Articles by Dan

Let’s take a quick little tour of the Xcode project file format. Anyone doing Mac OS X or iOS development in a team environment (i.e. just about everybody) has had to deal with the joy of merging project changes in version control (subversion, cvs, git, you name it).

An Xcode project, of course, is actually a package, a directory with bundled files within. A typical project might look like this:

MyProject.xcodeproj/
	danwr.mode1v3
	danwr.pbxuser
	project.pbxproj

The first two files have your username. If multiple users use the same project, you will have a set of .mode1v3 and .pbxuser files for each user. These files contain various user settings (preferences, really) that are associated with the project, for example:

• the size and position of the project window
• which groups are open and which are closed
• which item in the project is selected
• …and more

This information is not normally shared among multiple users, so it is neither necessary nor desirable to add these (.mode1v3 and .pbxuser) files to your repository. If you already have, go ahead and remove them (svn remove if you use subversion, for instance). I’ll wait.

Project.pbxproj

This leaves us with the project file itself, project.pbxproj. Oh, look how smug it is, all undecipherable and all. Or… is it? Let’s open up that bad boy in a text editor and see what it looks like. First, the overall layout:

// !$*UTF8*$!
{
	archiveVersion = 1;
	classes = {
	};
	objectVersion = 45;
	objects = {
            [[ : snip! : ]]
	};
	rootObject = 29B97313FDCFA39411CA2CEA /* Project object */;
}

We start with a comment indicating the text encoding used in the file, UTF-8. Presumably other encodings are possible, however in practice all project files use UTF-8. You will notice other comments (C-style: /* ... */) sprinkled throughout the project file. While presumably Xcode’s lexer handles multi-line comments, Xcode itself does not generate multi-line comments. If one were attempting the read or write project.pbxproj files, the parser would need to be able to handle multi-line comments, while ideally avoiding writing them (unless preserving existing comments).

A set of brackets { } enclose a record of key-value pairs. The keys are:

• archiveVersion; It is set to 1 in all versions of Xcode that use the file format described here.
• classes; A list of classes, usually empty.
• objectVersion; This relates to which object types are used in this project.pbxproj, and which keys are defined. This changes based on the version of Xcode that wrote the project; it is controlled by the “Project Format” popup menu in Xcode’s “Project Info” window. The value 45 corresponds to “Xcode 3.1-compatible”.
• objects; This is a list (actually, a record, or hash) of objects in the project. This is the meat of the file format.
• rootObject; This identifies the root object, the object that represents the project itself.

Now let’s take a look at the objects in general. There is an object for every file, group, target, build phase, and so on. Each object is identified by a UUID. If they are universally-unique, then any number of projects from any number of original machines can be opened at the same time by a single copy of Xcode without any problems. These UUIDs are 12 bytes—24 hexadecimal digits without any separating hyphens. Each object has a set of properties, one of which, “isa” specifies the class of the object. The other properties are determined by this class.

Object classes

The object classes supported depends upon the objectVersion. Here is a partial list:

• PBXBuildFile
• PBXFileReference
• PBXFrameworksBuildPhase
• PBXGroup
• PBXNativeTarget
• PBXProject
• PBXResourcesBuildPhase
• PBXSourcesBuildPhase
• PBXVariantGroup
• XCBuildConfiguration
• XCConfigurationList

The most important one here is PBXFileReference; every file referenced by the project (source files, headers, libraries, frameworks, xcconfig files, other projects…)¹ is represented by a PBXFileReference.

	089C165DFE840E0CC02AAC07 /* English */ = {
		isa = PBXFileReference;
		fileEncoding = 4;
		lastKnownFileType = text.plist.strings;
		name = English;
		path = English.lproj/InfoPlist.strings;
		sourceTree = "";
	};

There are two different types of files: input (e.g. source files) and output (e.g. the output application or library). lastKnownFileType is present and set for input files. The list of possible values can be found in Xcode in the file “Get Info” window. Additional values are, of course, possible.

	8D1107320486CEB800E47090 /* MyProject.app */ = {
		isa = PBXFileReference;
		explicitFileType = wrapper.application;
		includeInIndex = 0;
		path = MyProject.app;
		sourceTree = BUILT_PRODUCTS_DIR;
	};

Output files always have the explicitFileType key, includeInIndex (typically set to 0, or false, for binaries and packages). Both input and output files have a path and sourceTree specified. Path names are not normally quoted unless necessary (for example, if the pathname includes a semicolon, space, or other special character).

In part 2, I’ll look at PBXVariantGroup, XCBuildConfiguration, and XCConfigurationList.

¶

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¹ There are exceptions; two of the most well-known are Info.plist files and precompiled header source (.pch) files. These two files are always identified either by absolute pathname, or by a path relative to the .xcodeproj project.

In which we ponder the Great Imponderables of text on Mac OS X, and the subtle, obtuse, and bone-headed means by which we may stymie proper anti-aliasing.

Fig. 1: Basic anti-aliased text, magnified 2x.

