At CodeMash 2013, I gave a talk entitled “Using Images in iOS.” Sorry for the delay, but here are some links for the contents of the presentation:
A while ago I noticed an interesting API for creating a
UINib object from data:
At the time I didn’t have a use for it, until this exchange occurred on Twitter:
Other things you can externalize for remote updates: Error messages and codes; refresh / download policies; AutoLayout visual language.— Matt Drance (@drance) January 2, 2013
@drance You can make a UINib from an NSData, so theoretically you could download remote nibs too.— Jeff Kelley (@SlaunchaMan) January 2, 2013
The resulting exchange was very fruitful, including this gem from ex-Apple employee Michael Jurewitz:
So I wouldn’t recommend using this in a shipping application, but I wanted to see if it worked. I created a simple app that loads a nib from a website, then tries to initialize a view controller’s view using it. You can view the whole project on GitHub, but here’s the relevant code:
Would I recommend using this in a shipping app? Absolutely not, given Jury’s recommendations. But it is an interesting idea for enterprise, in-house, or jailbreak apps, and I can see the possibility for some very cool stuff to come out of it.
Images in Cocoa Touch, represented by the UIImage class, are a very important subject. Apple’s iOS platform prides itself on visual appeal, with Retina Displays, custom UI in many top apps, and a focus on photos with apps like Instagram. To that end, it behooves you as an iOS programmer to know a bit about working with images. This post won’t discuss everything you need to know about using the UIImage class, as that’s more appropriate for a book than a blog post—though maybe a series of blog posts would do—but instead will focus on one advanced topic: working with pixel data. You can find the basic stuff in the UIImage documentation, anyway.
Turning an Image Into Data
One of the first things you might want to do with a UIImage object is to save it to disk. To do that, you’ll need to save it to an image file. There are built-in functions to get properly-formatted data from an image, in both PNG and JPEG functions:
- UIImagePNGRepresentation(), which returns an NSData object formatted as a PNG image, taking a pointer to a UIImage object as its sole parameter.
- UIImageJPEGRepresentation(), which returns an NSData object formatted as a JPEG image. Like the previous function, its first parameter is a pointer to a UIImage object, but it has a second argument: a CGFloat value representing the compression quality to use, with 0.0 representing the lowest-quality, highest-compression JPEG image possible, and 1.0 representing the highest-quality, lowest-compression image possible.
Once you have the image data represented by an NSData object, you can then save it to disk with various NSData methods, such as -writeToFile:atomically:.
Getting Raw Pixel Data
While the above functions are great for saving images, they aren’t so great for image analysis. Sometimes you need to analyze the pixel data for a given pixel, down to the values for the red, green, blue, and alpha components. To get that kind of granularity in an image, we’ll be using a lot of CoreGraphics functions. If you haven’t used CoreGraphics before, know before going in that it’s a C-based API à la CoreFoundation, so you won’t be using the Objective-C objects you know and are used to. Instead, there are opaque types (represented by CFTypeRef, which is analogous to Objective-C’s id) representing objects grafted onto C, complete with manual memory management—no ARC for you. That’s neither here nor their, however; let’s talk about pixel data.
The color space of an image defines what the color components of each pixel are. Represented by the CGColorSpace type, you’ll typically use either an RGB color space or a Gray color space, which have red, green, and blue components or a white component, respectively. For this example, we’ll be using the RGB color space. We can create an instance of it with the CGColorSpaceCreateDeviceRGB() function, which returns a CGColorSpaceRef type—think of it as a pointer to a CGColorSpace object.
What does using this color space get us? We now know that the pixels of our image will have three color components, and in what order. This will come in handy later on when we need to query the data.
A graphics context, represented by the CGContext type, is analogous to a painter’s canvas—it’s what you draw into. For the purposes of drawing an image, you’ll create a CGBitmapContext, the ideal type of context for this data. You create a context with the CGBitmapContextCreate() function, which return a CGContextRef type. Let’s look at the declaration of that function (from CGBitmapContext.h):
CGContextRef CGBitmapContextCreate ( void *data, size_t width, size_t height, size_t bitsPerComponent, size_t bytesPerRow, CGColorSpaceRef colorspace, CGBitmapInfo bitmapInfo );
So, that’s pretty simple, right? It’s actually fairly straightforward, despite its appearance. Let’s break it down into more easily-digestible components. It’ll make more sense if we don’t go top-to-bottom, so we’ll go in the order I think makes the most sense.
First is the bitmapInfo parameter. The CGBitmapInfo type is a bitmask that represents two options: the alpha component, which contains transparency information, and the byte order of the data. We’ll talk about the alpha component here; byte order is another topic altogether. On iOS, only some pixel formats are supported. Looking at this chart in the documentation, we can see that, for all supported pixel formats on iOS in the RGB color space, these are the CGBitmapInfo constants we can use:
We can do two things with the alpha component: skip it, or use it in a premultiplied format. The premultiplied flag tells the system to multiply the individual red, green, and blue components by the alpha value when storing it. So, instead of RGBA values of 1, 1, 1, and 0.5, it’s stored as 0.5, 0.5, 0.5, and 0.5. This is a performance-saving measure on iOS devices, and is done automatically to all of your PNG images by Xcode when you build for a device.
So, for the bitmapInfo parameter, I generally pass kCGImageAlphaPremultipliedLast.
The penultimate parameter, colorspace, is a CGColorSpaceRef pointing to a color space you’ve created. This informs the context about the number of color components. Keep in mind that there’s one extra component for the alpha information if you’re not skipping it, so an RGB color space uses 4 components including alpha.
