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Image Optimization 7 mistakes (and how to correct them) Stoyan Stefanov Yahoo! Exceptional Performance Velocity, June 2

Image Optimization 7 mistakes (and how to correct them) Stoyan Stefanov Yahoo! Exceptional Performance Velocity, June 24th, 2008, San Francisco stoyan@yahoo-inc.com yslow@yahoo-inc.com. About the presentation. Image optimization for the non-designer

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Image Optimization 7 mistakes (and how to correct them) Stoyan Stefanov Yahoo! Exceptional Performance Velocity, June 2

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  1. Image Optimization 7 mistakes(and how to correct them) Stoyan Stefanov Yahoo! Exceptional Performance Velocity, June 24th, 2008, San Francisco stoyan@yahoo-inc.comyslow@yahoo-inc.com

  2. About the presentation • Image optimization for the non-designer • 7 mistakes all sites make, even the big ones • Trivial to correct, serious savings • Easy win • improve page load time • save bandwidth • save disk space • save servers (and energy and the planet) • … without compromising quality

  3. About the presenter • Yahoo! Exceptional Performance (research, consulting) • YSlow lead developer • Open Source contributor (PEAR, Firebug) • Blog http://www.phpied.com • Articles and books author

  4. Hmm, images? Q: Is this really important? A: Let’s survey the global top 10 sites.

  5. What % of page weight is images? • Average 45.6% • Not exactly half, but pretty close • Your site may be different, amazon.com for example is 71% images • huge potential

  6. Mistake #1: Using GIF when PNG is smaller

  7. GIF vs. PNG * some IE < v.7 issues

  8. PNG transparency and IE Truecolor PNG IE 7 and up IE 6 and earlier PNG8 IE 7 and up IE 6 and earlier Verdict: IE7+ is OK; IE6 supports GIF-like transparency

  9. GIF vs. PNG • PNG8 can do everything a GIF can do, and more (sans cheesy Web 1.0 animations) • PNG8 works across all A-Grade browsers Q: And what about the size? A: Let’s survey the top 10 sites and convert all GIFs to PNGs to check if there are savings

  10. GIF-to-PNG • Average 20.42%savings

  11. Take away #1: Choose PNG over GIF LOSSLESS!

  12. Mistake #2: Not crushing PNGs

  13. About the PNG chunks • PNGs store information in "chunks" • Most chunks can safely be deleted • Most image programs DO NOT optimize • Command line tools: • pngcrush http://pmt.sourceforge.net/pngcrush/ • pngrewrite http://www.pobox.com/~jason1/pngrewrite/ • OptiPNG http://www.cs.toronto.edu/~cosmin/pngtech/optipng/ • PNGOut http://advsys.net/ken/utils.htm • Example: > pngcrush -rem alla -brute -reduce src.png dest.png

  14. Crush top 10 and check for savings • Average 16.05%savings

  15. Take away #2: Crush your PNGs LOSSLESS!

  16. Mistake #3: Not stripping JPEG metadata

  17. JPEG metadata • Comments • EXIF • camera information • thumbnail! • audio!?! • … • Application specific (e.g. Photoshop)

  18. Lossless JPEG operations • jpegtran (http://jpegclub.org/) • Free command-line tool • Loseless operations such as crop, rotate • You probably have it installed already Example: > jpegtran -copy none -optimize -perfect src.jpg dest.jpg

  19. Running jpegtran on top 10 sites • Average 11.85%savings

  20. Take away #3: Strip needless JPEG metadata LOSSLESS!

  21. Mistake #4: Using truecolor PNGs when palette PNG is good enough

  22. Truecolor vs. palette PNGs • Palette PNGs (PNG8) have 256 colors • Truecolor PNGs could have millions • In practice, truecolor PNGs often have less than 1000 colors • Otherwise it would mean they are photos and should be JPEGs • Human eye is not that sensitive

  23. Truecolor vs. palette PNGs • Convert truecolor to palette • How about 50% savings? • Solves IE<7’s little alpha transparency problem • Warning 1: LOSSY • But no one will ever know • Warning 2: automation is hard when there’s alpha

  24. Truecolor vs. palette PNGs Command-line tools: • pngquant (http://www.libpng.org/pub/png/apps/pngquant.html) • pngnq (http://pngnq.sourceforge.net/) Example: > pngquant 256 src.png

  25. Take away #4: Make all PNGs palette PNGs * * but manually check the result (or wait for someone to complain, chances are you'll be waiting in vain ;)

  26. Mistake #5: Using CSS alpha filters

  27. CSS filters • Back to the problem with truecolor PNG alpha transparency in earlier IEs • AlphaImageLoader CSS filter to fix the issue • IE proprietary • Blocks rendering, freezes the browser • Increased memory consumption • Per element, not per image! • CSS code for sprites becomes messy

  28. Avoid filters • Best: Avoid completely, use gracefully degrading PNG8 • Fallback: use underscore hack _filter not to penalize IE7+ users • Yahoo! Search saved 50-100ms for users of IE5&6

  29. Take away #5: Avoid AlphaImageLoader, try PNG8 or at least _filter

  30. Mistake #6: Serve generated images “as-is”

  31. Dynamically generated images • GD library doesn't do what pngcrush does • Generated images are bigger • From big to small:generated GIF > generated PNG > crushed PNG • Server resources on every request

  32. Recipe for generated images 1st request: • generate • write to disk • pngcrush • serve 2nd request: • serve cached • Estimate5-30%savings

  33. Case study: Google charts API

  34. Case study: Google charts API

  35. Take away #6: Crush generated images LOSSLESS!

  36. Mistake #7: Not combining images

  37. … CSS Sprites CSS: HTML:

  38. CSS Sprites • Size of the combined image is usually less • You save HTTP requests • Article: http://alistapart.com/articles/sprites • Tool: http://csssprites.com

  39. Optimizing sprites

  40. Take away #7: Use CSS Sprites Stay within the palette number of colors (use PNG8)

  41. Bonus mistake: Improper favicon.ico • www.example.org/favicon.ico • Necessary evil: • The browser will request it • Better not respond with a 404 • Cookies are sent • Cannot be on CDN • Interferes with the download sequence • Make it small (<= 1K) • Animated favicons are not cool • Set Expires header (but not “forever”) • Tools: imagemagick, png2ico • Case study: Yahoo! Search - favicon.ico is 9% of all page views

  42. Bonus mistake #2: serving images from your app server • Option 1: use a CDN • Option 2: setup a static server • stripped down Apache or LightTPD • no libraries, no PHP, no Perl, etc • serve images (and other static components) • request -> find on disk -> serve • "Never expires" policy • no cookies

  43. Wrapping up You can save 10-30% in unnecessary image size!

  44. URLs – Exceptional Performance YUI blog http://yuiblog.com/blog/category/performance/ YDN (Yahoo Developer Network) http://developer.yahoo.com/performance/ YDN blog http://developer.yahoo.net/blog/archives/performance/ Mailing list (Yahoo! Group) http://tech.groups.yahoo.com/group/exceptional-performance/ Feedback http://developer.yahoo.com/yslow/feedback.html

  45. Thank you! stoyan@yahoo-inc.comyslow@yahoo-inc.com

  46. Credits • http://svs.gsfc.nasa.gov/vis/a000000/a002600/a002680/ Apolo 17 Full-Earth Photograph • http://www.w3.org/Graphics/PNG/inline-alpha.html W3C PNG Alpha Test • http://www.sitepoint.com/blogs/2007/09/18/png8-the-clear-winner/ PNG8 - The Clear Winner

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