Photoshop Tip #1: Adding Layer Masks

Photoshop's layer masks allow one to create complex, multilayered images. Using masks, one layer can modify only a part of another, or one layer can blended with another. This is a necessary component of a technique for creating depth blurring, which can give a rendered three-dimensional image more verisimilitude.

Layer masks in Photoshop aren't always intuitive, though. Even though you can use the standard drawing tools to modify one, you can't paste an existing grayscale image directly into a layer mask. This is a big problem if you're working with 3D renders. Many 3D rendering applications can generate a separate channel that contains depth information, which is essential for correct blurring. This depth mask can be made into a layer mask, but it takes a special trick. This tip will show you how.

The first step is to create a 3D image with a grayscale depth mask. These were created in Ray Dream Studio 5; Bryce can also render depth. In the depth mask, the closest points are black, while white indicates the farthest.

Rendered image [original image] [image mask] Depth mask

We'll work with the 3D image. Our goal is a blurred effect, so make a copy of the image layer and then use the Gaussian blur filter on it (Filter/Blur/Gaussian Blur...). We'll rename this layer Blurred copy to distinguish it from the original image layer.

Next, add a layer mask to the blurred layer (Layer/Add Layer Mask), then click on the mask to select it.

[layer mask added to blurred layer]

The intuitive thing but wrong to do is to copy the depth mask to the clipboard, select the layer mask, and then try to paste the depth mask into it. Even though both the source image and the destination are 8-bit grayscale images, Photoshop doesn't allow this. Instead of copying the depth mask into the selected layer mask, it creates a new layer and pastes the depth mask into it. Here's what it looks like if you do it wrong:

[mask pasted into new layer]

What isn't obvious (or, as far as I can tell, documented) is that the layer mask is actually a channel. If we select the layer Blurred copy and switch to the channel palette, we find a channel labeled Blurred copy Mask. This is what we need to paste the depth mask into.

Select the Blurred copy Mask channel, make sure it's visible (), then paste.

[channel palette]

The depth mask is now in the right place. If you look at the layer palette again, you'll see the depth mask in the layer mask.

[depth mask in layer mask]

That's it. The depth mask is now a layer mask, and our jiva now has depth blurring. In this case the effect's somewhat subtle; it's most pronounced in the lower left corner. Compare the original with the finished version.

Original [original image] [masked final image] Processed

Last updated 7 June 2004
All contents ©2000-2002 Mark L. Irons

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