Compositing and Rendering: Discussion By Colin Litster Discussion Exercise: A Steam Gauge Although other chapters have contained a more academic discussion section, the compositor lends itself to an extended example. Its usefulness comes not from individual settings, but from combining components and practical understanding. If you are interested in a blow-by-blow rundown of Render settings, they can be found in a third section to this chapter: Render Settings: Discussion. This section, however, will show you how to approach the compositing process, as well as several common compositing setups. To do this, you'll become the compositing and rendering department of a virtual movie studio, tasked with finishing a shot from the highly anticipated feature film, "Blender Hothouse." The modeling, texturing and lighting have all been completed by other departments. Your job will be to devise the look and feel of the scene by compositing its various elements into a completed 10 second animation without altering the models, lighting or materials.

A steam gauge from the "movie" Blender Hothouse From the screenplay:

An oily brass steam gauge within a dark steam room. The dial initially increases from zero to a quarter of its travel as though steam pressure is rising. The dial hovers at this position, gradually becoming more agitated, as though the pressure is about to blow. The dial then rapidly turns to maximum and shakes: pressure is at its highest level and explosion is imminent...

The node network for this composite. The Composite Node network for this exercise appears to be staggeringly complex. However, it can be broken down into four distinct portions, each one covered here in depth. There are a number of files that will help with this chapter, all found in the "examples" folder on the included CD. The first exercise will require you to create the node network from scratch, but later ones will use pre-made files for you to examine and play with. Let’s start with the file as delivered to you from the production department. Load “CompositeStage1.blend” and perform a test render (F12). If you find that the render takes longer than a couple of minutes, you may want to consider working with the renderer set to 50% size in the Render buttons.

The file as provided. If you examine the scene, you will see that some of the materials use raytracing. A simple textured plane outside of the camera's view provides something for the dial's face to reflect. Creating the Source Renders, Scenes and Components to Composite Change the wide window at the top of the screen from a 3D view to a Node Editor. Switch to Composite Nodes with the face icon on the header, and make sure the Use Nodes button is enabled. The window on the left side of the screen shows the Render buttons. Both windows at the bottom have been set to UV/Image Editors for showing Preview and Composite result nodes. You'll be focusing on the main gauge for the majority of this discussion, so disable Layers 2 and 11 to hide the background and wall elements.

Shift-LMB click on these two layer buttons.

A good setup to begin compositing. When turning on Use Nodes for the first time, the default is to have a Render Layers node connected directly to a Composite node. However, you are not limited to a single render input. It is possible to set up different Render Layers, each with their own node input, pulling elements from various modeling layers and scenes which can then be dealt with separately in the Compositor. These controls are found in the Render Layers tab of the Render buttons, nested into the same panel as the Output tab.

The Render Layers tab. Render Layers From this tab it is possible to control exactly what will be rendered, as well as what will be passed to the compositor for processing. In short, a Render Layer is a selection of scene layers that will be rendered in a single pass: a set of Layers that will be rendered together. Each Render Layer can have its own Input node, allowing you to perform different composite operations on different sets of objects from your scene, as you'll see later. Let's look at the controls on the Render Layers tab: Scene

Scene layer buttons This refers to the set of layer buttons at the top of the panel. These controls are a duplicate of the layer buttons found on the 3D view headers, and are included here as a convenience. As you will be indicating which layers should be included with which render input in this same tab, it is nice to be able to check the contents of layers without leaving the panel. Below this is the name and selector for the active Render Layer. Like other name popups, new Render Layers may be created by selecting Add New and may be removed by clicking the "X" to the right of the control. Layer Figure RCD.05.2: Layer Layer buttons Further down is the Layer control, which again shows the familiar layer selector. Unlike the one above, which controls what displays in the 3D view, this selector is the one that determines which scene layers will be included in this Render Layer. When a Render Layer is created, it defaults to including all scene layers. Why would you need separate access to so many scenes and layers? You could, for example, divide a scene between background and foreground objects, sending the background objects to the compositor in a different Render Layer for blurring. It’s also possible to have part of your project in a completely different Scene, allowing you to composite objects with completely different render settings.

