Blend

The Blend transformation combines an overlay image with the current image. It supports a number of blending modes to combine the images in different ways. The overlay image must have the same dimensions in pixels as the input image.

A more powerful but more complicated version of this transformation is available as the Composite transformation.

Amount

The amount control lets you control how much of the blend transformation is applied to the input image. You can apply a percentage of the transformation to the entire image, or you can specify an amount mask to restrict the effects of the transformation to only part of the input image.

Overlay

This control lets you select the overlay image or you can select a solid color.

Operation

Blend -- the overlay image is blended with the input image with a transparency determined by the amount control. The result is similar to a multiple exposure.

Lighten/Darken -- the overlay image is blended with the input image, but only where it lightens or darkens the input image.

Filter -- the overlay image is applied to the input image as a filter (similar to the way the Filter transformation works). The result is like sandwiching two transparencies.

Subtractive Filter -- the overlay image is applied to the input image as a subtractive filter (computed as the negative of filtering the negative of the base image with the negative of the overlay image). This is similar to the normal filtering except that light areas of the filter are opaque and dark areas transparent.

Soft Light -- similar to the soft light blend mode in other image editors.

Reverse Soft Light -- same as Soft Light, but with the input and overlay images swapped.

Hard Light -- similar to the hard light blend mode in other image editors.

Reverse Hard Light -- same as Hard Light, but with the input and overlay images swapped. This mode is also known as Overlay in other image editors.

Negative -- a special effect is created where the overlay and input image are combined so as to reverse each other.

Add -- the overlay image is added to the input image. This can generate out-of-range pixel values which are then clipped.

Subtract -- the overlay image is subtracted from the input image. This can generate out-of-range pixel values which are then clipped

Reverse Subtract -- the input image is subtracted from the overlay image. This can generate out-of-range pixel values which are then clipped

Absolute Difference -- similar to Subtract except that negative differences (i.e. where the overlay image is brighter than the base image) are converted to positive instead of being clipped to black.

Offset Difference - the result is a mid-level gray plus the input image minus the overlay image.

Color Burn -- similar to the color burn blend mode in other image editors.

Color Dodge -- similar to the color dodge blend mode in other image editors.

Linear Burn -- similar to the linear burn blend mode in other image editors.

Exposure Compensation

This slider is only displayed if the Operation is set to Filter or Subtractive Filter. It acts as a multiplier to lighten (for Filter) or darken (for Subtractive Filter) the result.

Strength

This slider is only displayed if the Operation is set to Subtract, Reverse Subtract, Absolute Difference or Offset Difference. It acts as a multiplier to exaggerate or reduce the differences between the two images.

Tips

To place a solid color background behind part of an image, set Operation to Blend and set Overlay to the background color. Then create an Amount mask to identify the areas where you want the background to replace the input image.

To simulate a multiple exposure with more than two images, use the Stack Images transformation instead.

Blending an image with a High Pass version of itself using the Soft Light or Hard Light mode is a way to bring out detail.

Formulas

These formulas are used to compute the various operations. To simplify the formulas, pixels values are assumed to run from 0.0 for black to 1.0 for white and amount values run from 0.0 for 0% to 1.0 for 100%.

A          Pixel value from the Input image

B          Pixel value from the Overlay image

X          Amount value

S          Strength

E          Exposure Compensation

The calculation proceeds in two stages:

First, the selected operation is applied to the entire input image to produce a temporary result image

Next, the temporary result image is combined with the input image according to the Amount setting

Step 1 -- Compute the result image (R) from the input image (A) and the overlay image (B), based on the operation

Blend: R = B

Lighten:  R = max(A, B)

Darken: R = min(A, B)

Filter:  R = A * B * E

Subtractive Filter: R = 1 - ((1 - A) * (1 - B)) * E

Soft Light: R = A * (A  + 2 * B * (1 - A))

Reverse Soft Light: R = B * (B  + 2 * A * (1 - B))

Hard Light: if (B < 0.5) R = 2 * A * B else R = 1 - 2 * (1 - A) * (1 - B)

Reverse Hard Light if (A < 0.5) then R = 2 * A * B else R = 1 - 2 * (1 - A) * (1 - B)

Negative: R = A xor B

Add: R = A + B

Subtract: R = (A - B) * S

Reverse Subtract: R = (B - A) * S

Absolute Difference: R = |A - B| * S

Offset Difference: R =  (A - B) * S + 0.5

Color Burn: if B = 0 then R = 0 else R = 1 - (1 - A) / B

Color Dodge: if A = 1 then R = 1 else R = B / (1 - A) 

Linear Burn: R = A + B - 1

Step 2 -- Combine input image (A) and result image (R) according to the Amount setting (X)

O = A * (1 - X) + R * X