public void setTargetIndex(int index) {
        if (index < 0 || index >= 16) {
            throw new IllegalArgumentException("Target index must be between 0 and 16");
        }

        if (colorBufs.size() < index) {
            throw new IllegalArgumentException("The target at " + index + " is not set!");
        }

        colorBufIndex = index;
        setUpdateNeeded();
    }
If MRT is not enabled ({@link FrameBuffer#setMultiTarget(boolean) } is false)
then this specifies the color target to which the scene should be rendered.
<p>
By default the value is 0.

@param index The color attachment index.
@throws IllegalArgumentException If index is negative or doesn't map
to any attachment on this framebuffer.
jme3-core/src/main/java/com/jme3/texture/FrameBuffer.java