"Halo Control" is used to avoid halo effects around light objects when sharpening too aggressively. For low ISO (low noise) images use 1000 or less, for high ISO images use 2500-3000 or even more. It is very similar to the USM Threshold parameter and has a high impact on the visual quality. Edge Tolerance, determines how much a pixel has to differ from its neighbor to be considered as an edge and not as noise.A higher radius slows down the image processing. If the noise is low, a lower radius can be used, and vice-versa. This is useful when sharpening noisy photos. If you activate " Sharpen only edges" then uniform areas will not be sharpened. The Amount parameter controls the strength of the sharpening. In the example screenshot, the blackest tones have no USM applied, then USM is applied to a broad range of tones from dark to light, and the strength of USM gradually drops off from maximal at the mid-tones to none at the whitest tones, so as to prevent noise amplification and haloing. The default threshold values will protect from over-sharpening and haloing in most cases and limit the sharpening effect to mid-tones. When moving the left pair of sliders to the right side, sharpening is reduced in the shadows and minimizes amplification of dark noise. When moving the right pair of sliders to the left side, sharpening is reduced in the highlights. Holding the Ctrl key while moving a point with the mouse allows for very fine movements. As mentioned in the tooltip, to move each of the points in the threshold tool individually, hold the Shift key before clicking on a point with your mouse. The horizontal axis corresponds to luminosity: select the tonal range that will get sharpened - the darkest tones are on the left, progressing to white tones on the right. The vertical axis corresponds to opacity: 0% at the bottom (transparent, sharpening not visible), 100% at the top (opaque, sharpening visible). The Threshold tool allows one to create a curve via which the sharpening is applied. The Threshold tool helps to suppress noise amplification and haloing and to confine sharpening to a desired tonal range. For low ISO images that are in focus and without motion blur a value of 0.5-0.7 is satisfactory. In general the quality of sharpening is best if the sharpening radius is smaller. The Radius determines the size of the details being amplified and, consequently, relates to the width of the sharpening halo. Though unsharp masking in other software is easily prone to causing halos, RawTherapee has a unique threshold slider which allows you to achieve a superb sharpening effect with a minimal risk of halos. It makes use of several phenomena of the human visual system in order to accomplish this effect, such as the Cornsweet illusion and Mach bands. Unsharp masking (USM) is a technique used to increase the apparent acutance (edge contrast) of an image, making it appear clearer, even though it technically does not really sharpen the image. Activate the "preview the sharpening contrast mask" button (in the toolbar above the main preview) to see this mask. The "contrast threshold" and "blur radius" sliders allow you to control a mask that decides which areas are affected by sharpening and which are left untouched.
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