Amount Control

Many transformations and tools whose output image is the same size as their input image have an Amount control that can use a mask to apply their effect either uniformly over the entire input image or just in certain parts.

The amount control consists of a readout, a slider and a button. The readout indicates the name of the settings – in this case “Amount” – and its current value (100%). As you adjust the slider, the readout percentage updates and the button color changes to a gray tone matching the setting.

If the amount is set to 0%, the transformation result is just a copy of the input image. If the amount is set to 100%, the transformation is applied full strength to the entire image. Intermediate values blend the input image and the result image accordingly. This can be useful if you decide the effect you are getting is too strong and you want to dial it back.

Mathematically, the output image is computed as a weighted average of the input image and the transformation applied full strength. To simplify the formula, assume that pixel values are scaled from 0.0 (black) to 1.0 (white), and that Amount percentages are also scaled from 0.0 (0%) to 1.0 (100%).

O = I * (1 – A) + R * A

Where O is the output value, I is the input value, A is the Amount setting, and R is the full-strength transformation value. If Amount is 0.0, the output image is a copy of the input image. If Amount is 1.0, the output image is the full-strength transformation. For intermediate Amounts, the output image is a weighted average of the two images.

Amount Menu - No Mask

Clicking the amount button pops up a menu. Initially, the menu includes the items Help and New Mask… followed by a list of any open images that are the right size and type to potentially serve as a mask for the input image – i.e. black and white images with the same pixel dimensions as the input image. Also included in the list are any existing masks that are the right size.

Selecting New Mask… brings up a Mask Dialog to let you create a new mask, starting with an empty mask.

Selecting one of the images from the list also brings up the Mask Dialog, but it is initialized to contain a copy of the selected image.

Double Slider

Once you have selected or created a mask, the amount button changes to display a thumbnail of the mask and the amount slider splits into two parts – a black slider and a white slider.

The white slider sets the amount value associated with regions in the input image where the mask is white. The black slider sets the value where the mask is black. Where the mask is gray, intermediate amount values are applied. In the example above, the transformation is applied full strength to the right half of the input image (where the mask is white) and not at all to the left half (where the mask is black). Reversing the positions of the black and white sliders is like inverting the mask – the transformation is applied full strength where the mask is black and not at all where the mask is white.

Mathematically, the effective Amount value at a given pixel location is determined by the mask pixel value and the values of the black and white sliders:

A = B + (W – B) * M

Where A is the effective Amount value, B is the black slider value, W is the white slider value, and M is the mask image value. Thus, if the mask is black (0.0), the effective amount is equal to the black slider value, and if the mask is white (1.0), the amount is equal to the white slider value. For intermediate mask gray levels, the effective amount is a weighted average of the black and white slider values.

Amount Menu - Mask Selected

Clicking the amount button when a mask is selected pops up a menu with some additional items.

Reset -- resets the mask, closes the mask dialog, if any, and reverts the amount control to a single slider.

Re-open Mask Dialog Box -- brings back the mask dialog box, if it is not already open.

Reverse -- swaps the positions of the black and white sliders.

Zero-Centered Amount Controls

Amount controls for some transformations are zero-centered. This means the slider runs from -100% to +100% with 0% in the middle. Accordingly, gray mask values correspond to 0%, black values to -100% and white values to +100%.

Amount Controls without Sliders

There is also a variant of the amount control without sliders. This version is used to specify just a mask -- it is just like a normal amount control except there is no Reverse menu option and shift-clicking on the mask does not do a temporary reset.

Tips

Shift-clicking the amount button temporarily resets the amount to 0% which effectively disables the transformation. This is a quick way to compare before and after versions of the transformation.

Ctrl-clicking the amount button temporarily resets the amount to 100% which enables the transformation at full strength over the entire input image, even if it has a mask. For zero-centered amount controls, both Shift- and Ctrl-clicking reset the amount to 0% which is the center of the amount scale.

You don't have to drag the sliders, just clicking where the value you want moves the slider there immediately. If you have black and white sliders, click above or below the horizontal line.

Sliders have a resolution of one part in 10,000, even though they are displayed with fewer pixels than this full resolution. You can make fine adjustments to a slider value by holding down the Shift key while you drag the marker.

Many transformation dialog boxes can be resized by dragging their corners or edges. Making the dialog box wider makes sliders and amount controls wider as well which can make it easier to make fine adjustments.