speaking of
alpha transparent images, even though it isn’t
technically accurate.
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Image Transparency
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Animation
• Animated images can be saved as GIFs, but not as
JPEGs or PNGs.
– Using images for animation is becoming uncommon, except
for GIFs on site’s like Tumblr.
– Animation is generally created using CSS Animations,
JavaScript, HTML5 Canvas, SVG (the dark horse of the
bunch), and Flash.
– Flash animations on the web are becoming rare due to iOS’s
lacks Flash support and the increased capability and support
for other technologies.
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Getting Images
• Ways to get images:
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Buy or download ready-made images.
Digitize photographs or hand-drawn images with a scanner.
Use a digital camera.
Draw images from scratch in an image editing program like
Adobe Photoshop.
• Once you’ve got them in your computer, you can
adapt them for use on the web.
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To Get Images
• Use Google to find images on the web.
– Even free images on the web have some restrictions.
• Stock photography and images can be bought for a
reasonable price.
– Purchased images can be used for most purposes, except
reselling.
– Read any disclaimers or licenses carefully.
• Create images with scanners and digital cameras.
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Creative Commons Licenses
• Creative Commons: Non-profit that developed
Creative Commons license.
• Creative Commons license: System of copyright
templates that let artists share their work in specified
ways without giving up all rights over their work.
– Many artists use Creative Commons licenses.
– Flickr asks users to designate a Creative Commons license
for each photo they upload.
– Google can do a restricted search based on usage rights.
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Choosing an Image Editor
• Most modern image editors have special tools for
creating web images.
– Photoshop is the industry standard.
– Adobe Fireworks is a very powerful program.
– Alternatives include Paint.NET for PCs and Acorn or
Pixelmator for Mac.
– Online editors, like Photoshop.com and Aviary.com, are
becoming more capable.
• Basic strategies for optimizing images for the web are
the same regardless of the software you choose.
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Saving Your Images
• Saving images is a
balancing act
between visual
quality and file size.
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Photoshop’s Save for Web
Command
• Visually compares
original image with up
to three versions.
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To Use Photoshop’s Save for
Web Command
• Open Photoshop and create image or open existing
image.
• Choose File > Save for Web. Save For Web dialog
appears.
• Click one of the following tabs:
– Optimized tab to see one optimized version.
– 2-Up tab to see one optimized version with the original.
– 4-Up tab to see three.
• Click an optimized version, if necessary.
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To Use Photoshop’s Save for
Web Command
• Choose desired format:
– Save images created on a computer in PNG-8 or GIF format.
– Save images with continuous tones in JPEG or PNG-24
format.
• Adjust additional settings until smallest file with
acceptable quality is reached.
• Click Save, choose directory, and name new file.
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Inserting Images on a Page
• Images on a webpage
appear automatically,
as long as the browser
is set up to view them.
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To Insert an Image on a Page
• Place the cursor in the HTML code where the image
will appear.
• Type <img src="image.url”
– Where image.url indicates location of image file on server.
• Type a space and then the final />.
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Offering Alternate Text
• Alt attribute adds
descriptive text that will
appear if the image
does not appear.
• To offer alternate text:
– Within the img tag, after
the src attribute and
value, type alt=".
– Type the alternate text.
– Type ".
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Why Images Might Not Appear
• An image may not appear for a variety of reasons:
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You coded the wrong URL in the src attribute
You forgot to upload the image to the web server.
Visitor might have a poor connection.
A browser can be set up to not load images.
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Specifying Image Size
• Specify image size in code and browser will reserve
its space. Text will fill in around image’s space as
image loads, and layout will remain stable.
• Use browser or image editing program to get exact
dimensions of image.
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To Find the Size of Your Image
with Your Browser
• Right-click the image. A
contextual pop-up menu
appears.
• Choose Properties or
View Image Info
(depending on
browser). A box
appears that shows the
dimensions of the
image in pixels.
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To Find the Size of Your Image
with Photoshop
• Open the image in
Photoshop.
• Choose Image > Image
Size. The Image Size
dialog displays.
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To Specify the Size of Your
Image in HTML
• Determine the size of your image.
• Within the img tag, after the src attribute, type
width="x" height="y"
– Use the values you found for the width and height to specify
the values for x and y in pixels.
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Scaling Images with the
Browser
• Change display size of an image by specifying a new
height and width in pixels.
• To scale an image with the browser:
– Type <img src="image.url"
• Where image.url is the location on the server of the image.
– Type width="x" height="y"
• Where x and y are desired width and height, respectively, in
pixels, of your image.
– Add any other image attributes as desired and type final />.
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Creating and Sizing Images for
Retina Displays
• Apple’s Retina display has four times as many pixels
in the same amount of space as in an otherwise
comparable normal display, resulting in sharper
images.
• In some cases, images need to be made with the
Retina display and similar displays in mind, otherwise
they will look blurry in a browser.
• To avoid blurry images, double the size of your
images, but display them at half that size.
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Scaling Images with an Image
Editor
• Most images are simply too big for a webpage.
– Image for print might measure 1800 pixels across.
– Images for webpages shouldn’t be wider than 600 pixels.
• Scale images up or down with an image editor.
• To scale an image with Photoshop:
– In lower-right portion of the Save for Web window, click W
(Width) box or H (Height) box in the Image Size section.
– Enter a new width or height in pixels, or a percentage, and
then press the Tab key to resize the image.
– Continue to adjust the size up or down until you’re satisfied.
The image is not resampled until you press Save.
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Scaling an Image
2816 pixels wide
400 pixels wide
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Scalable Vector Graphics
(SVG)
• The SVG graphics language allows you to create
graphics that can scale up or down without
compromising quality (and more).
• You can include SVG in your webpages; gradually,
we’re seeing more of that, because all modern
browsers provide basic support.
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Adding Icons for Your Website
• Favicon are small icon in address bars, tabs, and
bookmarks.
– Short for “favorites icon.”
– Favicons are small, measuring 16 x 16 pixels.
– Because browser often displays icon over gray or other
colors, make icon’s background transparent.
• Touch icons display when a site is added to the
home screens of Apple and other touch devices.
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To Add an Icon for a Website
• Create 16 x 16 image an save it in ICO format.
• Create 114 x 114 image for touch devices. Save in
PNG format.
• Place the icon images at the root of your website.
Browsers look for those specific file names at the root
automatically.
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Favicons
Favicon in browser display
Apple-touch-icon
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