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The Magnetic Lasso tool is one that I find interesting, because it's sort of a balance between a manual selection tool you might say and an Automatic Selection tool. What I mean by that is it's manual in a sense that I need to trace around the object that I want to select, but it's automatic because it automatically detects the edge of that object. Let's take a look at how the Magnetic Lasso tool works. To get started I'll select the Magnetic Lasso tool from the toolbox. It's hiding, as it were, underneath the regular Lasso tool. So I'll click and hold my mouse on the Lasso tool's button and then from the fly out menu that appears I'll select Magnetic Lasso Tool. The Magnetic Lasso tool is, in some ways, a Brush tool. It has a brush size or a width setting that determines which area of the image is being evaluated.
Let's take a quick look at the basic operation of the Magnetic Lasso tool. I'll simply move out over the image and increase the brush size. And then I'm going to click on the edge of the object that I want to select. So I'll just click here on the edge of this portion of the building and then I move my mouse along that edge. I don't need to hold the mouse button down. I just move my mouse along that edge. And what's happening is that small anchor points are being added along that edge. Those are defining the actual edge so each of those anchor points are being connected.
And as I move my mouse, Photoshop is looking inside the circle to see where the highest point of contrast is. Within that circle. So it's an Edge Detection tool, but one that requires me to trace the edge of the object I want to select. Let's take a look at the various settings that relate to the use of the Magnetic Lasso tool. At this point I'll just press the Esc key to cancel out of the selection, and we'll take a look at the Options bar. We have first the option to create a new selection or to add to an existing selection.
To subtract from an existing selection or to intersect with an existing selection. We can also feather our selection as we're creating it, but I recommend leaving this at a value of 0 pixels, and instead, apply the same effect as feathering later in your workflow when you put the selection to use. I also recommend leaving the anti alias check box turned on, so that jagged edges in the selection will be smoothed out a little bit and then we turn to the key settings for the Magnetic Lasso tool. The first is width and, as we've already seen that essentially is the size of the brush that we're using.
In order to trace the edge of the object that we're trying to select. We can use the left and right square bracket keys to adjust the size of that brush, so the right square bracket key will increase the size, and the left square bracket key will reduce the size. You could also enter a specific value for width, or you could take advantage of the scrubby slider, simply clicking on the word Width and dragging to the right to increase the value. Or to the left to decrease the value. The approach I recommend in setting the width value is to consider the object you're selecting, and to sort of compromise between how detailed you need to be when you're tracing along that edge.
You can get a more accurate selection by using a very small width and tracing very carefully along the edge of the object you're selecting. If you use a relatively large width, then you're able to move quite quickly, because it's very easy then with such a large circle to keep the edge inside of that circle. So the best approach is to compromise a little bit, to find a setting that works reasonably well, where you can trace carefully but quickly. If they're nearby high contrast edges, so in other words if there have been another edge that had high contrast near the high contrast edge I'm trying to select, then I would probably want to use a much smaller width setting, so that I'm able to keep that high contrast edge taht I want select inside the circle.
Without any other competing high contrast edges getting inside that circle. We can also adjust the contrast. Under normal circumstances, when the magnetic Lasso tool is a good tool for a particular selection, you can probably just leave the contrast at the default value of 10%, or frankly have it set to just about any value, because it won't make a huge difference. But you can also increase the value, which essentially is increasing the amount of contrast that is required for an area to be considered an edge. So for example, if I were using an especially large brush for some reason, and that allowed another edge to get. Inside the circle then it would be possible to use a high-contrast setting in order to ensure that only the highest contrast edge were treated as the actual edge being selected.
In this case for example, I have a reasonably good amount of contrast so, let's take a look at the difference we might see with a low setting versus a high setting. I'll go ahead and take this value down to 1%, and then I'll just click. And drag along this edge, not being careful at all. And you'll see that the selection is not all that good, because it's finding a one percent contrast all over the place inside of that circle as I move. And so it's very difficult for Photoshop to identify which particular contrast edge I'm really looking for. I'll press this Escape to cancel that selection and I'll take the contrast value up relatively high.