A render composited from two scenes. The ocean scene used standard render settings, while the mine and buoy scene used the Edge settings. Render Process RCD.05.3: Render process buttons Below the Render Layer selector are toggles for which portions of the renderer to use. Blender treats different types of objects in different ways, and each of these can be enabled or disabled here. For example, if you were to turn off the Solid button, no objects with solid faces would be rendered, leaving only the background. The other buttons can be used to disable rendering of Halos, Edges, Transparent (zTra) objects and the Sky background or BackBuffer image, on a Render Layer by Render Layer basis. Just below the render process buttons are two text fields: Light and Mat. If the name of an object group is entered in the Light field, the Render Layer will use lamps from that group, ignoring any other lamps in the scene. A material name entered into the Mat field will cause all objects in the Render Layer to be rendered as though they were temporarily linked to that material. These fields are useful for doing test renders and special effects. For example, you might need to substitute a simplified lighting rig and material to test object placement without actually replacing lamps and materials throughout your entire scene. Render Passes RCD.05.4: Render passes buttons At the bottom of the Render Layers tab are the controls for render passes. As Blender renders an image, it performs a number of calculations that are combined to deliver the final color of the rendered pixel. Render Passes allow you access to each stage of these calculations individually from within the Compositor. For instance, you could separate the Diffuse, Specular, and shadow calculations, and recombine them in the compositor. By adjusting the way they mix, you could make the shadow darker or blur and lighten the specular highlights. Using a work flow like this gives you the freedom to drastically improve and alter the look of the final output without re-rending, potentially saving enormous amounts of time.

On new Render Layers, only two render passes are enabled: - Combined, which delivers the final RGB and alpha results; and - Z, the depth information of objects from the camera's viewpoint. Each pixel in a render has a Z value, which refers to the distance between the camera and the face that was rendered. If you look in the Node Editor, you will see that the Render Layers node has three outputs: RGB, Alpha and Z. These outputs correspond directly to the Render Pass settings. Enabling any of the other pass buttons adds additional outputs to the associated Render Layer node. The other twelve passes are: Vec: Provides vector motion data for the rendered geometry. Mostly useful for calculating fast, vector-based motion blur. Nor: Provides the Normal information from objects in the render layer. If looking at the output of this pass in a Viewer node, the strange colors are the visual encoding of the Normal. UV: The UV information from objects that have UV mapping. This pass makes it possible to replace the colors on objects that use UV mapped textures, without re-rendering or changing the materials directly. IndexOb: You can assign any object an index value in the Object buttons and use this to create selection masks. Col: Provides an un-shaded color pass, as though everything had been rendered with a Shadeless material. Diff: The diffuse shading of objects, including colors, but without shadows or specular highlighting. Spec: Specular shading. Shad: A pass representing shadowing information. This pass is Multiplied with others to get a final image, so non-shadowed areas appear in white, with shadowed areas being progressively darker. AO: The result of Ambient Occlusion, without any materials applied. Refl: The reflection pass, if Ray is enabled on the Render panel and an object has a reflective material. Refr: Refraction, if Ray is enabled on the Render panel and an object uses ray refraction. Rad: A radiosity pass. Radiosity is another method of lighting that is not covered in this book. If you would like to see the actual outputs from any of these passes, it's as simple as connecting their output sockets to a viewer node and re-rendering. Of course, if you already rendered after the different passes were enabled, no re-render would be needed.

The Col, Diff, Spec and Shad passes. Recombining Passes For the first part of this exercise you will recombine the Diffuse and Specular passes to make the brass of the gauge's body a little brighter and shinier. You can either follow the simple instructions to set this up yourself, or if you prefer, load up the completed stage for examination. The file "CompositeStage2.blend" can be found in the "examples" folder. If you want the practice of building your own, here's what to do: In the Render Layers tab of the Render buttons, enable the Diff and Spec passes.

Enabling the Diff and Spec passes. Add a color Mix node (Add->Color->Mix), connecting the Render Layer node's Diff output socket to Mix's upper image input and the Spec output to the bottom image input. Create a View node with Add->Output->Viewer, and connect the output socket of the Mix node to the Viewer node. If something was already connected to the Viewer node, that connection will automatically be replaced by the new one you make. NOTE: The nodes systems doesn’t like loops or ambiguity, and will frequently delete connections when you replace them with others, or warn you of problems should they occur. The Mix Node The Mix node is one of the most frequently used and important nodes in the compositing system. It defines how color passes or images from two separate inputs will be blended into a single output.

