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Sally Jane Photographic Art

Adobe Photoshop Tutorial


Adobe Photoshop Tools - Marquee Tools

Magnetic Marquee Tool

See also Polygonal Marquee tool.

If the subject you want to select has clearly defined edges but loads of curves you might be better off using the magnetic marquee tool to select it rather than the polygonal marquee tool as previously discussed. this tool requires a little patience and a bit of practice as it can be like pushing a shopping trolley with a wonky wheel. It tends to have a mind of it's own and appear to go off on a tangent at times. No worries, there are ways to control it.

OK so here's my image. As you can see, curvy but reasonably well defined - just like me :-)) and again, just like me it's a English rose! Umm, maybe I should stick to the script before I make you all sick.


Just like before when I talked about the polygonal marquee tool I have prepared by duplicating the layer and creating another layer in-between which I have filled with white. Once again I find it a bit simpler to remove the outside unwanted area roughly first. You could use the free hand marquee tool or polygonal tool to do this quickly. Make sure you have the right layer active which in this image would be the top one called Layer 1.

rose selected

Now select the magnetic marquee tool from the tools pallet. You will notice just below the menu bar there are some options available for this tool that were not available with the other marquee tools.

magnetic tool options

The magnetic marquee tool works by searching for contrasting edges and snapping to the nearest one. The width box here ask you to select how far from your cursor you want the program to search for the edge. the higher the number of pixels the wider the snap to area will be and so the more chance there will be for it to go astray. The smaller the number the narrower the search field will be so you will need to be more accurate with guiding your cursor for it to find the relevant edge. The contrast box is asking you to tell the program how much of a contrast is required to define an edge. The higher the value here (a max of 100%) the stronger the contrast supposedly needs to be for the tool to find it but in practice changing the setting here seems to have little effect. As you work around the edge the program will add anchor points. The frequency at which these anchor points are added is set by adding a value in the Frequency box. the higher the value the more frequently the anchor points are added but a high number requires more processing power then a low one. If the edge is very complicated a higher number would be of benefit while a simple edge won't need so many anchor points.

magnetic marquee tool selection

Zoom into your image so it is at 100%. With the magnetic marquee tool selected click on your image close to the edge you want to work along and then follow the edge round. You will see the line with be drawn to the edge of your subject even though your cursor may drift off. If you get to a corner it is often best to force an anchor point by clicking exactly where you want it to go. If an anchor point gets added where you don't want one click on the Delete button on your keyboard and it will be removed. Hitting the Delete button again will remove the next one back and so on. Don't worry if the selection is not exact as we will need to tidy this up at the end. If you are using a graphics tablet and stylus pen you do not need to have the pen actually touching the tablet but can hover it just above. This is useful because then to click an anchor point in place all you need to do is to touch the pen on the tablet in the right place. Double clicking automatically joins up the ends of your selection and makes it active.

Note: If your image is larger than your screen you can scroll across it in two ways. Moving your cursor off the edge of the image will automatically scroll it in that direction although this can be a little hap hazard. The best way is the hold the space bar down on your keyboard and you will see your cursor will change to a hand symbol. all the time you are holding the space bar down you can now use your mouse or stylus pen to move the image on your screen. As soon as you release the space bar your cursor will switch back into the magnetic magnetic marquee tool selectionmarquee tool.

If your subject is reasonably simple you may want to try and do it all in one hit as I will here. If it is a complicated outline it would be better to do it in sections as I did with the polygonal marquee tool.

Once you have finished the selection you will probably need to refine it especially if your subject contains areas where the focus is softer in places. You could simply feather the whole selection but that would then create soft edges all the way around. Better is to use the quick mask tool and then modify the edge of the selection by alternating between the eraser and the pen tool both set to a soft enough edge so as to match the softness of the edge of your subject.

magnetic marquee tool selectionWork all the way around your selection varying the hardness of your brush or eraser to match the edge your are working on. Once done, exit the quick mask. In order to erase the background you will now need to inverse your selection because currently it is the rose that is selected here and I don't want to delete that. Go to Select on the top Menu bar and click on Inverse. You will know if you now have the background selected because instead of there just being a dotted line around your subject you will also have one around the whole image. Now hit delete and you are done.

magnetic marquee tool selection

If you need to make any further adjustment you can do so by using the eraser or the history brush just like I explained before with the polygonal marquee tool.

Note: When working with the Magnetic Marquee tool you can change to the other marquee tools on the fly by holding down the Alt key which will change it to the Free Hand Marquee tool or holding down the Alt key and clicking which will change it to the Polygonal Marquee tool. Releasing the Alt key returns it to the Magnetic Marquee tool. There is similar functionality with the other marquee tools too.


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