Adobe Photoshop Tutorial. Copyright Michael B. Karbo.
10. The magnetic lasso
In the previous chapter you tried to isolate a figure, and that was done with a selection.
You can select in many ways – the magic wand is just one of the tools. Now you will see another tool, the magnetic lasso.
Just like in the previous chapters you will do a number of exercises. You will be testing the magnetic lasso (and it is almost genial), but of course we also use the opportunity to introduce different other tools and functions.
Start by retrieving the picture pplys.jpg from the home page for this booklet. That is a photograph, which needs loving treatment.
1. First save the picture as peter plys.psd. Then doubleclick on the layer Background, and rename it to plys:
2. You might want to zoom to 200% (press Control++).
3. Use the hånd tool to move the picture, so you see the top of the bear’s head. You do that by holding the space bar down and drag the picture itself down with the mouse:
Figure 22. Use the hand tool to navigate in greatly enlarged pictures.
Start the lasso
Now you are ready to separate the teddy bear.
2. The magnetic lasso has the property to automatically cling to areas with high contrast. Start in the settings line by adjusting the Frequency to 80:
3. Now click once with the tool in top of the bear’s head, to the right of the left ear:
4. Then drag the mouse to the right along the edge – without pressing the mouse button!
5. Can You se that the selection follows the contour of the bear’s head. That happens automatically while you drag:
6. Now try to hold the space bar down. Then the mouse cursor changes to a hand. The lasso tool is also temporarily deactivated:
7. Use the hand (still holding the space bar down) to drag the picture a bit to the left. In this way you can navigate in a greatly enlarged picture. When you release the space bar, the cursor returns to become a lasso, and you can continue working with your selection:
8. Try to drag the tool all the way around the bear. When you return to the starting point, the mouse cursor shows a small circle. This indicates that the selection can be concluded with a click (or a double click). Try that:
9. If your selection goes totally awry, don’t worry about it. You just have to get used to the magnetic lasso, it is rather special. Read through the next sections, then try again!
Use the tool correctly
It is rather easy to select the whole figure with the magnetic lasso. But it can help to know a couple of small tricks:
1. You can always interrupt the selection with Escape. Then you can start over. Try to do that now, and follow these comments and instructions:
2. Drag the mouse slowly; so you see the selection follow.
3. Notice the small anchor points, which are inserted on an ongoing basis:
4. Try to press Delete a couple of times. Each time you press that, the last anchor point is deleted. Continue the selection.
5. If you employ enlarged viewing, you will encounter a window frame. To be able to continue the selection, you need to hold the space bar down (to activate the hand tool) and drag until you have a fresh section to work on.
6. Besides, it is a good idea to use the space bar to make a pause in the selection, if you get a spasm in your hand.
7. Down by the bear’s paw you will encounter a somewhat difficult area, where there are heavier shadows, and the edge may not be so easy to recognize (the contrast is lower). There you could insert extra anchor points by clicking. Try that:
8. Work slowly. Click on your way as needed. If it goes awry, you drag the cursor backwards and press Delete to delete anchor point(s).
9. Continue the selection. In some places it is wise to insert additional anchor points – such as where the ears start – to make the selection as precise as possible:
10. You may also need a bigger enlargement of the picture: use Control++ while you hold on to the selection with the tool – that can be a tease, so try that!
12. Then click with the mouse, and the selection is completed. You can also double click to force a conclusion.
Now review the selection carefully – if you are very unsatisfied, you can start over. But if there are only minor errors in the selection, they are easy to correct:
1. Choose the regular lasso tool:
2. Now you can expand the selection with the Shift- key and reduce it with the Alt key. That is important – remember that!
3. See the figure below. There the selection is too big. The arrow shows the small area, that needs to be removed from the selection:
4. You do that with the regular lasso and by holding the Alt key down while you start a selection outside the existing selection. Then let the new selection ”cut” the unwanted area away.
5. You encircle the area with the lasso, so the new selection overlaps the existing. The common area in the two selections is removed. Notice the small minus symbol by the lasso. This indicates that the selection is being reduced:
6. When you release the mouse button, the unwanted area is removed from the selection. That is so simple. Here you see the result:
7. The reverse problem is that the selection is too small. That is corrected by adding an area to the selection in the same way as before, but with the Shift- key down.
8. Notice the small + symbol that follows the mouse cursor. It indicates that the selection is being expanded. Notice also that the new selection actually starts inside the old, which is crosses to get out and select some more:
9. Now you should be able to make a really good selection around the teddy bear.
You have tested the magnetic lasso. If you look in the settings line, there are five parameters, which can be adjusted for the tool:
Here is a brief review of the parameters:
Feather gives a softening on both sides of the selection, so the transition between the selected area and the background gets less abrupt. Feathering actually creates some transparency in the border area. Threfore there is also a small loss of picture details. It is best to avoid feahering during the selection. Wait with a potential feathering until the whole selection is in place. Then you have the option to cancel and try again with other values.
Anti-alias makes the edges less choppy. Smoothes the color transitions in the edges of the selection. Effects only pixels in the edge. Should alwsays be activated.
Width indicates in how wide a belt the lasso will register edges. You can values between 1 and 256 pixels (try to start with 20).
Edge Contrast indicates how big the contrast has to be before anything has to be perceived as an edge – use low values in pictures with low contrast contours.
Frequency indicates the frequency of automatically inserted anchor points. The value can be between 1 an 100. The default value is 57, but we increased it to 80 in the teddy bear exercise.