Comic Arts in Games, Asset Production, and Rendering
Non-photorealistic rendering (NPR) is a topic under computer graphics trying to achieve some level of stylization in 3D with the use of geometry, lighting, shading, and special effects. Authors are specializing in recreating the aesthetics of comic arts in video games via implementing the research already done in the area and also adding new research topics.
Comic art stylization characteristics include toon shading, outlines, line art, hatching, and inner lines. The study of authors started with a literature review and implementing some of the real-time-related works to unreal engine. Unreal engine has been chosen mainly for its rendering quality, and the ease of access to the source code if any change in the renderer is required to create the desired effects.
Hatching Shading Method and Line Art
Comic book art often uses different hatching techniques unique to its artist. Our goal was to minimize the texturing efforts in video game production pipeline. After trying different methods, the tonal art map pattern introduced in real-time hatching (Praun et al. 2001) seemed like the best option for creating seamless hatching patterns. Unfortunately, it didn’t match the desired quality in the case of a repeated use especially on flat surfaces. We decided to limit the use of hatching shading on background organic objects.
Furthermore, the stylization of inner lines on hard surface models is achieved by adding a distortion on the UV outlines. This is a very similar technique to the one used in WYSIWYG NPR: Drawing Strokes Directly on 3D Models (Kalnins et al. 2002). This can be done either real time because the additional cost to GPU is negligible or while baking the texture so it can be modified manually if necessary. As UV outlines can be detected using an edge detection, the distortion process doesn’t require any effort from the artist.
Outlines are an important part of comic art. An edge detection algorithm is used to draw outlines. Edge detection is a post process; therefore, all of the methods create artifacts on their own. It is not possible to perfectly distinguish the object in the front from the background every time unless an inverted hull method introduced by “Guilty Gear X’rd technical presentation” (Motomura 2015) is used. The inverted hull requires the geometry to be drawn a second time; therefore, it is more costly.
Our edge detection draws the line to the pixel outside of the object, thus creating a proper silhouette of the object. The distance of the object from the background and the distance between the object and the camera are reflected in thickness and color of the line drawn.
Also inlines are drawn with a lighter line color similar to the technique used in suggestive contours for conveying shape (DeCarlo et al. 2003).
Comic book artists also consider lighting and geometry of the object while drawing the outline. This is a further research area for NPR.
The outline color can also reflect the color of the light source enlightening the object or it can be used as a second outline layer.
Most art directors use toon shader to create paperlike characteristic. In video games, it is mostly difficult to achieve a good looking toon shader because of polycount limits. Hard transition between toon shading levels often gives away the low-poly geometry of objects. One solution to the problem is editing the normals of the model, but it requires time and effort. Our solution was to use smooth transition between toon shading levels.
Conclusion and Possible Exploration Topics
There are many further research topics including a better hatching pattern and technique, possibly using a deep learning methodology to analyze different comic art styles; an outline method reflecting light intensity and geometry; and a solution for easily editing the normal maps for better looking toon shading.
Comic art is one of the many possible traditional art styles one might try to imitate in 3D. Painting methods such as water painting, oil on canvas and many more, and also all the art movements, these techniques have been used with provide a very rich soil for non-photorealistic rendering research.
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