Images are quickly becoming one of the most critical factors for web performance. On one hand, users are demanding more visual websites, driving an increase in the number of images on a page and making background images cool again. On the other hand, technology trends such as Retina displays and RWD are making it much harder to choose the right image to download at any given time, avoiding the download of excess bytes. In this talk, I go over what you can do to maximize the impact of every image byte. I explain the concept of Image Compression, understand how it applies to different image formats, and show the tools and techniques you should use to communicate the best visuals with the fewest bytes. Lastly, I show how to combine image compression and Retina displays, and discuss some newer image formats and how you can take advantage of them today

1. Faster ForwardTM
PUTTING YOUR IMAGES ON A DIET
Guy Podjarny (@guypod)
CTO, Web Experience, Akamai
Everything you wanted to know about image compression… and then some.

2. Faster ForwardTM
We have an image problem…
Images appear simple
Images seem friendly
Images look like they can’t hurt
But while you weren’t looking…
Images Have Been
Taking over
The World
(wide web)

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Images make up 63-66% of Page Bytes
17%
3%
5%
9%
3%
1054,
63%
Desktop Bytes
Scripts CSS Flash Other HTML Images
23%
5%
0%
1%
5%
552, 6
6%
Mobile Bytes
Scripts CSS Flash Other HTML Images

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Image Byte Increase over last 3 years
It’s not more images,
It’s BIGGER images
2.2x more Image Bytes in 3 years
32% more
bytes
in 1 year

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What is there to optimize with images?
- Today’s focus: Image Compression
- Optimizing image formats & encoding
- There are other ways to optimize images
- Optimize Image Loading
- Optimize Image Delivery
- Responsive Images
- …

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LET’S START AT THE BEGINNING…

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GIF – Graphics Interchange Format
- 27 Years Old (1987)
- 256 Colors
- “Simple” Transparency
- No partial (Alpha) transparency
- Supported Everywhere
- Supports Animation
- Previously Patented
- Patents expired by now

8. Faster ForwardTM
PNG – Portable Network Graphics
- 18 Years Old (1996)
- Supports 8-32 bit color palettes
- PNG8 and PNG24 Most Common
- Supports Alpha Transparency
- Good browser support
- Certain issues in IE 6-8
- Non-patented
- Supports Animation (MNG)!
- No browser support

9. Faster ForwardTM
Lossless PNG Compression
- Delta-Filter Encoding
- Write pixels as delta from neighbors
- Compress using LZ77
- Reference past sequences
- Compress using Huffman code
- Identify repeating sequences
- Code popular ones with fewer bytes
= 1,2,3,4,5,
0x20
= 1x25
= 1,2,3,4,5,
(-5,5)x5
Up Delta
Left Delta
Source: Sergey Chikuyonok, Smashing Magazine

10. Faster ForwardTM
Lossless PNG Optimization
- Delta Filter Optimization
- Per row, choose comparison target: None, Left, Up, Average, Paeth
- Convert from PNG 24 -> 8
- Possibly using multiple PNG 8 layers
- Tune LZ77 distance/length parameters
- Tune Huffman buffer size
- Trade “Alpha” transparency with simple (100%) transparency
- PNG Optimization Tools: PNGCrush, OptiPNG

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JPEG – Joint Photographic Experts Group
- 22 years old (1992)
- Supports RGB Colors (24 bit)
- Supports custom color palettes
- No Transparency Support
- Doable with HTML5 Canvas Hacks
- No Animation Support
- Not impressed…
- It’s a Lossy Format

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JPEG Dominates Page Bytes, But PNG Is Growing
17%
25%

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JPEG Compression - Simplified
1. Divide image into 8x8 pixel blocks

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JPEG Compression - Simplified
1. Divide image into 8x8 pixel blocks
2. Split Luminance & Chrominance
Y Cb Cr

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YCbCr (YUV) Visualization – Grand Tetons
Source:
IE Blog

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Chroma Subsampling
- Chroma diffs are less noticeable
- Subsampling “merges” adjacent pixels
- Most popular is 4:2:2, 4:2:0
- Reduce Chroma bytes by 2x or 4x
- Tools often deduce subsamplling
- Based on quality tier
- Photoshop Example:
- no subsampling for quality 51-100
- 4:2:0 subsamplling for quality 0-50
Source: JPEG For The Horseshoe Crabs

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JPEG Compression - Simplified
1. Divide image into 8x8 pixel blocks
2. Split Luminance & Chrominance
3. Chroma Subsampling
4. Write pixels as ~delta from average
• and round (DCT + Quantization)
Y Cb Cr

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JPEG Compression - Simplified
1. Divide image into 8x8 pixel blocks
2. Split Luminance & Chrominance
3. Chroma Subsampling
4. Write pixels as ~delta from average and
round (DCT + Quantization)
5. Write as line, compress (Huffman)
= -9,-1,0(x62)

