STF readouts work by clicking on any view (main view or preview) of an image window in any of the readout modes (black, midtones, white point) to set the STF parameters accordingly. In this mode, while the mouse button is held down on an image, readout values are calculated for the cursor coordinates according to global readout options, and sent to the STF interface window. In other words, after clicking on this icon, hover over an image and click on it — the black point of all channels will be set to the readout values computed from the pixels where you just clicked.

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