In this case around 85% or so, and then I will do the exact same thing just moving not very carefully along that edge. And you'll see I get a little bit better result, not perfect but improved over what we saw just a moment ago. So again the contrast setting tends not to be terribly critical, but it does have some effect on the behavior of the magnetic Lasso tool, so in some cases it may be worth adjusting. We can also set the frequency. We've already seen that the Magnetic Lasso tool places anchor points in order to define the actual edge of the object we're selecting.
The frequency determines how frequently those anchor points are being placed. I'll go ahead and press Escape to cancel my selection and I'll take this frequency down to the minimum value of zero. And then I'll click along one of the edges in the image. And I'll move my mouse slowly along that edge. And you'll notice because the frequency is set to zero, I'm not getting all that many anchor points. Certainly anchor points are being added as needed in order to define that edge, but they're not occurring very frequently. I'll press Escape to clear that selection and then I'll increase the frequency to its maximum value of 100 and I'll repeat that same process, clicking on that high contrast edge and then slowly moving along the edge.
And you can see very clearly that I'm getting many more anchor points and the result is a little bit more accurate selection. Because I'm getting a greater number of those anchor points and therefore subtle changes in shape of the object I'm selecting will be followed a little bit more accurately. So, with my settings established, and I think at this point, I have pretty good settings for this particular selection, I'll go ahead and press Escape so that I can start over, and then I'll click along that high contrast edge. And once again, I'm just clicking and releasing. I'm not clicking and holding the mouse button and dragging.
And then I can move the mouse along that high contrast edge. And I can move at a reasonably fast pace. I really just need to make sure that I'm keeping the contrast edge of the object I want to select. Inside that circle at all times. Because that's where photoshop is looking for the contrast. If I move a little more slowly, then generally you'll see that I get a little bit more accurate results in large part because the frequency is set to a high value. And therefore those anchor points will be placed more frequently.
As long as I'm not moving the mouse too quickly. Of course, for this particular object, it extends outside of the frame. And that is actually one of the shortcomings of the Magnetic Lasso tool. It essentially doesn't realize that the object is extending out of the frame, and so when I get to the edge of the frame, and move my mouse outside the image, and then follow all the way around to the point where I originally started, you'll notice that while it followed the edge for the most part, it also jumped in just a little bit in a couple of spots. Nothing too terrible, but enough that the selection is less than perfect and we'll need some clean up in those areas.
But then I will re-enter the image at that high contrast edge up toward the top of the photo. And continue to my original starting point. Once my mouse is hovered over that point, you'll see that my mouse pointer changes to an icon representing the Magnetic Lasso tool. With a circle at the bottom right corner indicating that if I click here it will close that selection, or complete the process of creating a selection. So I'll go ahead and click, and you can see that my selection is now created. It's probably abundantly clear that the Magnetic Lasso tool tends not to create perfect selections in many cases. But it does make it easy to create a selection where a high contrast edge exists.
It doesn't tend to be the first tool I look to for creating a selection, but it can be helpful in a variety of situations. There is one additional option available for the Magnetic Lasso tool that is especially helpful if you inadvertently move your mouse away from that contrast edge. I'll go ahead and press Control+D on Windows or Command+D on Macintosh in order to deselect that selection. And then I'll go ahead and start creating the selection again. I'll click and then move my mouse. And if I follow that edge I should have no trouble But if I move away from the edge and back in. Of course, Photoshop is still going to create anchor points for the selection, so now I've made a mistkae.
Well, I could certainly continue on. And then fix that mistake through other means. For example using one of the other selection tools and subtracting from the selection. But I can also move back. I can essentially back up along this path that I'm defining. The first thing that I'll do is move my mouse back to the point where I last had a good anchor point. Because that's the point where I'll want to continue from. And then I can press the Delete key. In order to delete the preceeding anchor point and I can keep pressing Delete until I've deleted all of the problem anchor points.
In other words until back to my current position and then I could continue moving the mouse along that contrast edge and anchor points will be placed for me. So as you can see, we have some options for fine-tuning the behavior of the Magnetic Lasso tool, but by and large, it is simply an Edge Detection tool. And it makes it relatively easy to make selections when we have a high-contrast edge to work with.
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