# 3D Viewer › Volume Rendering

## How to work with volume renderings

It is possible to edit a volume rendering. VoltexGroup provides a method fillRoi(), which takes as input parameters a ROI and a fill value. The ROI (in canvas coordinates) is projected onto the volume, and the covered part of the volume is filled with the specified fill value.

Because fillRoi() is a method which only applies to volume renderings (orthoslices being an exception), this method is not located in Content.java, but in VoltexGroup.java.

VoltexGroup is a subclass of ContentNode, and can be retrieved from a Content (one which is displayed as a volume rendering) with getContent():

    // Add the image as a volume

// Retrieve the VoltexGroup
VoltexGroup voltex = (VoltexGroup)c.getContent();

// Define a ROI
Roi roi = new OvalRoi(240, 220, 70, 50);

// Define a fill color
byte fillValue = (byte)100;

// Fill the part of the volume which results from the
// projection of the polygon onto the volume:
voltex.fillRoi(univ.getCanvas(), roi, fillValue);


One thing worth to keep in mind is that also the original image is changed, and can in this form be saved, if desired.