Tab Bar Icons (part 2)

In part 1 of this article I talked about general guidelines for the icons in a tab bar view. In this part, I will go into specifics as to how to actually create tab bar icons. One thing I noticed is there is little documentation or help out there on the Web regarding tab bar icons, so hopefully this information will be beneficial. Since I wrote part 1, I have found that the Apple documentation may be incorrect when it states that the icons should be 30x30 points. By making icons exactly that size, I found the icons were too big and they were truncated. I have since changed over to icons the size of 30x30 pixels and that seems to work well. In fact, the iPhone Human Interface Guidelines does specify "about" 30x30 pixels. There definitely is extra space to grow, but 30x30 pixels will be my starting point and only increase in size if I need the extra space. As a reminder, we're looking to create these type of icons:

1. PNG
2. 30x30 pixels
3. no color, as alpha values determine the image and shading
4. transparent background

Drawing without color is basically impossible, so I use black & white. Even the black and white colors are thrown out since the tab bar just uses the alpha channel data. By using black and white, it allows me to draw without fussing with colors. One important thing to realize is that tab bar icons are inverted. Things that you draw black come out white; things that you draw white are black. If you're old enough to remember film negatives, it is the same concept. You might be tempted to make your icons already inverted so that they look "normal" in the tab bar, but I believe this would be a mistake. Judging by the Apple application tab bar icons (You Tube and iPod), it is expected for the icons to be negatives. Another important point is that when you draw a black line, you are not applying a black color on top a white canvas. Drawing with alpha channels is a bit of a paradigm shift. Think of this way: you have a black canvas. You draw on this canvas using white paint. If you use really thick paint (a high alpha value), then this will show as bright white. If you thin your paint (a low alpha value), then some of the black from the background will bleed through, making is less bright and more gray. This is exactly what an alpha value is: it is the degree of transparency. A low alpha value means it is completely transparent. Using an alpha value of 0 means none of the white shows. As you increase the alpha value, the white color becomes less and less transparent, thus becoming brighter. When the alpha value is at its max of 255, it is completely opaque and you cannot see through the paint at all. Thus it shows up as white. Hopefully you aren't totally confused by now! An example will help demonstrate some of these concepts. Let's say we want to create a typical "documents" icon, three pages of paper stacked on one another. The top page could have some super tiny text on it. A "normal" icon for this would look like this:

When this is displayed in the tab bar, it looks like this:

See how this is a negative of our original icon? Everywhere you see white means those lines have an alpha value of 255. If this negative effect drives you crazy, you probably could draw using white paint on a black background, and then remove the background just before exporting the image. Again, the black and white colors are meaningless here; you could use purple and pink if you wish. It is the alpha values that are important, so make sure your white paint (or pink) has a high alpha value. So, how do we make such an icon? Icons can be created with numerous tools, but I will focus on one that I feel is best suited for this task, and the price is right (free). Inkscape is a Windows, Linux, and Mac OS vector graphics editor. It is freely available and can be found on the web here: Inkscape homepage. If you are installing Inkscape on Leopard, you may have a problem with it hanging while caching fonts. This is a bug that is easily fixed by going into the terminal and typing this:

$ mkdir ~/.fontconfig

After this, it should start normally. I didn't experience this when I installed version 0.46, so this may be fixed already. When you fire up Inkscape, you see a blank canvas that looks like this:

Edit the properties of this to get what we need. Select the menu File>Document properties... Set the size in the "Custom size" box. Make sure the units is "px" (pixels) and the width and height both to 30. You might also want to disable snapping, which I find annoying when trying to position drawings exactly where I want them. This is the "Enable snapping" checkbox on the snap tab. Close the properties window. Now, your canvas is probably really small, so select the menu option View>Zoom>Page to zoom the canvas to fit the window. To get into black and white mode, select grayscale from the color palette on the bottom. You can bring up the color palette window by clicking on the little triangle I circled in red in the lower right of the picture above. Inkscape automatically has a transparent background, so there is nothing more you need to do there. The area inside the gray border is your canvas. Even though this is white, remember it will be black on the tab bar. So the first thing we want to do is create the top page. Choose the "rectangle and square" tool on the left tool bar, or hit f4. Click where you want the top left corner and drag. Put the rectangle a little low and to the left to leave room for the other pages. We can center everything later. Once you have your rectangle, we need to fix it up. Select menu item Object>Fill and Stroke The fill is what is inside the object, and the stroke is the outline around the edge. What we want is a stroke with a high alpha (255) so that the border of the page shows as white, but the fill should be black, meaning an alpha of 0. The diagram below shows setting the fill alpha to 0:

Alpha values are the A at the bottom of the RGB tab. At the far left of the alpha scale it shows a checkerboard pattern. This indicates transparency. As you go higher, the checkerboard disappears and black replaces it. You could also hit the "X" button to remove the fill altogether, which will result in a transparent fill. Now we show the properties for the stroke paint. We want the stroke to have a high alpha value so it looks white in the tab bar:

You can also go to the "Stroke style" tab and play around with the line thickness and line pattern. Now we need to put pages in the back. You might be thinking that since this editor can do layers, you can simply copy the top page, shift it over some, and then put that layer underneath. Well, that won't work (at least I don't know how to make it work). Because the top page has a transparent fill (alpha of 0), the border of the bottom page shows through. What you end up with is this:

This is not what we want. Here lies a problem with using alpha values. It makes it very difficult to layer objects. It's best to think of creating a "flat" picture. All impressions of depth are fake. So instead of layering another page below the first one, we cheat and just draw the outline of the page edges next to the top page. When you're working with icons, it's also best to turn the grid on, which makes drawing lines easier. View>Grid. Use the Bezier curve tool to draw straight lines. Click once to select the starting point and move your mouse (don't hold the mouse button down). When you want to change direction, click to set another point. Double click to create your final point. For the right angle shown below, it consists of:

