## Classification

In this volume renderer classification is done via two transfer functions. The first one is applied on the density values and the second one is applied on the gradient magnitudes of the voxels.

### Histogram

The window for defining the transfer function shows the histogram of the currently loaded volume. For transfer functions used for the gradient magnitudes, the histogram of gradient magnitudes is shown. The histogram can be scaled with the slider left to the transfer function.

### Editing the transfer function

A transfer function defines the classification between a scalar value and a color value as well as an alpha value. The lowest density value (normally 0) is defined at the left edge of the transfer function and the highest value (normally 4096) is defined at the right edge. The same is true for the gradient magnitudes, the value for the lowest gradient magnitude is defined at the left edge.

The transfer function is interpolated between the nodes which are defined by the user. The y-position of a node defines the alpha value (completely opaque = top of the window). A new node can be created with a left mouse click on an empty space. A dialog for selecting the color will be opened. To move a node click with the left mouse button on the node and move the mouse while the mouse button is pressed. To erase a node, click with the right mouse button on the node. To change the color of a node, double click the node with the left mouse button. The rendered view of the volume is immediately updated when the transfer function is edited.

LMB on empty space | Creates a new node
RMB on existing node | Deletes the node
LMB on existing node | Move the node by drag and drop
Double click with LMB | Opens the dialog for selecting a color