1. A method of adaptive maximum intensity projection ray casting, comprising: fragmenting a sampled 3-D dataset of a scalar field into a plurality of sub-volumes of different sizes, each sub-volume associated with a set of data value parameters characterizing the data value distribution of the scalar field within the sub-volume; defining a screen transfer function that is dependent upon data values of the scalar field; selectively casting a plurality of rays towards the sampled dataset, each ray having an initial data value record and a cross-section, wherein the step of selectively casting is performed on a computer processor and comprises, for each ray, selecting a subset of the plurality of sub-volumes that are located along a path of the ray; identifying the maximum data value on the ray path that is within the selected subset; and converting the maximum data value into a pixel value in a 2-D image plane according to the screen transfer function; and using the pixel values determined for the cast rays to estimate other pixel values at other locations in the 2-D image plane wherein the step of selectively casting a plurality of rays towards the sampled dataset comprises: subdividing the 2-D image plane into a plurality of mini-planes; and for each of the plurality of mini-planes, estimating four pixel values at the four corners of the mini-plane; and recursively subdividing the mini-plane into multiple sub-planes, and estimating pixel values at centers of the sub-planes by casting rays towards the sampled dataset until the maximum pixel value differential of each sub-plane is below a predefined imaging error threshold; and wherein the predefined threshold is modulated by an image rendering speed specified by a user and a zoom factor in the case of parallel projection or a perspective angle and a perspective distance between the image plane and the sampled dataset in the case of perspective projection.