This post was written by John Engelhart, an iOS developer at Scribd and author of the JSONKit library.
So you have a lot of PNG images in your iPhone app…
When I started here at Scribd, we were just a few weeks away from launching our first iPhone app– Float.
Being a new hire obviously meant that I didn’t know the code base. Being just a few weeks away from launch obviously meant that there was a strong focus and getting something out the door. So the first thing I did is start making huge, sweeping fundamental architecture changes like swapping out all the XML REST stuff with JSON, and switching the JSON parser that was currently being used with JSONKit, because JSONKit is really, really fast. Just look at those graphs! Does it happen to parse JSON correctly? Are numbers arbitrarily and silently truncated to 32 or 64 bits haphazardly? Are floating point values preserved correctly when round tripped? Who cares! That simple graph tells me everything I need about those complicated technical issues: it’s fast! Anyone who suggested this had anything to do with the fact that I was the author of JSONKit was quickly silenced…
Oh, no, wait… that’s right, that’s not the way it happened… In reality it was obvious that no matter how much I might like to contribute to getting the app out the door, odds were that I would either slow things down or screw something important up because of my unfamiliarity with the code base. One thing that caught my eye was that the application had a lot of
PNG image assets, and in my various adventures in the great city of life, I knew that you could often easily make
PNG images even smaller.
This seemed like a good project that I could work on:
- It was independent of what everyone else was doing, so no one would have to stop and explain how something in the app worked.
- It was something that would probably either work or it wouldn’t. It would also be pretty unambiguous about whether or not it was causing problems.
- It could be easily and trivially backed out if a problem was found, even up until the very last second… as long as you kept the original
PNG images, which seemed pretty obvious.
- I could actually contribute to the app that was going to ship in a few weeks, even if it only meant that I “saved a few bytes that the end user has to download and takes up on their iPhone”.
In astronomy, you first enjoy three or four years of confusing classes, impossible problem sets, and sneers from the faculty. Having endured that, you’re rewarded with an eight-hour written exam, with questions like: “How do you age-date meteorites using the elements Samarium and Neodymium?” If you survive, you win the great honor and pleasure of an oral exam by a panel of learned professors.
I remember it vividly. Across a table, five profs. I’m frightened, trying to look casual as sweat drips down my face. But I’m keeping afloat; I’ve managed to babble superficially, giving the illusion that I know something. Just a few more questions, I think, and they’ll set me free. Then the examiner over at the end of the table—the guy with the twisted little smile—starts sharpening his pencil with a penknife.
“I’ve got just one question, Cliff,” he says, carving his way through the Eberhard-Faber. “Why is the sky blue?”
My mind is absolutely, profoundly blank. I have no idea. I look out the window at the sky with the primitive, uncomprehending wonder of a Neanderthal contemplating fire. I force myself to say something—anything. “Scattered light,” I reply. “Uh, yeah, scattered sunlight.”
“Could you be more specific?”
Well, words came from somewhere, out of some deep instinct of self-preservation. I babbled about the spectrum of sunlight, the upper atmosphere, and how light interacts with molecules of air.
“Could you be more specific?”
I’m describing how air molecules have dipole moments, the wave-particle duality of light, scribbling equations on the blackboard, and…
“Could you be more specific?”
An hour later, I’m sweating hard. His simple question—a five-year-old’s question—has drawn together oscillator theory, electricity and magnetism, thermodynamics, even quantum mechanics. Even in my miserable writhing, I admired the guy.
While “saving a few bytes” might seem trivial, small details like that matter to me. Whether or not someone is willing to pay attention to the small details can say a lot about them. The above quote from Clifford Stoll’s The Cuckoo’s Egg: Tracking a Spy Through the Maze of Computer Espionage is sort of like the culmination of a lot of small details– the sky is blue for a reason, often for seemingly trivial, small details… but those small details form a long, causally related chain. I think it also eloquently illustrates that while small details matter, knowing which small details matter is just as important, and the causal relationship between them. Just knowing that “Why is the sky blue?” is an interesting question can reveal just as much about someone.
