Pixologic :: Interview :: Artist in Action

Technique #5: Easy Multipass Rendering

One of the cardinal rules in creating a demo reel is to only show your best work. That means you need to get your still renders and animations looking their best. ZBrush contains a number of powerful tools that let you fine tune your renders to in order to achieve outstanding results. However most professionals prefer to tinker with their renders in a dedicated post-processing tool like Photoshop or After Effects. They do so using a technique called "multipass rendering."

Multipass rendering simply means to generate multiple renders and then blend them together to get the results you want. It requires more work up front, but allows you to avoid the "tweak-render-tweak" process typically associated with creating a finished image.

Creating multipass renders in ZBrush isn't difficult. A simple type of multipass render can be generated using the Create Maps button in the Render menu. This option will generate Color, Shadow, Ambient Occlusion, Mask and Depth passes. Simply make sure that you have enabled Shadows and Ambient Occlusion in the Render menu. Then press the BPR button to create a Best Preview Render. Once the render is complete, you'll see small icons in the Render menu depicting the various render passes. Clicking on each of these will bring up a file requester, allowing you to specify a name and location to save the image.

The Create Maps option is great when you simply want to tweak with the shadow density or amount of ambient occlusion. However if you want full control, you'll need to set up the multipass render by hand. Here's how to do so on a still render. The video at the bottom of the page will show you how to set up a multipass render for an animated sequence.

Before you begin, make sure you've posed your object and oriented your camera to an angle that best shows off its form.
Note: You may find it helpful to add a keyframe to the Timeline to store the camera's position.

We'll start by setting up our lighting. In this case, it will be a standard 3-point light setup. Go to the Light menu and dock it to the tray. The first light is already selected and by default is set up as a Sun light with the ability to cast shadows. This will be perfect for our key light.

Drag the orange dot on the preview sphere in the Light Placement Window of the Light menu. Position it in the upper left corner of the preview sphere. Set its Intensity slider to 1.0 and then set the Ambient light slider to 0. This allows us to see the effect of this light without any ambient light interfering with it.
Note: Depending on what materials you are using on your object, you may or may not see the effect of the key light on your object. MatCap materials (such as the default Red Wax) already have lighting information embedded in them. For this reason, if you are using MatCap materials, you will not see any changes to the physical lights in your scene (such as position or intensity). Therefore, to make sure that you can see the effect of the lights, we will temporarily disable the material and texture information and use one of the Standard materials.

Open the Tool | SubTool menu and click on the primary SubTool. (For example, if you are working with a character, this would be the body or head as opposed to the clothes or accessories.)

Next, go to the Tool | Polypaint menu and make sure that the Colorize button is turned off. This will ensure that any embedded texture and material information is disabled while we set up our lights.

Now go to the Tool | Texture menu and make sure that the Texture On button is not highlighted. This ensures that any UV texture maps are temporarily disabled.

Click on the Material palette on the left side of the interface, then select the Basic Material 2. This material has basic diffuse shading with a bit of specular highlights. You should see your object filled with a light grey. If you do not, double check steps 5 through 7.

If needed, adjust the key light in the Light menu's Light Placement Window until you are happy with it. When you are finished, click the Key light bulb icon to turn it off. This will allow us to set up our fill light without any interference from our key light.

Now it's time to set up our fill light. Click the light bulb icon to the right of the key light in the Light menu. It should turn orange, indicating that the light is now enabled. (If the light does not turn orange, click it again. The first click will typically select it. The second click will enable or disable it.) Set its position in the Light Placement Window so that it is in the lower right corner. Remember that a fill light is designed to fill in the dark regions caused by the key light. Adjust the fill intensity to 0.3. (Traditionally, the fill light is set to roughly 30% of the key light.) When you are finished, click the light bulb icon for the fill light to turn it off.
Note: Shadows for the second light in ZBrush are off by default. That's okay. Leave the shadows off. We only want shadows coming from our key light.

Lastly, we need to set up our rim light. Click on the third light bulb icon (just below the key light). It should turn orange, indicating that it is on. You should notice that this light has a slightly different effect on your object than the previous two. That's because by default, this light is set up to work as a fill light. If you look at its settings, you'll notice that its intensity is set to 0.18. It's using a Sun light type with Shadows turned off. However it's also using the Radial option. The Radial option causes a light to affect the surfaces facing away from it. To properly use this type of light, you would position it in the same general location as your key light. With the Radial option turned on, it will shine light on the opposite direction of your key light, which is exactly what a fill light is designed to do. The Radial option is great, but we already have a Fill light. We need a rim light. So turn the Radial option off. Then set the light intensity for this light up to 1.5. In the next step, we'll position it.

A rim light is designed to create a "halo" effect around the outer edge of your object. This separates it from its background and can help define its silhouette. In a typical 3D package, the Rim light would be positioned above and behind your object and pointing back towards the camera. Currently, our rim light is facing towards the object in the same direction as our camera. To change this, simply click on the orange dot in the Light Placement Window. Make sure not to move your mouse as you click. You will notice an immediate change in the appearance of the preview sphere. Now drag the orange dot in the Light Placement Window so that it is positioned just below the top of the preview sphere. Adjust the placement as necessary.

