March 5, 2009

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!

9 comments:

  1. Great tutorial! Oh how I wish I would've found this page about a week ago when I was blindly searching for tabbar icon tutorials, downloading inkscape, figuring out controls, trying to figure out tabbar icon sizes, etc...

    Thanks and I'm sure this will help others out a ton!

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  2. Excellent post! Thank you very much, exactly what I was looking for.
    Panoma

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  3. Great article!I was wondering, how many icons could fit in the bar? or if it depends on just how many 30px icons can fit in the bar itself..

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  4. The tab bar will fit up to 5 icons. I don't think the size of the icons matters, as the SDK will resize them. If you have more than 5, then only 4 will be put in the tab bar and the 5th will be a "more" icon. When you click on that, you see your remaining icons in a table.

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  5. This was a "light in the dark" article.
    Thank you very much.

    It helped a lot.

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  6. thanks for the great post. I am struggling to understand if the png should be optimized at 163 dpi cause the screen resolution is 163??

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  7. Thank you Thank you Thank you!!!! What a fabulous tutorial. You are a great teacher!

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  8. What more can I say?
    Thank you.

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  9. Much better to convert your image to greyscale, invert and paste as alpha channel

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