There’s a lot of small, trivial details involved in something as simple as “optimizing an iOS devices PNG images”. For example, once Xcode.app has built the app, you can not modify any of the files in the applications bundle because that will invalidate its code signing. There’s also the small detail that the
PNG images that end up in your applications bundle aren’t PNG standard conforming, but are actually an Apple proprietary
Turning iPhone PNG optimization up to eleven
Xcode.app has a build setting that you may not be aware of–
Compress PNG Files, and for new Xcode.app iPhone projects it is set to
Yes by default.
For the vast majority of projects the only time it is ever set is when the project was initially created… which is probably one of the reasons why you’ve never heard of it. If you did happen to notice the
Compress PNG Files build setting, the only other option is
No. Given these two choices, who wouldn’t want their
PNG files compressed?
What it does
When you build your project, and the target is an iOS device, not the simulator, the
Compress PNG Files build setting causes any
PNG resources that are copied in to your applications bundle to go through a preprocessing step that optimizes them for iOS devices.
Apple has not published any of the details as to what it specifically means to “optimize a
PNG image for iOS devices”, but others have reverse engineered at least some of it:
- Extra critical chunk (
- Byteswapped (
BGRA) pixel data, presumably for high-speed direct blitting to the framebuffer.
zlib header, footer, and CRC removed from the
- Premultiplied alpha (
color′ = color * alpha / 255).
gzip -9, except this one goes to
PNG optimization tools tend to perform optimizations at the
PNG level, such as:
- Color reduction (i.e., 24-bit
RGB to 256 indexed color conversion, etc).
- Bit depth reduction (i.e., 8-bits per Red, Green, and Blue to 4-bits per).
- Optimizing some of the
zlib libraries user tunable settings.
PNG filter optimization.
PNG standard specifies a number of predefined filters that can be applied to an image that can often improve compression. It’s difficult to tell in advance which filter will give the best results for a particular image, so
PNG optimizers usually try several of them. As you can probably imagine, the number of combinatorial permutations of different options grows rather quickly, so there is usually an option to specify how many of the different permutations will be tried in an effort to optimize the
PNG images size. As is often the case with such brute force techniques, the amount of time it takes to try the different permutations tends to grow exponentially, and the improvements gained for the extra effort tend to shrink inverse exponentially– the dreaded diminishing returns, where more and more work gets you less and less of an improvement.
PNG optimization tool stands apart from the rest, however: the
advpng optimizer from the AdvanceCOMP recompression utilities. This
PNG optimizer does most of its optimization at the
zlib level– instead of using the standard
zlib library, it uses the RFC 1950 (the standard that defines the
zlib compression format) implementation from 7-Zip / LZMA compression engine instead. Most of the time, the 7-Zip / LZMA RFC 1950 /
zlib compression engine is able to do a better job, and thus produce a smaller compressed result, than the standard
zlib library at its maximum compression setting.
advpng tool does not perform any of the optimization strategies that the common
PNG optimizers use, and in fact will undo any of the optimizations that they performed when it recompresses the result using the 7-Zip / LZMA compression engine. And you can forget about using it on quirky, proprietary
PNG image formats that aren’t
PNG standards compliant…
What would be great is…
The majority of a
PNG image is contained in the
IDAT chunk– it contains the actual pixels that make up the image. The
IDAT chunk is compressed using standard RFC 1950 / zlib compression. What’s really needed is a tool that just recompresses the
IDAT chunk chunk using the 7-Zip / LZMA compression engine, while leaving everything else unmodified.
Well, Good News, Everyone! Just such a tool exists: the
advpngidat tool, which is part of Scribds AdvanceCOMP fork on github.com. Not only that, it happens to work correctly with Apples non-standard
PNG format! This means you can make the
PNG images in your iOS applications bundle even smaller. Naturally, your milage may vary, and it wont be able to make every
PNG smaller, but it can usually compress your iOS
PNG images an additional 5% – 7%.
Turning Xcode.app up to eleven
So how do you turn your iOS projects
PNG compression up to eleven using Xcode.app? You use Scribds Xcode.app
PNG optimizer enhancement, also available on github.com.
