Lesson B: Knockout & Clip Mask Blending

Let’s learn two features that will let us erase some chalk drawing and leave a bit of residue.

Knockout

Use a layer’s content as a mask, a kind of layer drill.

• Open the file “concept- knockout.psd.” Parts should look familiar.
• Double-click near the top layer’s name (“scanner”). This opens the layer’s Blending Options in the Layer Style dialog.
• Lower the Fill Opacity to about 33%. Of course, the green content of the layer diminishes, but as we saw before, it leaves the effects intact. That’s why the stroke around the edges remains. Now here’s the new thing…
• Choose each of the two items in the Knockout menu (a little lower in the dialog than the Fill Opacity). Right now, both Shallow and Deep yield the same result: a view of the Background layer. Shallow requires layer groups to exist in order to do something else. We’re about to make one of those, so leave Knockout set to Shallow, then click OK.

Knockout now uses the scanner layer to drill through all the layers below it, but it’s unable to go through a Background, if one exists. Otherwise, it would go all the way through to transparency. We don’t want to go that far. We want to drill through the Peachy layer only.

• To group layers and limit the Knockout, we need to highlight them. So click near the name of the Peachy layer, then shift-click near the name of the scanner layer. Choose Layer > Group Layers or use the shortcut ⌘-G/Ctrl-G. The two layers are now in a folder-like construction in the Layers panel called “Group 1.” You’ll see the view through the scanner layer is different too.
• Activate the Move tool and, in the Options Bar, set Auto-Select to Layer.
• Use the Move tool to drag the greenish “scanner” around the image. The group will expand and the scanner layer will be highlighted thanks to Auto-Select.

What you see are the layers below the group. That is, the scanner layer is now drilling through only the layer that is in its same group: Peachy. Elsewhere, Peachy continues to obscure those other, disturbing layers.

Soon, we’ll use this to feature to have a chalk erasure layer. Anything we paint on it will use Knockout to hide the fancy chalk drawing revealing the blackboard below. When I shared that with my inquisitive student, he came back wanting some “residue” left from the erasure. Thus, we have a bit more to learn.

An interesting feature of groups is that they have their own opacity, blend modes, etc. They can have effects and be masked like layers. For more, see “Groups” (page 152).

Clip Mask with Blending Options

Clipping or clip masks use the visible extent of a layer, group, or Smart Object to control the visibility of other layers. So, if a photo is “clipped” to a type layer, the photo is visible only where the type is. It feels like the opposite of Knockout. The “base layer” of a clip mask is a support or foundation for other layers, where a layer used for Knockout is a hole punch.

• Open the file “concept- clip mask and blending.psd.”
• With the layer called “clip” active, experiment with different Fill Opacity settings (note that it’s called “Fill” at the top of the Layers panel). As it’s lowered, the hot pink disappears, the bevel effect remains, and the cracks in the ground show through.
• Restore the Fill Opacity to 100% again and make the top layer (“tortoiseshell”) visible. Since it uses the Multiply blend mode, we can see through it like we can tortoiseshell.

To make the word “clip” appear to be made of tortoiseshell, we will clip the top layer to the one below it. They’ll mostly remain independent, but not entirely, as you’ll see.

• Hold down the option/Alt key as you hover your cursor above the line between those two layers. You should see the icon illustrated below. When you do, click.

The word clip retains its effects and has adopted the tortoiseshell as its new content. We know that tortoiseshell is translucent and that we’d see the cracks in the ground through it. Now what feature did we use a little while ago to reveal those cracks? Fill Opacity, wasn’t it?

• Lower the clip layer’s Fill Opacity all the way to 0. Bummer, isn’t it?

Recall two things: Fill Opacity lowers the opacity of a layer’s content (but not the effects), and clipping renders the clipped layer as the content of the other. That’s why the tortoiseshell disappears. But this is Photoshop, the program with a setting for everything.

• Double-click near the clip layer’s name to open its Blending Options in the Layer Style dialog box.

Yes, the same place we found Knockout, Advanced Blending, is where we’ll find the solution to this dilemma. Its phrasing is a bit opaque, but we’ll work it out.

Locate the Blend Clipped Layers as Group checkbox. Hovering over it reveals an “explanatory” tooltip. Maybe it helps. It includes the word “before.” What they are trying to say is that the clipped layer is added to (blended with) the content and then the Fill Opacity is lowered, affecting everything. Unchecking that little box reduces the Fill Opacity first, hiding the hot pink, and then the tortoiseshell is added with its Multiply blend mode, allowing us to see all the way through to the cracks in the ground.

Combining the Two

I hope you’re feeling more informed than abused by all this! If the latter, please blame me. If the former, thank my talented former student Thom Head for the inspiration.

The result is that we now know how to see the content of a layer that’s been clipped to one with 0 Fill Opacity. That layer with 0 Fill Opacity can be used to Knockout one or more layers below it.

Our erasure layer will be the one with 0 Fill Opacity knocking out the chalk drawing while serving as the base layer to the mostly erased version. All we’ll have to do is paint on that layer and it will look and act like actual chalk erasure with some residue left behind.