Our lights are now set up. Enable all three lights in the Light menu so they are active. Now let's create the first passes. Begin by going to the Tool menu and enabling any Polypaint and/or Textures that were disabled for the primary SubTool in steps 5 and 6. Your object should be in its finished posed and textured state.

Now open the Render menu and click on the Create Maps button. This will allow ZBrush to store the diffuse color, depth, shadow, ambient occlusion and mask passes the next time we do a BPR render.

In the Render menu, click the MGray button. This will allow ZBrush to calculate the shadow a and ambient occlusion passes with a broader dynamic range, giving us more control over the shading in post.

Make sure the Shadows and AOcclusion buttons in the Render menu are highlighted. Then click the BPR button to do a BPR render.

When the render is finished, go to the Document menu and choose the Export button. Save the image to a new folder on your hard drive and call it "BPR – Document Beauty.psd" This is the finished render from ZBrush and we will use it for comparison as we set up our multipass renders in Photoshop later on.

Now we need to save out each render pass. Open the Render menu. You will see small icons for each pass just below the Create Maps button. Click each one, then save the resulting image to the same folder you placed the "BPR – Document Beauty.psd" file into. Note that each pass will automatically be named according to what it is.
Note: After creating the Beauty passes, it's a good idea to save your scene and the light setup you've created. You can save the scene by going to the File menu and choosing Save As (or pressing the (Ctrl)+(S) hotkey combination).

The Create Maps button allowed us to save out the shadow and ambient occlusion for our object. But you might note that the Image pass contains both raw color and diffuse shading. This prohibits us from adjust the lighting of our object in post. Therefore, we must create separate passes for the raw color and the diffuse lighting.

To create a color pass, you must fill all of your SubTools with the Flat Color material. The Flat Color material is specifically designed to show off your object without any diffuse shading. You can quickly and easily fill your object (and all SubTools) using the SubTool Master plugin (available from the Pixologic website). You can also do this manually by selecting each SubTool, choosing the M button at the top of the shelf, then going to the Color menu and choosing the Fill Object button.

Once all the SubTools in your object are using the Flat Color material, we can do a render. Go to the Render menu, turn off Shadows and Ambient Occlusion, and disable the Create Maps button. Then press the BPR button. When the render is finished, go to the Document menu and choose the Export button to save the render as a PSD file. Call it "BPR – Raw Color

Now that the color pass is complete, let's save out our diffuse lighting. Make sure that your key, fill and rim lights are all enabled. Then Fill your object (and all SubTools) with the Basic Material and a white color. Then do a BPR Render and save the results, calling the file "Diffuse.psd.

Now let's create lighting passes. Select the Basic Material 2 from the Material palette, fill your object with this material and adjust its diffuse and specular properties as seen in the image below.

Now go to the Light menu. Turn off the fill and rim lights, leaving only the key light active.

Do a BPR render. When you are finished, go to the Document menu and export the image as a PSD file. Name it "Lighting – Key Light."

Repeat the process for the Fill and Rim lights, disabling all but the current light, and saving the resulting image after each render. When you are finished, you should have the raw color pass, the diffuse pass, three lighting passes, and the various passes created via the Create Maps button.

Now it's time to begin combining each pass in Photoshop (or any other image editing program that supports layers and blending modes).

Begin by loading in each separate pass into your image editor. Then copy each separate image and paste them into one master document. I prefer to copy each pass into the "BPR – Document Beauty.psd" file, however it doesn't really matter so long as all of the images are in the same document.

Now comes the fun part. Experimenting with combinations of blending modes, layer opacity and adjustment layers to get the best looking image possible. There are no absolutes here, however there are a few guidelines. The most important thing is the final image, so use the information below as a guide to help you achieve great results.

As a general rule of thumb, your diffuse render should be at the lowest level of the stack. Its blending mode should be set to Normal. Your Raw Color pass should be at the top of the stack and its blending mode should be set to Overlay.
Note: If you are using the BPR Image pass (which contains both diffuse and color information in one image), place it at the lowest level of the stack and disable both the diffuse and raw color layers.

The lighting passes should have their blending modes set to one of the lighten modes (Lighten, Screen, Dodge, etc.).

The shading passes (Shadow and Ambient Occlusion) should have their blending mode set to one of the darken modes (Darken, Multiply, Color Burn, etc.)

Note:There are two "utility" passes. The first is the Mask. It can be used to separate your object from its background. The second is the Depth pass. It is typically used to create a Depth of Field effect with the Lens Blur tool in Photoshop. However you can also use the Depth pass to add "punch" to your lighting. While not a traditional use for this pass, you may find the results pleasing. Simply place the Depth pass at the top of your stack and set its blending mode to Overlay.

Play with the blending modes, masking, opacity and level adjustments for each layer until you are happy with the results.

Video: Easy Multipass Rendering

(click to view)

Summary

Employers are deluged with demo reels and great artists can often get overlooked by ignoring a few key rules. If you're not already using these techniques, I would encourage you to put them into practice for your next reel. You may find that it gives you the edge over the competition you were looking for.

About the Author

Steve Warner created his first computer animation in 1980 on a Timex Sinclair 1000. He began working with 3D software in the 1980s and became a professional artist in 1991. He has authored three best-selling books on computer graphics and now works as the Executive Director of the Digital Animation and Visual Effects School (DAVE School) in Orlando, Florida.