The pixel cross-dissolving of 2D morphing suggests a linear . Indeed, it works well for blending the color information of and . However, it fails to interpolate opacities in a manner such that the rendered morph sequence appears smooth. This is due to the exponential dependence of the color of a ray cast through the volume on the opacities of the voxels it encounters. This phenomenon is illustrated in the morph of figure 5. In particular, the morph abruptly snaps into the source and target volumes if a linear is used: figure 5g shows that at time , very early in the morph, the empty space towards the front of the human head has already been filled in by the warped orangutan volume.