Blend If and Blending Options explained

Masks control where a layer shows. Blend If controls where it shows based on brightness, either its own or that of the layers beneath. It's one of the less known tools in layer editing and one of the most useful, because it lets you hide the dark or light parts of a layer without painting anything. In Paint by Zaweba it lives in Layer > Blending Options.

What the dialog holds

Blending Options brings mode, opacity and fill together in one place, along with the Blend If sliders. There are two slider bars. One is for this layer, the other for the underlying layers. Each has a black handle and a white handle.

Drag the black handle of This Layer to the right, and the darkest parts of the layer vanish. Drag the white handle to the left, and the brightest parts vanish. On the underlying layer bar, the handles hide this layer wherever the picture beneath is dark or bright.

You can set Blend If on grey, which uses overall brightness, or per channel, which uses red, green or blue.

Smooth transitions

Moving a handle on its own gives a hard cut. Pixels on one side of the value show and pixels on the other don't. Alt+drag splits a handle into two halves, and the layer then fades gradually between them. Almost every practical use of Blend If needs this, because hard cuts look rough.

Jobs it does well

Texture on dark areas only: put a grunge or paper texture on top, then on This Layer drag the white handle left and split it, so the texture fades out in its light parts.

Text or a logo that sits behind something bright: with the logo above a sky photo, use the Underlying Layer white handle so the logo disappears where the clouds are bright, and it looks tucked behind them.

Dodging and burning on a grey layer: Blend If can keep the effect out of pure highlights and shadows.

Line art scanned on white paper: drag This Layer's white handle left and the paper drops out, leaving the lines.

Advanced Blending

Layer > Advanced Blending adds settings most people use rarely but are glad to find. Knockout lets a layer punch through others: none, shallow or deep, deciding how far down it cuts. That's handy for text that reveals a photo below several layers of design. Channel restriction makes a layer affect only some colour channels. There are also options to blend the layer's effects as a group and to blend clipped layers as a group, which change how effects and clipped layers combine with the mode.

When Blend If confuses

Forgetting Blend If is set is the most common trap. A layer that seems to have holes in it for no reason, with no mask, may have Blend If sliders moved. Open Blending Options and check.

Using hard handles is the second. Always try Alt+drag to split before deciding the effect looks bad.

And remember the difference between the two bars. This Layer looks at the layer itself; Underlying Layers looks at what's beneath. If moving a handle does nothing, you may be on the wrong bar.

Blend If does with a slider what would take a careful mask, and it adapts when the picture changes. Once you've used it in Paint by Zaweba for a texture or a sky, you'll find more uses for it.