CARTOON RENDERING IN POSTPRODUCTION USING GIMP Cel shading tutorial by Mauro Bisiani. NOTICE: The following instructions are meant for Gnu Image Manipulation Program (GIMP), but they can be easily adapted to other software as well (ex. Photoshop, Paint Shop Pro) This tutorial is meant to help you create images like the one following:
tweaking default values in the DAZ Studio [Render Settings] pane, or by mean of specific plugins or shaders (ex. Visual Style Shaders from DAZ3D store, by DraagonStorm & TheNathanParable). Using “toon” figures, like the ones that you can buy from DAZ3D, is useful but not a necessary requisite, as rendering engines (3Delight, Iray, Octane, etc.) look at figures as just a collection of geometric primitives. While perfectly good NPR renderings are possible this way, it usually involves a lot of tedious, time-consuming tweaks and adjustments, and making mistakes in the process is easy and frustrating. A different method relies on post-production methods, namely 2D image filtering techniques. This is often a more viable (and cheap) option. The basic principle is that of producing a “standard” render, and then applying one or more filters to the final image from within a 2D photo editing software, like the Gimp. Filtering techniques When it comes to digital image processing, there are typically a bunch of different ways through which a desired effect can be achieved. In short, as image filtering is math operations at the core, different (chain of) filters produce the same (or very similar) effect. The following pictures illustrates that
A bit of history This graphic style (cel shading) is typical of animated movies and comics, both western and Japanese one. The term “cel shading” comes from the early days of animation, when every frame was hand-drawn onto cels that are celluloid sheets. The most distinctive feature of cel shading is that of color simplification. In 3D context, cel shading is also termed “NPR”, Non-photorealistic Rendering. The following picture set clearly shows the difference between these two rendering styles:
While in photorealistic renderings colors are smoothly shaded over the 3D surfaces, in NPR renderings color transitions are greatly reduced. A typical NPR style employs just two shades of every different color in the image. That is the style we are focusing on in this tutorial. Rendering VS Filtering You can achieve all sorts of graphic style at render time, 52 • March 2016 • DS CREATIVE
The picture on the left is obtained through the use of “Threshold” filtering. The picture on the right is obtained through the use of “Contrast” filtering. As you can see, the results have a similar graphic appeal. Threshold Filtering As cel shading style is based on a reduced number of color shades, threshold filtering is a fast (albeit not so accurate) way of obtaining it in post-production. Since colors in digital medium are represented by integer numbers (typically in the 0-255 range for Red, Green and Blue components), it’s easy to reduce that number using a threshold. That means that every color (color number) in the image that is above or below a specific value will be rendered to white or black, or a specific color (Posterization). Starting with the following image