Important: Scribds Xcode.app
PNG optimizer enhancement directly modifies configuration files that are private to Xcode.app!
While the Xcode.app
PNG optimizer enhancement modifies private Xcode.app files, the changes it makes are relatively benign:
- It modifies some
.xcspec files that are used to enable the
Compress PNG Files build setting in the GUI by changing the build setting from a
boolean to a multiple choice.
- It modifies some related files to modify and add descriptions that are displayed info help and info displays.
- It modifies some
perl and shell scripts that perform the actual copy and “optimize the
PNG image for iOS devices” so that, depending on the additional build setting options, pass the optimized
PNG image to
advpngidat for additional compression.
The end result is this: The
Compress PNG Files, which was a simple
No boolean setting, turns in to a multiple choice build setting:
| Identical to the unmodified
Compress PNG Files
| Identical to the unmodified
Compress PNG Files
Yes setting. This uses the Apple proprietary version of
pngcrush to optimize
PNG files for iOS devices.
| The compressed
PNG files from the
Low setting are further optimized by the
| The same as
Medium, except a handful of carefully chosen
-m compression methods that work much better in practice are used instead of the default heuristic used by
| The same as
pngcrush is passed the
-brute option which tries all of the compression method permutations.
Warning: This can take a very long time!
It even goes to twelve, but your puny iOS device can’t handle it…
Unfortunately, you should not use the
Extreme settings. While iOS versions < 5.0 had no problems with
PNG images compressed with either setting, iOS 5.0 will not correctly display
PNG images compressed at either
Extreme. Although it depends on the particulars of the image, some images will be displayed using the wrong colors. Of course, there could be other problems as well, as the image format is an unpublished, non-standard
That being said, the
Medium compression setting seems to work just fine– the only optimization it does is recompress the
IDAT chunk using a better RFC 1950 / zlib compression engine. Everything else in the
PNG file is passed through unmodified.
Help fight random entropy!
Take a look at Scribds AdvanceCOMP fork and Xcode.app
PNG optimizer enhancement (which requires the
advpngidat tool from the AdvanceCOMP fork), both available on github.com. After reading the documentation, and assuming you’re comfortable with modifying some of Xcode.apps private files, install them both.
Once installed, simply set your Xcode.app iOS projects
Compress PNG Files build setting to
Medium, and do your part in the fight against random entropy!
Just how many useless bytes were saved?
|| Size (bytes)
As previously mentioned, a problem was discovered with iOS 5.0 with some images compressed using either
Extreme. This is most likely due to the fact that the Apple proprietary “optimized for iOS devices” format seems to only use a PNG filter setting of
None. This means that the decompressed result can be used without any additional per-pixel filter processing.
So, in the end, we were only able to use the
Medium setting, which only optimizes a
IDAT chunk, leaving the rest of the bytes completely unmodified. Still, this resulted in a savings of 7.9%, which translates in to nearly 753K-bytes shaved off the final application bundle.
One more thing…
advpngidat compression tool isn’t just for “optimized for iOS devices”
PNG images, it can be used on regular
PNG images too. This can be a useful addition to any work flow that passes
PNG images through one of the common
PNG optimization tools (i.e.,
pngcrush). As an example, any web site that has a large number of static
PNG images can use a simple shell script to process all of the static
PNG images with something like
optipng, and then process the
optipng results with
In fact, the
advpngidat tool effectively does what is on the roadmap for the
… which is exactly what
advpngidat does today– the only “optimization” it performs is it recompresses the
IDAT chunk using the “powerful 7zip deflation” compressor. If the recompressed result happens to be bigger than the original, then the
PNG image is left unmodified. Otherwise, the
PNG image is replace with the smaller, optimized result.
This is really something that every web site with static
PNG images should do. You only need to perform the “optimization” on an image once, and every request for that
PNG image after that point will use the smaller, optimized result. You don’t have to be a rocket scientist to figure out the benefits: less bytes to send means pages load that much faster, and if you happen to pay for the amount of bandwidth you use… it means a simple, one time run through
advpngidat can save you real money.