The width and height parameters are pretty simple: the number of pixels wide and high to make the context. Keep in mind that for Retina displays, you may need to double the values. You can use the scale property of the main UIScreen object as a quick “am I on a Retina device?” check.
Next, let’s talk about the first parameter: data Here you have two options: to pass in a pointer to a region of memory you’ve allocated for the image data, or to pass NULL and have the graphics subsystem create it for you. If you’re trying to access pixel data, however, it’ll help to have a pointer to the data, so here you’d pass in memory you’ve allocated. How do you know how much is enough? Let’s look at the bitsPerComponent parameter. I usually use 8-bit components—again, see the chart linked above for valid options—so I would pass 8 for bitsPerComponent. Once you know that, you can determine bytesPerRow easily:
size_t bytesPerRow = (bitsPerComponent * width) / 8;
And then, finally, we can determine how much data to use. I use the uint8_t data type to represent this, as it’s an unsigned 8-bit integer, perfect for our needs.
uint8_t data = calloc((width * height) * numberOfComponents, sizeof(uint8_t));
The entire stack might look like this:
The only thing in this code that we haven’t gone over so far is the call to CGContextDrawImage, which (surprisingly) draws the image. It takes three parameters: the context to draw into, a CGRect defining where to draw, and a CGImageRef for the image. You can obtain a CGImageRef from a UIImage using its -CGImage method.
Now that the image is drawn in our context, the rawData array will be filled with real, live image data! You can access it like so (modify the values of x and y as suits your needs):
int x = 0; int y = 0; int byteIndex = (bytesPerRow * y) + (x * bytesPerPixel); uint8_ t red = rawData[byteIndex]; uint8_ t green = rawData[byteIndex + 1]; uint8_ t blue = rawData[byteIndex + 2]; uint8_ t alpha = rawData[byteIndex + 3];
And there you have it! Now that you’ve gotten the data out of your image, do whatever you want with it. Just remember the blog authors you read along the way when Facebook buys you for a billion dollars.
Note: The venerable Mike Ash published a similar article while this one was half-done in my drafts folder. I thought about scrapping it altogether, but since mine is iOS-specific, and with some prodding from a co-worker, I decided to press on. Go read Mike’s blog, too. It’s awesome.
Yesterday I gave a talk on concurrency in OS X and iOS at CocoaConf Columbus. As promised, here are the slides and code:
- Code—NSOperationQueue example
- Code—GCD Example (Note: The master branch does more than we covered. To see the end state we got to, check out the dispatch-barrier-example branch.)
A while back, I was working on an application for a client with a very specific requirement. Since it collected personal data, the application could only run on iOS devices that were protected with a passcode. This requirement, seemingly very simple from the client’s perspective, was a bit of a hassle to implement on the programming side of things. There’s no simple method on
UIDevice to determine if a passcode is set, nor is there a way to force that programatically. In fact, there’s no way to force most things like that. The iOS device isn’t the programmer’s, it’s the user’s. Except when it isn’t.
One of the things that you can do is to use the iPhone Configuration Utility to make a configuration profile. These profiles can support a range of things, from requiring a passcode (or even an advanced, non-numeric passcode) to WiFi settings, VPN to CardDAV settings. Creating a configuration profile that requires a passcode, then installing that configuration profile onto the device is a no-brainer. But how do you ensure that the application will only run in that case?
Disclaimer: Before I go any further, you should know that since this was an in-house project, none of the code that I wrote made it into the App Store. Therefore, I don’t know if this is kosher in an App Store app, nor do I recommend this approach for that.
One thing that you can do is to include a self-signed certificate in a configuration profile. Those of you familiar with OpenSSL may be groaning as you realize where this horrible, horrible workaround is headed. I created a new certificate authority. With that new certificate authority, I signed a separate certificate that I had created. Verifying this certificate, then, requires that the verifying party accept the certificate authority’s certificate as valid. Well, since you can set that in the configuration profile, I did, along with the passcode requirement. Then, in the app, I bundled the certificate that I had signed with my CA.
When the app starts up, it attempts to verify the certificate. In the case where the configuration profile is installed and the CA’s certificate is in the system’s keychain as trusted, this is no problem: the certificate checks out and my app is free to go. If that validation fails, however, then I know that the certificate from the CA is in the system, so I know that the configuration profile is installed, as well.
Why this works for a passcode so well is that to install a configuration profile on a device without a passcode when the profile requires one is that you can’t install it without setting a passcode in the process. For the client, this was Good Enough, and the app shipped and worked properly. It’s worth noting, though, that the less the end-user knows about this process, the better. To circumvent the passcode restriction, all one would have to do would be to modify the configuration profile to still include the CA’s certificate, but not the pas code requirement. For that reason I can’t recommend this for anything like EMR or tax records, but for minor demographic information like we were collecting, this sufficed.
I realize this didn’t include any code, but the individual portions aren’t that hard, and I don’t have access to the original code so I’d have to re-write them all. Here they are in a nicely-formatted list for those keeping score at home:
- Create a new certificate authority with OpenSSL.
- Create a new certificate, then sign it with that certificate authority you just created.
- Create a configuration profile in the iTunes Configuration Utility with the settings you would like to enforce.
- In the “Credentials” section in the iTunes Configuration Utility, add your CA’s public-facing certificate to the configuration profile.
- Add the certificate you signed with your CA to your application’s bundle.
- In your application, verify the certificate you included.
- Distribute the configuration profile along with your application to end users.
Like I said, this is far from perfect. But when you’re working with an enterprise client who has Big Needs, this is one trick to keep in your back pocket when you’re up against a deadline.