19. Faster ForwardTM
JPEG Compression - Simplified
1. Divide image into 8x8 pixel blocks
2. Split Luminance & Chrominance
3. Chroma Subsampling
4. Write pixels as delta from average
and round (DCT + Quantization)
5. Write as line, compress (Huffman)
Q=10 Q=50 Original

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Original PNG: 574 KB JPEG, Q=90: 105 KB

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Original PNG: 574 KB JPEG, Q=75: 53 KB

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Original PNG: 574 KB JPEG, Q=40: 30 KB

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Original PNG: 574 KB JPEG, Q=20: 18 KB

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Original PNG: 574 KB JPEG, Q=10: 11 KB

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Original PNG: 574 KB JPEG, Q=5: 7 KB

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Lossy Compression – Not for Everyone
GIF, 11 KB
JPEG, Q=80, 8 KB
JPEG, Q=30, 4 KB
Average Color not
representative
PNG, 7 KB

27. Faster ForwardTM
JPEG Optimization
- Control Quality
- 75-85 tops
- Quality level ≠ percentage
- Tools interpret quality differently
- Tune Huffman Buffer Size
- Use Arithmetic Encoding (vs Huffman)
- Low adoption, no browser support
- Tools: jpegtran, ImageMagick
0
50
100
150
200
250
300
350
0 20 40 60 80 100 120
Quality vs File Size

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JPEG Metadata
120KB
Thumbnail

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JPEG Optimization
- Control Quality
- 75-85 tops, try to go lower
- Tune Huffman Buffer Size
- Remove Metadata
- Tools: jpegtran, ImageMagick

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JPEG Baseline vs. Progressive
Baseline
Progressive
http://www.bookofspeed.com/chapter5.html
Pro-Tip: Configure how blocks are
“interlaced” (The “Coefficient”)
- Controls minimum quality level
- Affects Size

31. Faster ForwardTM
Baseline vs Progressive – Visual Progress
Baseline
Progressive
Patrick Meenan:
“… Looking at the
Speed Index, we saw
median improvements
of 7% on Cable and
15% on DSL …”
Loading…

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JPEG Baseline vs. Progressive
Baseline
Progressive
http://www.bookofspeed.com/chapter5.html
1 2
0 0
1 0
0 0
3 -1
0 0
1 2 0 0 1 0 0 0 3 -1 0 0
1 1 3 2 0 -1 0 0 0 0 0 0
1 1 3 2 0 -1 EOB
Mo’ zeros, Mo’ compression

33. Faster ForwardTM
Baseline vs Progressive – File Size
Baseline – Progressive Size for 100,000 JPEGs
Stoyan Stefanov:
Images of size 10K+
likely smaller as
Progressive.
Median delta is 3%,
Up to 10%
common, 250KB gap
extreme.
Progressive Smaller
Baseline Smaller

34. Faster ForwardTM
Progressive vs Baseline – Current Reality
Ann Robson:
“… In a thousand-image
sample, 92.6% are
baseline …”
Patrick Meenan: “… I crawled all 7 million JPEG images
that were served by the top 300k websites in the May 1st
HTTP Archive crawl and came out with....wait for it.... still
only 7% …”

35. Faster ForwardTM
Lossless JPEG Optimization
- Control Quality
- 75-85 tops, try to go lower
- Tune Huffman Buffer Size
- Remove Metadata
- Consider (and default to) Progressive over Baseline
- Tune Progressive Scan coefficients
- Tools: jpegtran, ImageMagick, jpegrescan (~1.2% better)

36. Faster ForwardTM
Retina Images
bg_header_retina.png – 56 KB
bg_header.png –
14KB
1 2
43
• Marketing-speak for 2x pixel ratio (or more)
• 1 CSS pixel = 4 real pixels
• Other ratios are possible (1.5x common)
3X

37. Faster ForwardTM
JPEG Retina Trick
- Retina images often 3-4x file size
- Regular images are “fuzzy” on Retina
- Double size, Lower Quality =
Sharper Image, Smaller File
- Discovered by Netvlies & Thomas Fuchs
- Popularized by Filament Group
- Not clear WHY it works
- Artifact of tools “Quality” level?
- Higher Memory Usage
- More on that later…

38. Faster ForwardTM
Lossless JPEG Optimization
- Control Quality
- 75-85 tops, try to go lower
- Tune Huffman Buffer Size
- Remove Metadata
- Consider (and default to) Progressive over Baseline
- Tune Progressive Scan coefficients
- Consider Retina Double-Size-Low-Quality Trick
- Tools: jpegtran, ImageMagick, jpegrescan, ImageOptim

