Today’s Question: I’ve been exploring the “Blend If” sliders in Photoshop, and so I reviewed the documentation on the Adobe website. Their terminology makes it very confusing such as by using both “blended” and “unblended” to describe pixels that will be visible. Can you explain what the “Blend If” sliders are actually doing?
Tim’s Quick Answer: The “Blend If” sliders found in the Layer Style dialog in Photoshop enable you to combine two image layers to create a composite where you can hide or reveal pixels on each layer based on brightness levels.
More Detail: First off, I should clarify that the “Blend If” sliders do not relate to blend modes, which can be used to combine layers such as for a multiple exposure effect based on specific mathematical formulas that use the pixel values of layers as the input. Rather, the “Blend If” sliders are used to simply determine which pixels will be visible in the composite image based on their tonal (brightness) value.
Let’s assume, for example, that you have combined two images into a layered document. The upper layer consists of a relatively dark tree set against a very bright sky, and the underlying image features scattered cumulus clouds against a deep blue sky. You could then select the upper image layer and click the “Add a layer effect” button (the “fx” logo) at the bottom of the Layers panel and choose “Blending Options” from the popup menu.
Initially, of course, the upper image layer will completely block the underlying layer from view, because the layers are opaque. But you can blend pixels from the upper and lower layers together based on their brightness values using the “Blend If” sliders.
At the bottom of the Blending Options section, you’ll find the “Blend If” sliders. The upper slider control is labeled “Current Layer”, but it would be more clear if it were instead labeled “Hide Current Layer If”. If you drag the white slider toward the left the brightest pixels in the current layer will be hidden from view. If you instead drag the black slider to the right the darkest pixels in the current layer will be hidden from view. You can also hold the Alt/Option key while clicking on one half of the slider handle to split it in two, which provides a degree of feathering for the transition between visible and not visible pixels.
The lower slider control is labeled “Underlying Layer”, but it would be more clear if it were labeled “Reveal Underlying Layer If”. If you drag the white slider to the left, you will reveal the brightest pixels from the underlying image layer. If you drag the black slider to the right, you will reveal the darkest pixels from the underlying layer. And again, you can Alt/Option-click on the slider handle to separate the controls to create a feathered effect.
With the example image layers noted above, you could, for example, drag the white slider to the left for the Current Layer control to hide the bright sky, revealing the scattered clouds in the sky image layer below. This would replace the bright sky with the more interesting cloudy sky. You could then drag the white slider for the Underlying Layer control to the left to reveal the clouds from the underlying image in all areas, making it look like the tree is enveloped in clouds or fog. You can also separate the sliders to provide some blending, such as to make the clouds revealed from the underlying image layer blend into the image more smoothly.
