Darkening the edges with an Adjustment Layer and gradient tool

Following on from my presentation on how I use layers, I have been asked to make a few notes on how I darken the edges of my pictures. I have put the following instructions together as guide for you to try. Please ask/email me if any points are unclear.

Thanks to Glynn for pointing out the colour shift that can occur when modifying with curves (I can see it now I no it is there!). I have added an extra step of setting the layer blending mode to luminosity to overcome this.

Open the picture to be modified.

Shrink the size a little so the picture is a little smaller than the window this makes it easier to use the gradient tool

Add an adjustment layer. Layer->New Adjustment Layer->Curves (or from the icon at the bottom of the Layers window) this brings up a curves window. Add a middle point and drag it down a little. This will darken the whole picture at this stage.

Set the layer blending mode to Luminosity. In this mode only the luminance is changed, overcoming the problem of colour shifts that can occur when Normal blending mode is used with curves. (In Normal blend mode, pixels that are an unequal mix of Red, Green and Blue will have each component modified by a different amount, thus shifting the colour)

The mask associated with the adjustment layer is initially fully on. Press CTRL-I to invert the layer mask from on to off. The mask is now fully off and the curves adjustment is prevented from modifying the picture.

Select the gradient tool

Set foreground colour to white

Set background colour to black

Set blend mode to Lighten

The gradient tool applies the gradient from the foreground colour to the back ground colour. In this case white where the left mouse button is pressed to black where it is released.

Starting at the middle of the left edge, use the gradient tool to add a gradient to the adjustment layer mask. Drag it perpendicularly in from the edge to a point about 20% across the picture.

Repeat for the other edges. Because the blend mode is set to 'lighten' as each gradient is applied they are combined. (In more detail: the 'lighten' blend mode conditionally modifies each pixel of the mask if the colour of the new gradient for that pixel is 'lighter' than the colour already there.)

If you wish the mask corners can be rounded off a bit by adding gradients at 45 degrees. I normally start these slightly in from the corners

You should now have a picture with darker corners. The mask created can be viewed by holding down the ALT key and pressing the left mouse button when the cursor is over the layer mask thumbnail for the adjustment layer in the layers window. Repeating the operation returns the view to normal.

Double clicking the layer thumb nail brings up the curves dialog box again allow the curves adjustment to be changed. This allows fine tweaking of amount of darkening of the corners.

To compare the before and after effect the adjustment layer can be turned on and off by clicking the 'visibility eye' for that layer in the layers window.

Hopefully these instructions have given you the basics to the use of adjustment layers.

-- MarkRussell - 26 Feb 2005

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