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Photoshop Selection by Brightness by Big Al
Submitted By: karenc Date: March 7, 2010, 5:18 PM Views: 644

Photoshop Selection by Brightness

by Big Al

There is a function under the Select menu (not in Elements) that allows you to select the brightest pixels, the darkest pixels or the mid-range ones. But it is not very flexible because it has no slider controls and the selection is 'feathered' according how bright the pixels are.

What I am going to tell you is a method of getting a hard-edged selection based on a brightness level of your choice. It will only be useful on certain photographs but, when it does become useful, it will save you a huge amount of work. It can be used on all versions of Photoshop although some Elements users will need to do a bit more work to benefit from the results it gives.

The best use for this technique is selecting the sky from a complex pattern of branches and leaves.

Selecting by Brightness

Before you start, create a copy of your Background layer (create two copies if you want to refer back to the original image later on.)

At the bottom of the Layers palette is a small (adjustment layer) icon represented by a circle divided diagonally into black and white. Click the icon to reveal the drop-down menu and select "Threshold..." A dialog box will appear (the three dots after the menu option always indicates a dialog control will result.)

With the "Preview" box ticked, move the slider control left or right until the object you want to select becomes a sharp silhouette. Don't worry if you cannot get all the object perfectly silhouetted because you can repeat the process using different levels and merge parts of all of them together. OK the dialog when you are satisfied.

From the Layers palette fly-out menu, select Merge Down (standard PS users can use Ctrl+E). This will turn the layer into a black and white image with absolutely no greys.

Pick the Magic Wand tool (W) and make sure that the "Contiguous" box is unchecked. It does not matter whether or not the "Anti-aliased" or "Use All Layers" boxes are checked because they will not affect this process.

Click in the middle of either the black or the white area (depending on whether you wish to select the darker or brighter pixels). All the black (or white) pixels are then selected.

Hide or delete your 'threshold' layer and pick your original layer from the Layers palette. You now have all the pixels above or below a particular brightness threshold selected.

Multiple Threshold Selections

If the selection you wish to make does not fully appear from a single brightness threshold you can repeat the process on multiple (layer) copies of the original image. Each black and white layer will have to be created from different Threshold settings.

Put neighbouring silhouettes on layers one above the other then erase the unwanted parts of the upper layer so that the two layers reveal the best of each.

When done, use Merge Down to combine a pair of silhouettes. Keep combining silhouettes until they become one full selection.

Minor amendments can be made by painting a silhouette with a hard edged black or white brush (or pencil.)

Now you can turn the silhouette into a selection using the Magic Wand as described before.

Softening Up the Selection

The current selection will be far too harsh a transition for most edits. You can soften it up by feathering the edges of the selection.

Having used the Magic Wand, chose the menu option Select > Feather... and choose a Feather Radius that is appropriate to the smoothness of the transition you want. The radius will depend on the overall size of the image and the severity of the changes that you will make to the selected area. It will be impossible to give any value to cover all situations but, in general, keep the radius small. Try different values until you get experience of guessing what will look right.

The 'feathering' works along the edge of the selection and in both directions. Thus, if you use a radius of 3, the 'strength' of the selection will fade to zero over 3 pixels outside the selection and into full strength over 3 pixels inside the selection. The 'feather' effect will be 6 pixels wide.

You may wish to change the selection so that the feather effect only works outside (or inside) the original selection border. To do this, choose Select > Modify > Expand... (or Contract...) and enter a value the same as the "Feather Radius" value you will be using. Do this before doing the feathering.

Using the Selection

If you are using Elements then I suggest that you duplicate your layer before you apply any effects using the selection and deselect after applying the change. That way you can selectively erase the edges of modified layer to reveal the original details underneath.

Other (non-Elements) users can immediately use the selection to create a layer mask - Layer > Add Layer Mask > Hide Selection (or Reveal Selection.) That way you can paint black, white or even grey to the mask to make minor changes to the 'selection' area if your image edits don't look quite perfect.

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