Aliasing is a term used in computer graphics to describe artifacts created when a shape is rendered into a grid of pixels; these artifacts are commonly known as “the jaggies.” Anti-aliasing, then, is the process of rendering graphics in a way that conceals these artifacts. Let’s take a brief look at how text is anti-aliased on Mac OS X, and then we’ll look at how things can go wrong.

Completely unaliased text looks horrible (Fig. 2); in the mid- to late-nineties, Mac OS became accustomed to anti-aliased text, as in Fig. 1 (above). LCD displays became widely available and inexpensive enough to push out CRTs at the same time, not just because they are lighter and thinner, but for excellent contrast and very sharp pixels—and sub-pixels.

Each pixel is actually comprised of three components, red, green, and blue, and on LCDs the operating system can take advantage of that fact to improve anti-aliasing through a method called sub-pixel rendering. ClearType is Microsoft’s patented method of sub-pixel rendering; it differs from the method used in Mac OS X primarily in its intent: ClearType is designed to make small text as readable as possible, while Mac OS X attempts to accurately maintain the shape of the individual glyphs. That’s all I will say about ClearType here.

Fig. 2: Aliased text has lots of jagged edges, and crooked curves. This is what most text looked like in 1995.

Fig. 3a: Text with sub-pixel anti-aliasing (Mac OS X). At low magnification (2x here), the eye sees smoother shapes rather than color fringing. Compare the horizontal bars in the letter ‘e’, or the descenders in the letter ‘g’, with those in Fig. 1.

It is possible to draw text on Mac OS X with anti-aliasing completely disabled, as in Fig. 2, by calling CGContextSetShouldAntialias(context, false). To draw text as in figure one, call CGContextSetShouldAntialias(context, true) and CGContextSetShouldSmoothFonts(context, false).

Now let’s take a closer look at sub-pixel anti-aliasing. This is the default drawing method. Figure 3a shows text zoomed to twice its actual size. To see how sub-pixel anti-aliasing works, we need to zoom in more—4x (Figure 3b) or even more (Figure 3c).

Fig. 3b: At 4x magnification, color fringing starts to become apparent, if mostly subtle.

Fig. 3c: 10x magnification. Color-fringing is now obvious.

At 10x magnification, we can clearly see the use of red pixels on the left side of glyphs and blue pixels on the right. But does it always work this way? Consider Figure 3d—white text on a black background.

Fig. 3d: Inverse—white text on a black background. Notice how the red and blue edges are reversed.

Blue on the left, red on the right! The question you might ask: Is this because the text is now white, the background is now black, or both? Well, what do we get if we change the text back to black, and change the background to a moderately dark color?

Fig. 3e: Black on a dark orange background. To the eye, the sub-pixels look similar black-on-white (c.f. Fig. 3c). A more careful look reveals differences: Are the right edges blue here?

We see here that the color of the text matters most. But if we look closely at Figure 3e, we can see that the left edge isn’t the same color as in Figure 3c, and the right edge pixels are very different. This leads us to important, if unexpected lesson:

Sub-pixel anti-aliasing depends on both the color of the text and the color of the background. It is impossible to correctly apply sub-pixel anti-aliasing unless we know what the background contains.

These brings us to the practical portion of this article. Under what conditions will sub-pixel anti-aliasing fail (not appear, or appear incorrectly)?

• The application has called CGContextSetShouldAntialias(context, false);
• The application has called CGContextSetShouldSmoothFonts(context, false);
• The application has called CGContextSetAllowsAntialiasing(context, false);
• The context has no background (or has been filled with a color with alpha=0)
• The application draws text first, then later fills in the background color (for example, to highlight text or show the selection). You should always draw highlighting before you draw the text.
• If you change a view to use a CALayer (for CoreAnimation), you will now be drawing that view without any background, unless that view draws its own background. In Interface Builder, check “Draws Background” and explicitly set the background color, usually to windowBackgroundColor.

There are a couple more “gotchas.” First, Mac OS X Snow Leopard does not recognize all third-party display as being compatible with sub-pixel rendering, and may therefore disable it system wide.¹ If you suspect this is happening to you, try the following command at a Terminal prompt:
defaults -currentHost write -globalDomain AppleFontSmoothing -int 2

Also, Core Graphics has, by default, a y-axis that extended upwards, rather than downwards, as in most other graphics systems. It is tempting to reverse the y-axis, draw offscreen, then flip it back when blitting to the screen. But there’s a catch: this breaks text anti-aliasing (the chief symptom is blurry serifs and other horizontal lines). Do not draw upside-down unless you intend the text to be viewed upside-down. I hope I don’t need to tell you that drawing text backwards and flipping will have terrible results.

¹ 10.6: Re-enable LCD font smoothing for some monitors

A new photo set, Carl with his mom, is now on Flickr.

Jon Stewart takes on the “birthers.”

Apple has now released iPhone OS 3.0. This is a free upgrade for all iPhone owners, and a $9.95 upgrade for iPod Touch owners.

As an iPhone/iPod Touch developer, I should tell you that Apple makes it difficult to upload fixes to old versions of applications to the App Store (that is, you can only update the current version, not older versions), which means that you may get shut out of bug fixes to your favorite apps unless you upgrade to 3.0.