Photoshop Selections (Part 1)

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Understanding Photoshop selection tools and how to the selection works is essential for a powerful Photoshop user.


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Selection Tools Inner Workings

Sure, you may use the software just to color correct your images with the various Adjustment layers, or maybe you just like playing with the Liquify Tool or maybe all you do in Photoshop is paint. This may be fine if you’re using Photoshop for personal use only, after all, your friends and family aren’t the greatest Photoshop experts and you can impress them with what you know already. But if you are working for a client or maybe you want to go to the next level in Photoshop expertise then you really should master (at least the basics) selections. Before you even touch a selection tool you should know the inner workings of the process.

Photoshop doesn’t know a thing about colors. It doesn’t see the orange, blue or red colors. Strange, huh? One of the best raster (and implicit color) image manipulation tools, it’s complete unaware of the colors it works with. But how does it do adjustments, selections and so on if it is color blind? Well, it may be color blind, but it is not Luminance blind. It works only with shades of black and white. Or grayscale. If you don’t know about channels check this tutorial, but as a quick resume know that Photoshop sees each channel as a grayscale image with 255 shades of black and white. So when it makes a selection it does so based on the difference between these 255 shades of black and white.

But wait, this is just one channel, and when I make selections all three channels are involved and that’s more than 255 shades of black and white, right? Wrong. Well, in a way! You see, concerning selections what Photoshop does is that it calculates the overall “grayscale image” by preserving the lightest pixel. Let me put it in another way. Let’ say a pixel is 100 levels red, 20 levels green and 240 levels blue. In the final grayscale composite that Photoshop uses for selections the pixels will be 240 levels luminance. The lightest channel wins. If you want to emulate this behavior simply press Ctrl + Shift + U to desaturate the image and make your selections using the Quick Selection Tool or the Magic Wand tool. The results will be the same as if you were using a colored image.

Ok, so let’s recap. Regarding selections Photoshop only sees 255 shades of black and white. Some selection tools use this model to select different parts of the image based on the difference between adjacent pixels (more on this later). You will also meet this number in many Adjustment tools (Levels, Posterize, Curves) and now you know why. These tools work with 255 shades of black and white as mentioned above.

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Now it’s time for you to meet the selection tools. Below you can see the Marquee selection tools.

To make rectangular selections we use the Rectangular Marquee Tool and to make elliptical selections we use the Elliptical marquee tool. Pretty obvious, huh? The Single Row Marquee Tool and the Single Column Marquee Tool are pretty useless and I seldom use them. They select a single pixel row or column.

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Let’s go back to the Rectangular and Elliptical marquee tools. You can see in the image below some selections I made using these tools. They may not seem of great use, but trust me, with the little tricks I am going to show you they are.

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To use the above mentioned tools simply Left-click and drag. When you are happy with the selection release the mouse button.

First trick it is the use of the Shift and Alt keys. Let’s say you made a square selection but you want to modify this selection by adding another selection to it. If you simply drag again the previous selection disappears. To add to a current selection hold Shift then make your selection. This way the previous selection is still in place and we just added another selection to it. The Alt key works like the Shift key except it allows us to extract from the selection instead of adding to it. So if I have a rectangular selection and I want to subtract an area from this selection I simply hold the Alt key and drag with the selection tool of choice to subtract from the selection. Note that this technique works with all selection tools. Let’s say you have to select a perfect circle, like a planet (below) or a perfect square. Then you would hold the Shift key while dragging with the Rectangular or Elliptical Marquee tool for a perfect circle or square.

Note that in the image below I didn’t center my selection because the selection would’ve been more difficult to see and because I want to introduce you to the second (little known) valuable trick

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Some of you may be familiar with the Quick Mask Mode but nevertheless a quick description is required. Basically when you enter the Quick Mask Mode (Q) your selected areas (or unselected areas) are shown with a red overlay. Like below (Note: If the red overlay in your image looks different than in the image below continue reading, I provide a solution in the following lines).

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Now if you select a brush and paint with white you will erase the red overlay and painting with black adds to the red overlay. Painting with a 50% brush will partially erase or add to the red overlay. When you exit the Quick Mask Mode (press Q again) the area with a previous red overlay will be selected. The partial red overlay (like when you paint with a 50% Opacity brush) will be partially selected. This behavior may be different on your machine so the red overlay may be representing the unselected areas. In order for the following trick to work double click the Quick Mask button and choose Selected Areas.

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Time to show you this little trick I have been talking about. Ok, so you’re in the Quick Mask mode, you have a selection (like the planet image above). How do you center and tweak the circle so it fits the planet? You can’t do that with the Elliptical Marquee tool (at most you can make a flatter ellipse, but that’s all) so what is to be done. Simple. While in Quick Mask Mode with a selection press Ctrl + T to enter Free Transform mode and voila, you can tweak your selection to your heart’s desire. In the case of the planet, I would simply Alt-drag the corners to make it fit perfectly and because I selected previously Selected Areas I transform the selected area and not the unselected area (that will be weird). After you’re done press Enter to apply the transformation and the press Q to exit Quick Mask Mode and voila, you have free transformed the selection.

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These kind of selections would be very hard to make without this technique. Of course when you are in the Quick Mask Mode you can Warp the selections or apply Filters. This is very powerful and it struck me first when I found out this was possible. You can apply a huge variety of filters to your selections for creative effects. In the example below, after I applied my transformation I applied Filter > Distort > Ocean Ripple to my selection.

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Then I exited the Quick Mask Mode (Q) and I made a new layer from selection by pressing Ctrl + J. Then I added a layer below and filled it with a light blue for the purpose of the demonstration. Crazy, right?

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Next we have the Lasso tool family. To make a selection with the Lasso Tool simply draw your selection (or “lasso it”) and release the mouse button. Personally I don’t use this tool very often because it lacks precision. Next we have the Polygonal Lasso Tool. This tool helps us draw polygonal selections and may be useful in certain situations. To use it simply click and move your mouse multiple times. This is easier done than said so go ahead, select it and use it on an image. A quick tip: you can temporarily access the Polygonal Lasso Tool while using the Lasso Tool by holding the Alt key.

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Next we have the Magnetic Lasso Tool. To use this tool simply click once somewhere on the image and then drag across the margins of the object you want to select. You will notice some lines and points that snap to the edges of the object you want to select but not always. It works by analyzing the contrast between the adjacent pixels and draws lines according to the settings shown below. When you’re done simply double click to transform these lines into a selection. I will not be covering this tool mainly because I consider it bulky and imprecise. And I know much better ways to make selections. And mostly it is a noob’s choice (and we want to do it the professional way, right?). Of course, if you feel lazy you can use this tool for quick results but for advanced selections it will simply not be enough.

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  • Yuri P.

    Actually, elliptical selections can be re-positioned without resorting to transformation, but by simply pressing “Space” while dragging the selection (during its original definition).

  • http://www.leog.biz Leo Gozbekian

    Thank you! I have been struggling with this for years.

  • brian mccann

    good so far

  • Alee

    Thank you so much for explaining the Quick Mask Mode selection; I’ve never really understood how to work with it. Applying warp and filter while in Quick Mask Mode was something new for me. Good article!