Expert: Scott Valentine - 8/27/2009

Question
QUESTION: Hi Scott,

I've been using PS for about six months. I'm still learning but sometimes, doing certain tasks can be very frustrating, which is why I'm asking for your assistance.

Here's what I'm trying to do: I'm trying to create a rectangular glass pane that can move around over a background image using the Move Tool while blurring any part of the background as the pane is moved.

So far, all I've been able to is a draw a rectangle using the Rectangle Tool on a New Layer and then lowering the Opacity way down. After this, I go into Blending Options and fool around with the Drop Shadow and Inner Shadow layer effects to give the pane some depth. If I try to apply the "Blur" filter, nothing happens.

Honestly, I don't remember all the steps because I've tried a lot of things such as merging layers, converting the background into its own layer but not really understanding what I'm doing, or only partially.

Anyway, can you help me with this?

Thanks.



ANSWER: Interesting question, Ed!

If I understand correctly, you want to use a layer as a kind-of lens, as if you were dragging a real diffuser lens over the image to selectively blur whatever is beneath. Is this correct?

Within Photoshop, you can not do this directly. The reason is that the blur filters do not behave in a 'live' way like the blend modes do. However, I do have a solution for you which you may find useful in other situations, too.

The answer is a Clipping Layer, and here's how you pull off the 'trick'...

Start with your base image and then duplicate it to a new layer. Ignoring any other layers you have, you should have these two layers right on top of each other. Select the top of these two and apply your Blur filter (Gaussian, smart or lens blur, whichever you like).

Above the blurred layer, use the Rectangular Marquee tool to drag out a frame or small box. Fill this with black.

Now, drag the box layer *between* the two image layers, so your layer stack looks like this:

Blur
Box
Original

Holding down the Control or Command key, move your cursor to the line between the Blur and Box layers. When the cursor changes to two overlapping circles, click. The Blur layer is now 'clipped' by the Box layer. You can then use the Move Tool to drag around the box (make sure its layer is selected), and you will get the effect you want.

Note that this will not work outside of Photoshop, so you can't export the image and have it 'live' in the sense of being able to move the box around. If you need to do this for a web page or other application, you'll need to investigate Adobe Flash, or using the Pixel Bender tool kit (also from Adobe)

Pixel Bender is available on labs.adobe.com:

http://labs.adobe.com/technologies/pixelbender/

You should be aware that neither of these tools is trivial to learn and implement. Depending on your needs and level of skill, you may have to solicit some help =)

I hope this answered your question. Please let me know if I can be of further help!

-Scott

---------- FOLLOW-UP ----------

QUESTION: Hey Scott,

I did everything to the letter but wasn't able to turn my cursor into two overlapping circles. It just remains as a hand with no change. The layers are stacked the way you stated.

What could have gone wrong?  

Answer
Well, it appears you didn't do anything wrong; I gave you the wrong key command =)

For the PC, you should hold down the ALT key (Option on the Mac).

I'm very sorry about that! I am using a Microsoft keyboard on a Mac Pro, so tend to forget to 'translate' the keys.

Let me know how it goes!

-Scott