Intro:

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I felt like covering a topic that is often looked over when it comes to the world of VFX. Sometimes, there are those clients who are after a very stylized effect for their production. Which may be challenging for the production company under them trying to create their style. This often happens in animated films, or films attempting to create a mix between live action and animation. And sometimes in science fiction shows as well as video games. So here are my tips for 3D to 2D effects, as well as tips for stylized effects.

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General Setups

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So....Where do you start when it comes to building a 3D effect that you plan on adding toon shaders to? Well, you start with a regular setup of any effect in Houdini. Whether that is FLIP, Pyro, Particles, etc. You'll want to start building the effects the way you would normally do in Houdini. The largest amount of time you'll spend developing your setup, will be making sure the output is right for your toon shader. 

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- Try and make sure your effects have a very defined shape. The more round, square, sharp, etc. they are, the better they will work with a toon shader. A lot of 2D effects need to appear as toon, or comic-like as possible. 

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- If you are working with pyro, the smoke object has a neat little option called Two Dimensional. Turning on this parameter will automatically turn your smoke/fire sim into a 2D plane. This is incredibly useful when it comes to creating 2D fire.

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- After making sure your pyro solver is outputting 2D plane, you'll want to work with the heat attribute from your pyro network. By lining up a grid with your 2D plane, coloring it black, you can then attribute transfer over the heat attribute onto the plane. Just make sure your grid has a lot of subdivisions. Ta Da! Your fire is now on flat geometry. Then you can make sure the heat values on the grid are linked to your CD attributes, and delete all the remaining polygons on the grid that are black. 

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- If you are creating a smoke simulation, I would suggest considering particles over pyro. Simply because, you can have quick controlled results with particles faster, and create easier abstract movement with them. Then you can copy a bunch of spheres to the points, and then rasterize them into VDBs, and then to polygons. Then you have a moving smoke mesh.

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- For magical effects, any mesh is a good mesh. Use your POP networks, Carve SOPs, Polywires, etc. You'll need an effect that has a great network of moving geometry to render well with a toon shader. Carve SOPs are a great way to create quick and easy animated lines and curves. Point VOPs are also a great idea when it comes to creating jagged lines for lightening effects or magic bolts.

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- If you are working with animated lines and curves in your setup, you'll really want to exaggerate their movement. Whether that means making their movement faster or slower, you'll need to consider adding retimes and time-shifts to get the job done. 

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- When in doubt, convert your simulation to geometry.

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Stylizing Effects 

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In order to create an effect that has its own sense of style, you'll need to make sure it carries its own energy. As well as making sure it is both impactful and exciting at the same time. A good effect has a build up, a main movement/climax, and end. These three steps need to be conveyed through the color, action, and surrounding motion. 

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- Tip: If you are having trouble making an effect vanish or disappear, you can always refine its fade away in Nuke with a merge node or Blur node.

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Make sure the animation of your effect is timed correctly. A simulation that has a good setup to its movement will not only behave correctly, but also look alive. You can make any keyframe changes you'd like in Houdini's animation editor. Include your key principles like squash and stretch, slow in, slow out, and secondary actions. 

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If you would like to create more depth in your effects, think about the color palette of your simulation. It can help the viewer note the primary and secondary effects in your shot. So feel free to experiment with complementary colors, or exaggerated color variations. Color is also important because it helps tell the viewer what exactly your effect is. For example, apples are not bright blue. They are red.

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Now...let's talk about motion blur. I've always had a love/hate relationship with motion blur. But playing with how long your velocity trails are, or removing them entirely can give your effects a very cartoony movement. The longer the trail, the more "road runner"your effect will look. But a lack of motion blur will make your simulation appear to float, drift through the air, or retain it's detail. it's really up to you what you prefer. 

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Now if you are really struggling with stylizing your simulations, there are few awesome courses you can take to improve your workflow. As well as a few tutorials.

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CMIVFX has you covered. They have a course that will teach you how to create Cartoon Effects In Houdini. (Check it out HERE) Hirai Toyokazu will walk you through creating a stylized explosion in Houdini. He talks about using The OBAKE Effect, which is a special type of motion blur effect that Japanese artists use for cartoons. As well as how to create a variety of outlines for unique cartoon effects. Plus explosions, smoke, and anything else you could think about under the 3D to 2D umbrella. It's well worth the watch.

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Toon Shaders

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These can be the bane of your existence. But they are the heart and soul to getting your 2D/3D effect looking right.

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Houdini has something called a Toon Color Shader, which you can access in your MAT networks or shader menus.

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This shader works by dividing the model into three regions. Highlights, lit areas, and unlit areas. Then it shades the model with a flat color. But you can adjust the colors for the model on the shader node itself. This shader works with all light types, but it works best with point and distant lights. There is one downside to this shader and that is that it doesn't work in PBR in Mantra.

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If you would like to achieve certain cartoon looks, you can turn off shadows on the lights in your scene. As you may have guessed this shader is VERY much based around how light affects your objects, so you might spend more time adjusting your lights than the shaders themselves.