The available mixing methods. A list of available mixing methods can be viewed by clicking the popup selector. In this case, choose Screen. Screen brightens an entire image, based on the image being mixed into it. Light areas brighten more, with white turning the other image white. Dark areas brighten less, with black leaving the image unaffected.

The Specular pass. The specular pass in the example is mostly black with some lighter areas, so it won't brighten the other image much. We would like you to enhance the specularity, though, so you need a way to increase the brightness of the Spec pass. It could be run through an RGB Curves node and adjusted, but there is an easier way. The Fac (Factor) spinner on the Mix node controls the strength of the bottom image in the mix. Values of 0 through 1 represent 0 to 100%. The mix factor can go as high as 5, though, meaning that you can mix the Spec pass at 500% of its actual intensity. Set the Fac spinner on the Mix node to 4.77.

The current node network.

Increased specularity following the Screen Mix node. As you adjust the mixing Factor, the Viewer node updates without having to re-render. The brass gauge now looks a lot shinier, but could still be better. To do that, you'll apply a common post-process effect: bloom. Bloom/Glow Real highlights, such a specular reflection, tend to behave differently than mathematically calculated highlights in a 3D package. Light intensity in the real world can cover a huge range that our eyes find difficult to view. As a result, very strong highlights can cause our eyes to actually overload in certain places, perceiving this dramatic contrast between light and dark as a kind of glow. Adding this subtle effect can make a render look more authentic and is a simple way of adding believability without the extra processing requirements of more complex raytracing algorithms. Load the file "CompositeStage3.blend" from the "examples" folder. The nodes will appear empty until you render (F12). Also, it's okay that the dial appears blank right now. The dial is seen through a refractive object and won't show until you make use of the Refract pass later.

The Bloom Node network. Viewer nodes have been attached so you can easily examine the various stages. As you can see from the node tree, we have collapsed some of the nodes from the previous section using the controls described earlier. That should make it easier to focus your attention on the bloom effect. Once you are happy with a section of a complex composite node tree, it is a good idea to collapse it like this to keep clutter to a minimum. Source of the Bloom Although there are several ways to produce a bloom effect, the simple approach we've taken is to use the Specular pass again, brightening it and applying some blur before mixing it back into the existing image. Using RGB Curves to Brighten an Image The network uses an RGB Curves node, taking its input from the same specular pass you used before. In the basic back-shadowing tutorial, you used the RGB Curves node to invert and colorize an image. Here it is used to brighten the Spec pass by drawing the top right point of the Combined curve over three quarters of the way to the left.

Here's a neat trick: enable the "Backdrop" button on the Node Editor's header. Now, clicking on a Viewer node shows that preview right in the background of the Node Editor. This is particularly useful if you are working with your nodes in a Maximized window (Ctrl-Up Arrow/Down Arrow), or if there is no room in your screen layout for a UV/Image Editor window. The backdrop preview may be moved around with Shift-MMB.

Backdrop enabled on the header. This preview shows the result of the RGB Curves node on the Spec pass. Blurring an Image After the RGB Curves node, you have a Blur node, which can be found under Add->Filter->Blur.

The Blur node with settings for the bloom effect. Although there are seven different blurring styles to choose from, two of the most commonly used are Gauss and Mitch. Gauss Gaussian is a good, general purpose blur. It provides an even effect across the image. Mitch Mitchell-Netravali blur gives a more accurate effect for bright objects. It does not reduce highlights by evenly spreading them like Gaussian blur. Because of that, this type of blur is excellent for working with highlights, as you are here. Of course, other blur methods like CatRom will produce similar effects, so the choice is yours. With the speed of the Compositor, it's easy to switch between different blur methods to see which works best in your final production. Blur Settings In the previous illustration, the Blur node's X and Y values are set to 35. Above the X and Y settings are two buttons that, although not used here, are worth explaining.

Bokeh This is a more complex blur setting that attempts to simulate optical blurring, the kind that would happen with an out-of-focus camera, as opposed to the simple mathematical blurring of other methods. This setting will slow renders and composite updates considerably, but when attempting to fake a camera blur effect, it is much more realistic. Gamma This setting will give bright parts of the blur precedence over darker portions, instead of averaging them. This will usually lead to a brighter blur.