1. click to start, then move mouse right
2. click to form angle, then move mouse down
3. double click to end

Draw the third page the same way. Or to make it easier, duplicate the second one via ctrl-D, then drag the third one into place:

I will leave the "text" on the top page as an exercise to the reader. (Hint: use the bezier curve tool to draw lines, then adjust the stroke style of each line to be dotted.) Let's save it and try it out. To save as a PNG file, it is a bit non-intuitive: File>Export bitmap... select "Page" for the Export area select "Browse" button and choose location to put file. Find folder and type in filename. Inkscape will provide the .png extension for you. Hit "Save" This doesn't actually "Save" the file. You then need to hit "Export" button What you have is an icon that looks like this:

When I insert this into my tab bar, I get this result:

Notice how the large icon doesn't look like much, but when it is shrunk down to icon size, it doesn't look too bad. You even can see the 3-D effect that is not apparent in the full-size version. This is due to the loss of detail when it gets smaller, and your eyes/brain fill in the gaps. Think of it as the "Monet affect", where his paintings just look like a bunch of dots up close, but as you step back, you lose detail and the picture morphs into something beautiful. To help you visualize your icon as you are drawing your image, Inkscape has an icon preview mode which shows your current drawing in various icon sizes. Select menu View>Icon Preview... to bring up that window. That should be a good start to get you going on creating your own tab bar icons. I strongly urge you to learn more about Inkscape; it is a very powerful tool and can also handle all your other icon needs. Good luck and create something beautiful!

Tab Bar Icons (Part 1)

Tab bars are for toggling between application views. This is a common controller on the screen-constrained iPhone. Not to be confused with toolbars, tab bars are the black bars at the bottom of the screen with gray unselected icons and one blue selected icon. You see tab bars in the phone and ipod and YouTube apps, among others. Here is an example, from the ipod app:

When you are creating a tab bar for your own application, you face the problem of providing the icons. You have several options here. Your first option is to use one of Apple's system icons. Like the navigation bar, you have access to some of Apple's system icons, and they even encourage you to use them. Unfortunately, there are a couple caveats with using these. First, the list is pretty skimpy. Second, the icons include the text as well. You cannot set the image and the text separately; it's both or nothing. Here is what is offered:

icon	text	enum
	More	UITabBarSystemItemMore
	Favorites	UITabBarSystemItemFavorites
	Featured	UITabBarSystemItemFeatured
	Top 25	UITabBarSystemItemTopRated
	Recent	UITabBarSystemItemRecents
	Contacts	UITabBarSystemItemContacts
	History	UITabBarSystemItemHistory
	Bookmarks	UITabBarSystemItemBookmarks
	Search	UITabBarSystemItemSearch
	Updates	UITabBarSystemItemDownloads
	Most Recent	UITabBarSystemItemMostRecent
	Most Viewed	UITabBarSystemItemMostViewed

If you can make use of them, you access them via an enum for each defined in the UITabBarItem class. Here is how you would programatically set your tab bar image to the magnifying glass:

self.tabBarItem = [[UITabBarItem alloc] initWithTabBarSystemItem:UITabBarSystemItemSearch tag:0];

You execute this in your UIViewController subclass that you are putting into the tab bar. This is best done in your view controller's initializer method:

- (id) init
{
self.title = "search";
self.tabBarItem = [[UITabBarItem alloc] initWithTabBarSystemItem:UITabBarSystemItemSearch tag:0];
return self;
}  

If the system icons don't meet your needs, some people have suggested doing a screen capture of an app that has an icon you like. Ignoring the legalities of this, this may not give you good results because Cocoa-Touch has already applied affects to the icons. Your next option would be to search for some free or pay-for icons. I noticed many freelance graphics designers putting their shingle up and offering their services. I didn't look long, but I did not find any suitable, free icons for iPhone tab bars. This leaves you one last option: create your own. So the first questions that come up are:

1. what format?
2. what size?
3. what colors?

After researching these questions, I found some conflicting answers and lots of confusion among the iPhone developer community. I will try to summarize and disperse the clouds. For format, this is pretty well agreed upon to be PNG, which is the image format of choice on the iPhone. You're not limited to this format, but PNG is the recommended format and gives best performance. The background should be transparent, else your icon will be a plain square. The size issue is a bit more contentious. I saw reports of sizes ranging from 20 x 20 all the way up to 57 x 57. Most people don't differentiate between pixel or point sizes, and they are different. Here is what Apple says in the reference documentation for the UITabBarItem:

If this image is too large to fit on the tab bar, it is scaled to fit. The size of an [sic] tab bar image is typically 30 x 30 points.

Note it doesn't say what the max size is before it gets scaled. One poster thought the max size was around 48 x 32 before scaling happens. But if Apple doesn't publish this information explicitly, I would hesitate to use it. You could explore yourself to find out what the max is, but obviously Apple would like to retain some latitude in this area to possibly change it in future firmware releases. I believe 30 x 30 is a safe size that will give you the best look. There also seems to be a great deal of confusion regarding colors. From my research, colors are basically ignored. Here is what the Apple document "View Controller Programming" has to say about it:

The unselected and selected images displayed by the tab bar are derived from the images that you set. The alpha values in the source image are used to create the other images—opaque values are ignored.

In other words, Cocoa Touch will create new images from your source images, using alpha values only. Alpha, in the PNG world, is the degree of a pixel's opacity. Maximum alpha values denote a completely opaque pixels. In the example below this quoted text, they have an example source image and the resulting unselected and selected icons. Color is irrelevant. In part 2 of this article, I will discuss various tools used to compose icons, specifically how to create tab bar icons.