Understanding the Sliders
When you drag the white slider to the left on the This Layer bar, you lose the highlights or whites in the current layer, so the corresponding area in the layer below shows through. You saw this technique at work in the previous example where we replaced the sky. On the other hand, if you drag the white slider to the left on the Underlying Layer bar, the lightest pixels in the layer underneath show through over the current image. In the sky example, this would lift white clouds from the sky layer above the building, as shown in Figure 9.
Figure 9 Using the white slider on the Underlying Layer bar brings in the lightest parts of the underlying layer; here, the clouds from the sky layer.
Likewise, dragging the black slider to the right on the This Layer bar makes the darkest pixels disappear so that the image on the layer below shows through. If you drag the black slider on the Underlying Layer bar, the darkest pixels in the underlying image (the blue sky pixels in the sky example) appear through the main image.
Blend If works best where there’s a clear difference in the brightness or color between the object and its background. Given the right image, Blend If can make the process of working with detail (such as the lace in the knock-out example) much easier than it would be if you had to make the selection by using another selection tool.