Using Volocity, I have enhanced my images by using the Auto Contrast function but my volume does not look as expected.

Technical Note: 236
Reads: 6228
Creation Date: 20/07/2001
Modification Date: 12/02/2004

If you drag a stack of grayscale images into a Volocity library, it is possible to adopt and then 'Auto Contrast' them all. However, when a volume is made from these images, the resultant XYZ view looks corrupted, with regions in the image that are expected to be dark appearing bright and vice versa. This happens because the Auto Contrast function modifies the LUT (look up table) of an image and maximises the contrast based on the data in that particular image. Therefore, if Auto Contrast is applied to an image stack, each image is likely to have a different LUT applied.

A volume made from images enhanced in this way would be expected to show enhancement in the XYZ or 3D view. However, when a volume is created in Volocity, any LUT applied to the first image in the stack is applied as the LUT for the whole volume. It is likely that the LUT applied to the first layer in the stack will not be appropriate for the other layers in the stack. The volume thus created will not therefore look as you would have expected from viewing the individual enhanced layers. However, you can correct the problem by applying Auto Contrast again, this time to the volume. During Auto Contrast of a volume, the data of the whole volume is taken in to account to generate the appropriate LUT.

Problems can be avioded altogether by only applying Auto Contrast to the volume and not to the individual images which make up the volume.