Interactive applications, whether AR on a smartphone or VR on a powerful computer, enable interaction and an emotional connection that cannot be achieved by simply watching movies or graphics. The user does not stand outside, but becomes a part of the experience by actively shaping it. They move and rotate products, change the paint job on their dream car, or unscrew virtual devices.


Polygon Models

However, while offline renderings, such as those seen in our BLOG "What is CGI", have to be calculated for hours, days or weeks in advance, an interactive application often generates 60 or more images per second without a noticeable delay. This allows it to respond at any time to changes such as a user's input. This level of performance can only be achieved through countless optimizations. Fortunately, the times when we moved an edgy Lara Croft through catacombs on the first PlayStation are long gone. But that doesn't mean that the models we use in interactive applications can be arbitrarily inefficient.


As we can already guess from models of old games, the 3D models we work with are made up of many small surfaces called polygons. The first Lara Croft counted about 230 of them. A modern graphics card, on the other hand, can process several million under the right circumstances.

Komplex models

Manual retopologization

Exhausting these possibilities seems unrealistic at first. However, when CAD data is converted into detailed polygon models, they actually reach such polygon density. If the complexity of these models becomes too high, it affects the performance of the application - especially if the budget does not include the purchase of correspondingly expensive hardware. We may also want to provide a downloadable WebGL application or app that each user accesses on their personal device. This use case makes it impossible for us to assume powerful hardware in principle. So while, for example, a scene with 20,000,000 polygons would be appropriate for a movie, a reduction is necessary there in the interactive domain. This is done via so-called retopologization (retopo), in which, based on the highly detailed surface, a simplified surface is created.


Depending on the degree of reduction, this process can be automated and only needs to be evaluated and corrected by CG artists. Currently, however, human pattern recognition is more flexible than the computerized alternative. An accomplished CG artist can recognize the relevant aspects of a model at a glance. Performing a manual retopo takes more time, but the automatically generated result is clearly surpassed in terms of aesthetics and performance.

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