39. Faster ForwardTM
Got Through JPEG!
Next Up:

40. Faster ForwardTM
WebP – Web Picture?
- 3 Years Old (2011)
- Pronounced “Weppy”
- Supports Lossless & Lossy Modes
- Very well documented
- See: https://developers.google.com/speed/webp/
- Related VP8 Google Patents “released” by Google
- Other patents licensed by Google for global use
- Nokia still claiming related Patents

41. Faster ForwardTM
WebP vs PNG
Google:
Lossless WebP 26%-34%
Smaller than PNG
Yoav Weiss:
Lossless WebP 33%-42%
Smaller than PNG

42. Faster ForwardTM
WebP vs JPEG
Google:
Lossy WebP 25%-
34% Smaller than JPEG
Yoav Weiss:
Lossy WebP 37%
Smaller than PNG

43. Faster ForwardTM
WebP – Impressed Yet?
- 26% - 42% Smaller than PNG
- 25% - 37% Smaller than JPEG
- Supports Alpha Transparency
- Supports Animation!
- Why did waste time talking
about the other formats?!
23+ 12.1+

44. Faster ForwardTM
JPEG XR (eXtended Range)
- 4 Years Old (2009)
- Smaller files than JPEG
- A big bigger than WebP
- Patented by Microsoft
- Under “Open Spec Promise”
- Jxrlib has BSD License
- Support: IE 9+ (incl. Mobile)
- Properly works in IE 10+
- Includes IE Mobile
10+

45. Faster ForwardTM
Similarity Tests
- Formats work differently
- Quality # means different
visual quality per format
- Similarity algorithms help
detect “similar” images
- Across image formats
- Sample algo: DSSIM

46. Faster ForwardTM
WebP vs JPEG-XR vs Optimized JPEG (Akamai Data)
Nick Doyle (Akamai):
1. WebP & JXR Both better than JPEG.
2. Beware the different Quality scales

47. Faster ForwardTM
JPEG 2000
- 6-14 Years Old (2000/2008)
- Uses DWT instead of DCT
- Along with other improvements
- ~30% Smaller than JPEG
- Based on initial Akamai research
- Patented, but free to use
- As agreed by ~20 patent-owning org’s
- Support: Safari & Mobile Safari
- iOS 6+, Mac Safari, not Win Safari

48. Faster ForwardTM
Processing Images Ain’t Easy…
- Images are 60% of bytes - likely <95% of memory usage
- Resized Retina images take up 4x the memory!
- Output pushed to GPU, often through slow Mobile BUS
- GPUs now starting to process YUV – faster with less RAM for JPEGs
- Network time is not the only thing that matters…
Typical Image Decoding Flow (Source: IE Blog):
30 KB
JPG Input
~300 KB
in YUV
(~10 bbp)
1 MB After
Upsampling
(~32 bbp)
RAM:
Often 300-800ms on Mobile

49. Faster ForwardTM
Mozilla still undecided
- Ran a comparative study: JPEG, JXR, WebP, HEVC-MSP
- Used 4 algorithms to find “comparable” image across formats
- HEVC-MSP deemed best, results inconclusive for others
- Note: Study used PNG’s as source
- Akamai’s studies used “live” website images
0.8
0.9
1
1.1
1.2
1.3
50 55 60 65 70 75 80 85 90 95
Relative File Size, Kodak Image Set, Y-SSIM
JPEG JPEG XR WebP HEVC-MSP
0.5
0.6
0.7
0.8
0.9
1
1.1
50 55 60 65 70 75 80 85 90 95
Relative File Size, Kodak Image Set, RGB-SSIM
JPEG JPEG XR WebP HEVC-MSP

50. Faster ForwardTM
New Image Formats - Summary
Format Size vs
JPEG
Proggressive Transparency
Support
Hardware
Decoding
Encoder Browser
Support
JPEG N/A N/A No No jpegtran Everybody
WebP -35% -35% Yes No cwebp
JPEG XR -30% N/A
In Spec, not
Browsers
Maybe jxrlib
JPEG 2000 -30% N/A
In Spec, not
Browsers
Maybe OpenJPEG
10+
12.1+4+23+
6+6+

51. Faster ForwardTM

52. Faster ForwardTM
When to use which format?
- For tiny images (e.g. 1x1 pixel) use GIF
- PNG-8 practically always smaller
- For most small images, use PNG
- Where possible, prefer JPEG to PNG
- Use WebP/JXR/J2K where possible
- Serve only to supporting clients
- For Animation, use GIF
- Seat back and reap the rewards…

53. Faster ForwardTM
A PICTURE COSTS A THOUSAND WORDS
Guy Podjarny (@guypod)
CTO, Web Experience, Akamai
Thank You!
Questions?