A medical processing apparatus comprises processing circuitry configured to: receive an image data set for rendering; for each of a plurality of a pixels or voxels in the image data set: set a region of interest around the pixel or voxel; determine a maximum data value and a minimum data value for pixels or voxels in the region of interest; and designate the pixel or voxel as visible or as non-visible based on the maximum data value and the minimum data value for the region of interest; and perform a rendering process using the pixels or voxels of the image data set that designated as visible.