Volume Renderings

In a volume rendering, each image voxel is mapped to an opacity and a colour. You can specify the details of this mapping, using the "transfer function" and other settings (see below).

To add a volume rendering to the 3-D display, select Load Volume Rendering ... from the File menu of the 3-D Viewer: Adding a 3-D volume rendering

A dialog will now appear for selecting the image from which the volume rendering will be created.

A dialog for adding a 3-D volume rendering

First, select the image you want to work with by clicking on the open icon, by typing in the folder (directory) and file name of the image, or by pressing the right mouse button and selecting from the menu of recently-used images.

Selecting an input image

This will automatically generate and display a volume rendering with the current settings. You can now make the following adjustments to change the rendering. Note that after any adjustments, it may take a few seconds for the display to update as the view is re-computed.

  • Rendering quality. Volume rendering uses large amounts of memory, and can be very slow when working with large images. You can speed up the rendering by downsampling the image to reduce the number of voxels to be rendered. This is done by setting the rendering quality. As the rendering quality is reduced, the individual voxels become more prominently visible.

The images below show the same CT scan rendered with the three different rendering qualities.

The three rendering qualities.
Volume rendering of a CT scan with low rendering quality Volume rendering of a CT scan with medium rendering quality Volume rendering of a CT scan with gradient opacity transfer function
Low Medium High

  • Background threshold. Any image voxels with intensity below the threshold will not appear in the rendering.

    3d_threshold_slider

  • Global opacity. The global opacity has a value between 0 (completely transparent) and 1 (completely opaque) and applies uniformly to all voxels. Change the opacity using the global opacity slider.

    Slider to set the global opacity

    Setting a lower value will enable you to see deeper into the volume. However, the global opacity is modified for individual voxels by the opacity transfer function.

  • Opacity transfer function. This can change the opacity of each voxel individually.

    • Uniform - all voxels have the same opacity set from the global opacity value above.
    • Intensity - the voxel opacity is proportional to the brightness (intensity) of that voxel, multiplied by the global opacity.
    • Gradient - the voxel opacity is proportional to the magnitude of the gradient in brightness at that voxel, multiplied by the global opacity.

    The images below show the same CT scan rendered with the three different opacity transfer functions.

    The three opacity transfer functions.
    Volume rendering of a CT scan with uniform opacity transfer function Volume rendering of a CT scan with intensity opacity transfer function Volume rendering of a CT scan with gradient opacity transfer function
    Uniform Intensity Gradient

  • Colour mapping. The colour mapping is set from the View menu and, together with the Contrast sliders, determines the colour of each voxel that is rendered. For the contrast sliders:

    Sliders to set the image contrast

    The min/max slider settings determine how the voxel intensities translate to colours in the colour mapping. Voxels with intensity at or below the min value map to the first colour in the colour mapping; voxels with intensity at or above max value map to the last colour in the colour mapping. The mapping is linear between the min and max intensities.

The effect of selecting the A check-box to cut away part of the volume check-box is dealt with in the section about general controls.

If you click the done_button button, the volume rendering will be removed from the 3-D viewer.