Order Independent Transparency In OpenGL

This article uses OpenGL and its GPU to implement order independent transparency.OpenGL stands for Open Graphics Library, it’s a cross-language, multi-platform API, and dueto these, it’s widely used in high performance graphics develop and interactive visualsimulation, especially in game development area. With the improvement of GPU and itspowerful concurrency feature, it also becomes more and more important in data processingarea.With the development of visualization technology and computer technology, the applicationof visualized technology limits to expand ceaselessly, many areas are increasingly dependenton the visualization technology, such as automobile industry, machinery manufacturing,scientific research field. In recent years, OpenGL improves rapidly, all kinds of technologyusing GPU emerge in an endless stream. As GPU being used in mobile phone, handheld gamedevice and game machine field, GPU professional knowledge in today’s competitiveenvironment appears more and more important. And order independent transparencyrendering (Order Independent Transparency) is one of the hot technologies.Transparency is an important rendering effect, it can significantly improve the reality of sceneand visualization system efficiency. For special translucent processing, in practicalapplications, such as games, are mostly adopted simulation way, which is only simulation,rather than correct rendering with high cost. The main reason is the correct rendering of highcost, low efficiency, and high complexity. These greatly restrict the algorithm’s application.However, as the hardware graphics chip performance upgrades, the domain will expandgradually, in a real-time system using a fully correct transparency rendering would becomepossible.With the improvement of GPU (Graphic Processing Unit) and programmable graphicsprocessor, a variety of rendering translucent object algorithm also appear, such astransparency rendering by CPU based sorting, depth peeling, dual depth peeling, weightedsum, weighted average, and GPU based sorting. Here we will discuss and implement dualdepth peeling algorithm.Before talking about dual depth peeling, first of all we have to explain depth peeling, becausedual depth peeling is based on depth peeling algorithm. Depth peeling is a robust solution tothe fragment sorting problem which renders the transparent objects multiple times, peelingone layer of fragments every time by the distance from the observer (Camera), from nearestto furthest, and then blend all the layers in correct order to get the final result. The major drawback of it is that it requires one geometry pass per peeled layer. Thus, to capture all thefragments of a scene of depth complexity N, depth peeling requires N geometry passes. Dualdepth peeling improves this algorithm, reduces the number of geometry passes from N toN/2+1by applying the depth peeling method from the front and from the back simultaneously.And in the best case, it can be nearly2times faster.