28. Efficient Rendering of Order Independent Transparency on the GPUs

Order independent transparency refers to the problem of rendering scenes using alpha blending equations, which requires the primitives in the scenes to be rendered according to their distances to the viewer. It is one of the key rendering effects in many graphics applications, thus has been extensively studied. Various techniques and systems have been proposed to render order independent transparency. These techniques can be classified into three categories based on their underlying methodologies: the primitive level methods, the fragment level methods, and the screen-door methods.This article provides a comprehensive review of these existing methods, with an emphasis on the advanced techniques that have been recently developed. The background of order independent transparency is introduced at the beginning of this review. Key contributions, advantages as well as limitations of each method are summarized in three following parts, respectively. The first part focuses on the primitive level methods, which tries to solve the problem by pre-sorting primitives, then rendering them from back to front using alpha blending equations. The second part reviews the fragment level methods, which performs fragment sorting and blending on the fly, or captures all the fragments per pixel and sort fragments in post-processing before blending. The performance and memory consumption analysis is presented as a comparison between these methods. The third part introduces another catalog of methods which approximates the rendering results using screen-door techniques, which is quite practical while rendering scenes with high depth complexities, such as grass and hair. Finally, a simple conclusion is given at the end of the review, indicating the direction of the future development of order independent transparency.