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One big tip for maximizing the use and customization of this shader is by setting color attributes on the primitive groups of your geometry. You can use the Toon Shader Attributes Geometry Node to do this, or just do it manually with Group and Attribute Create SOPs. If you use the Toon Shader Attributes Geometry Node, you will need to set the color high, colormid, and colorlow attributes up where you would like them to be.

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Cell Shading

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Now that we have talked about the toon shaders in Houdini, let's mention the art form that it is based around. This cell shading technique is still used in the film industry to this day, but it has had many evolutions over time. It is also used in video games.

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Cell shading is a type of rendering that is designed to make 3D computer graphics appear flat. This type of shading is also used to mimic the style of comic books or cartoons. However, it can also make less traditional 2D shading effects. Such as making your effects appear as part of an oil painting or ink sketches. The name for this texturing technique comes from the celluloids(cells) that were painted on in classical 2D animation films. Cell shading in the film industry can be dated back into the early 21st century. 

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Cell shading is pretty much the only way to produce a traditional hand-drawn animation look from computer polygons. Sometimes a scene is too difficult to replicate by hand, and this process of cell shading is both a less expensive route, and a less challenging one. 

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Basically, the appearance and textures from cell shading can be described into a few elements. 

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The shading of the object/character/asset will have:

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- Less gradient than a regular texture.

- Less shading color. (less tonal range)

- Fewer tints.

- As well as a rough outline.

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Comp and Render Tricks

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So there are a lot of cool things you can do to improve or "fake" your 2D effects. Here are some general tips for doing so. Keep in mind, I'm mainly a Nuke user, so these tips are based around that software.

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• Play with your alpha channels. You can shuffle copy edits onto your image's alpha channel layer, as well as override other RGB channels with your alpha. Why is this useful? Well I find alpha channels best represent 2D outlines. Because they operate with black and white in their opacity ranges, you can have more freedom grading, color correcting, and keying this channel than any other. You can also add a luminance key down to clamp white and black ranges on your render. They also work with cell shading effects very well.

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• If you have rendered something with a toon shader, try and focus on where the light should be centered or reflecting off the shader. Then grade it as such. As we have talked about above, stylized toons are designed to be highlighted differently, and appear almost comic book like. Try to balance the colors to seem more comical, and play with which part of the character the viewer should be watching. You want the viewer staring at the character's face, not the lines that hold him together.

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• If you have additional channels (normal, opacity, lighting, etc.) from your Houdini renders, play around with them. Some might be interesting to multiply or merge over renders. If your render doesn't include motion blur, you can add motion blur in Nuke and make it more toony.

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•  If you are creating a magical 2D effect, try adding more glow! Add more glow!

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Exploring Houdini's COP network with your renders is also a great idea. You can also experiment with COP nodes to create Toon Filters for your effects. However, it is a challenge to create clean lines with the system. But I'll leave exploring that up to you.

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References:

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2D Still to 3D animation. Tips?: https://www.reddit.com/r/vfx/comments/v9wbv/2d_still_to_3d_animation_tips/

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Why 2D art skills help you create better 3D VFX: http://www.cgchannel.com/2015/10/why-2d-art-skills-help-you-create-better-3d-vfx/

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2D Art in 3D: Tips for Sketch & Watercolor Effects: https://80.lv/articles/2d-art-in-3d-tips-for-sketch-watercolor-effects/

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10 tips for moving from 2D to 3D animation: https://www.creativebloq.com/audiovisual/2d-3d-61411925

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2D VS 3D VFX SOLUTIONS: https://www.realtimeuk.com/blog/2d-vs-3d-vfx-solutions-for-your-tv-production/

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10 2D/3D Animation Tips & Tricks to Bring New Ideas to You: https://filmora.wondershare.com/animated-video/animation-tips-to-bring-new-ideas-to-your-video.html

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Visual Effects Today: Basics, Trends, and Tips: https://3d-ace.com/press-room/articles/visual-effects-basics-trends-tips

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HOW ‘THE WILLOUGHBYS’ USED 2D EFFECTS IN A 3D WORLD: https://beforesandafters.com/2020/04/22/how-the-willoughbys-used-2d-effects-in-a-3d-world/

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Toon Color Shader shader node: https://www.sidefx.com/docs/houdini/nodes/shop/tooncolorshader.html

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Toon / Stylized Animation using Houdini: https://bubblepins.com/blog/toon-stylized-animation-using-houdini

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Houdini Cartoon Effect: https://cmivfx.com/houdini-cartoon-effect

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Cel shading: https://en.wikipedia.org/wiki/Cel_shading#:~:text=Cel%20shading%20or%20toon%20shading,gradient%20or%20tints%20and%20shades.

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Cel Shading: https://tvtropes.org/pmwiki/pmwiki.php/Main/CelShading

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WHAT IS CEL SHADING? EVERYTHING YOU NEED TO KNOW: https://okuha.com/what-is-cel-shading/

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From Realism to Stylization: Game VFX Production: https://80.lv/articles/from-realism-to-stylization-game-vfx-production/

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Stark Houdini Animated Pyro: https://www.sidefx.com/tutorials/stark-houdini-animated-pyro/

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Stylized FX in Houdini: https://www.reddit.com/r/Houdini/comments/b18rdg/stylized_fx_in_houdini/