Blending Options - Blend If, Part 1

The Blend If sliders found in the Layer (Style) Blending Options dialog box are a powerful layer compositing tool. The Blend If sliders allows one to blend and mask layer content, based on "Gray" values or the individual colour mode channels of the file. This command is often overlooked in favour of using a layer mask, as it is usually obvious when a layer mask is required. Unfortunately, it may not be apparent to many users of layer masks when Blend If sliders may be used to full effect. It is advised that you take your time to explore the Blend If sliders, as you will find many situations where they provide a quick and elegant solution, in lieu of or in addition to layer masks.

The following article is the first in a three part series, exploring the theory and application of this often misunderstood layer blending feature. Part 1 covers the basic theory of using the Blend If settings in "Gray" mode. Part two will apply this knowledge to common image editing tasks, while Part 3 will introduce individual colour channel blends, instead of or in addition to basic "Gray" blending.

Blend If: Gray

Blend If defaults to blending the current layer [This Layer] with the lower layer/s [Underlying Layer] based on the "Gray" values of the composite colour channel (or to be precise, RGB luminosity values, as found on the RGB master curve). One also has the option of selecting individual colour channels instead of or in addition to "Gray", which is often a distinct advantage in LAB mode where tone and colour are handled very differently than in RGB and CMYK modes.

This Layer: Shadow Slider

To begin, select the blending options for the upper layer by double clicking in the space to the right of the layer's name in the layer palette (or use the Layer palette menu, Blending Options).

Move the upper left This Layer shadow slider inward one level, so that 0 now reads as 1. All pixels with the luminosity value of 0 will be blended as transparent, allowing content in the lower layer/s to be fully visible (screen blend mode is another common method of blending black as transparent). If desired, the shadow slider can be moved further to the right to exclude a greater range of darker luminosity values than pure black. The Blend If option uses a 256 level count for manipulating the "Gray" luminosity levels, even when the document is in Grayscale or CMYK mode where one may prefer to work in percentages rather than levels (unlike the curves interface, there is no option to switch between level and % display in Blend If). 

The animation above shows a ten step grayscale bar in the upper layer, while the lower layer is filled with cyan. Moving the This Layer shadow slider from 0 to 1 simply removes all solid black tones from the upper layer.

Splitting the This Layer Shadow Slider

As the tonal blending offered by the sliders is abrupt, one may wish to have a smoother transition of tone in the final blend. The triangle slider can be split in two by holding down the alt/opt key while dragging the inner half of the left hand shadow slider to the right. The slider will split in two, offering a gradation of tone in the blend between these two split points.

Move the right hand half of the split This Layer slider inward one level, so that 0 now reads as 0/1. The pure black 0 value will be halved, resulting in a value of 50% of the upper layer shadow blending into the lower layer/s. Setting the split shadow slider further to the right toward the highlight point, will gradate the blend further revealing less of the upper layer and more of the lower layer to blend through. The split tonal blending behaves in a non linear fashion, with the slider position at 0/2, there is a blend of 33% black into the underlying solid cyan layer resulting in 67% cyan and 33% black in the extreme shadow in the upper black layer.

This Layer: Highlight Slider

The highlight slider offers the same options and behaviour as the shadow slider. One can drop out all pure white pixels simply by moving the highlight slider 1 level to the left from 255 to 254 (multiply blend mode is another common method of blending white as transparent). One can also split the highlight slider in two by 1 level to 254/255 or move the split slider further left to exclude a greater range of lighter image tones than pure white.

Inverting the This Layer Sliders Default Positions

Swapping the sliders original positions so that the shadows read as 255 and the highlights read as 0, will remove all luminosity values from 1% to 99% found in the upper layer between pure white and black (resulting in 0ck highlight, 100c tones from 1% to 99% and 100k shadow for an upper black layer and lower cyan layer). Splitting the inverted sliders offers a greater range of darker and lighter tones than pure black and white while still restricting the majority of the tonal range from the blend.

Underlying Layer: Shadow Slider

The Underlying Layer sliders offer the same options as the This Layer sliders, with different behaviour. The sliders force ranges of tones from all lower layers to blend into the target layer (This Layer). The layer stacking order has been reversed for the following example. Moving the Underlying Layer shadow slider inward one level, so that 0 now reads as 1, results in the underlying layer's luminosity value of 0 blending into the upper layer. If desired, the shadow slider can be moved further to the right to include a greater range of darker luminosity values than pure black.  

 

Splitting the Underlying Layer Sliders

For example, move the right hand half of the split Underlying Layer shadow slider inward one level, so that 0 now reads as 0/1. The upper layer of 100% cyan will be halved, resulting in a value of 50% of the lower K layer shadow blending into the upper layer.

Inverting the Underlying Layer Sliders Default Positions

Swapping the sliders original positions so that the shadows read as 255 and the highlights read as 0, will include all luminosity values from 1% to 99% found in the lower layer between pure white and black (resulting in 100c highlight, gray K tones from 1% to 99% and 100c shadow for an upper cyan layer and lower black layer). One can also split the sliders to refine the tonal transition between upper and lower layers.

Underlying Layer: Highlight Slider

The Underlying Layer highlight slider offers the same options as the other sliders.

 

To be continued.