Transfer function is very important for volume rendering. One common approach is to map the gradient magnitude to opacity transfer functions. However, it catches too many small details. Gradient vector flow (GVF) vectors have large magnitudes in the immediate vicinity of the edges, where the GVF vectors keep coordinate with the vectors of the gradient of the edge map. While in homogeneous regions where the intensity is nearly constant, the magnitudes of gradient vectors are nearly zero and GVF diffuses the edge gradient. Because of these aspects, we extend GVF to color space and apply it for opacity transfer functions. Experiments show that our method enhances edge features and makes a visual effect of diffusing along the edges.
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