The histogram of the image shows the range of voxel intensities in the currently loaded image data. If a label is selected in the Segmentation widget, the upper and lower thresholds of the label are represented in the histogram as red and green vertical lines, respectively. You can left-click and drag these lines to change the respective threshold values. Note: the label thresholds cannot be changed while the contrast function is displayed.

To change the contrast of the images in the 2D and 3D windows, check the show contrast function box to display the piece-wise linear function that controls the contrast. Left-click on a control point and drag it to change the contrast for the voxel intensities in that area. If you left-click away from a control point, a new control point will be added where you clicked. To remove a control point, middle-click on it.