The most widely used image fusion method is based on the intensity-hue-saturation (IHS) transform, and many derivations have been developed from the basic IHS image fusion method. The recently developed generalized IHS adaptive (GIHSA) approach, which includes a parameter optimization procedure for the IHS transform based on a linear regression between panchromatic (Pan) and multispectral (MS) data, is among the best of the image fusion methods based on the IHS transform. However, GIHSA is only a semi-adaptive approach because only half of the parameters for fusion are adaptively determined in it. Therefore, we propose a fully adaptive image fusion approach under the general scheme, as an improvement for GIHSA. In the proposed approach, all parameters for fusion are adaptively determined following a two-step optimization procedure. The proposed approach is applied to Enhanced Thematic Mapper Plus (ETM+) images, and the improvements are verified by fusion results.