Using the Mix node in Screen mode again. Mixing the Bloom Effect You need to mix this brightened and blurred image with the results of the diffuse and specular combination from earlier. Notice how the title of the Mix node has changed to "Screen," making it easier to tell the mix type at a glance, even on a collapsed node.

A new Mix node, in Screen mode. We have used a Screen Mix node (Add->Color->Mix), as once again you have to blend an image that needs to brighten another. In this case, the Factor has been adjusted to 0.37, but try taking it as high as 0.80 to see if you like it better. Reflection/Refraction Adjustment So far, you have improved the look of the metal and added a nice bloom to the highlights. You're still missing the reflection and refraction, though. Load “CompositeStage4.blend” from the disk and render to fill all the buffers and passes.

The additional nodes for reflection and refraction. You have both a reflection and refraction pass available because the brass material uses raytraced reflections and the glass dial uses raytraced refraction. To see what these passes produce, follow the connectors to their associated viewer nodes and select each in turn.

The reflection pass.

The refraction pass. Neither is much to look at in this state, but there is a lot of information hidden in these apparently dark passes. You need to properly mix these passes with the output from your previous compositing result. To do this, you'll need to use another Mix node.

The Mix node in Add mode. The Add Mix node's lower Image socket receives its input from the Refract socket the whole way back on the Render Layers node. This makes it the "primary" image — the one being layered over your previous Diffuse/Spec/Bloom network result. Setting the Factor to 0.50 means that the Refraction pass is only used at half-strength, de-saturating the dial. This is actually ideal, because if mixed at full strength (1.0), the reflection pass would not be visible. Try setting the Add Mix Factor to 1.0 and see what it does to the final composited image. Once you've done that, change it back to 0.5. This is a great example of how using the Compositor can improve an image without re-rendering. Modifying a Pass Before Recombining One of the down sides of using raytraced reflections is that they are razor-sharp. In the real world, unless a reflection comes from a polished mirror, it will have some degree of blur. If you were not using the Compositor, the only way to accomplish blur in a reflection would be to use high Filter settings with Env Maps (see Chapter 9). By now, though, you should be able to see a simple solution to this problem. Examine the Blur node that is connected to the Reflect output of the Render Layers node. It uses the Mitch blurring method and only a small value, 4, for X and Y.

The Blur node set to affect the Reflection pass. The blurred Reflection pass is then composited with the Diffuse/Specular/Bloom/Refraction result with a Mix node in Add mode, set to 0.84. Varying this factor will increase or decrease the amount of reflection and apparent oily film on the brass. Experimentation is the key, and you are encouraged to modify any of these factors to see how it affects the composite.

The render with reflection and refraction passes added. Adding the Shadow Pass and Color Correcting the Image Load the file "CompositeStage5.blend" and render to fill the passes. The final pass to recombine is the shadows.

A Mix Multiply node to blend the shadow pass into the rest of the image. Mixing the shadow pass is relatively straight forward. It uses a Mix node set to Multiply, with a Factor of 1.0. The lower input socket runs the whole way back to the Shadow output of the Render Layers node. The upper input socket connects to the previous Add Mix node that is producing the final composite. Multiply mode is the opposite of Screen. When multiplied, black turns the underlying image black, while white leaves it untouched. As you can see in the next illustration, areas not in shadow on the shadow pass are white, so they will not affect the final image when blended in Multiply mode.

The shadow pass. Screen and Multiply offer superb after-render control of both shadowing and highlights. If you like, increase or decrease the Mix Factor to change the way the shadow blends into the final image. Color Correction As it is, the final composite looks a bit milky. Let's raise its contrast with an RGB Curves node. Although the RGB Curves node was used in the basic tutorial to completely change the color of the drop shadow, it can be used for more subtle effects too. One of those is enhancing contrast. In fact, this technique is so common and useful that it has its own name in the world of image processing: the "S Curve."

The Contrast S Curve. As you can see, it’s a subtle alteration to the default diagonal line, but when applied to an image it can significantly enhance its appeal. It works by darkening the already dark areas (pulling down the bottom left of the curve) and brightening already light areas (pushing up the upper right of the curve), while leaving the middle areas alone.

A detail of the final rendered, sweetened image. Even though you have split apart the different passes, tweaked them and put them back together into a much better image, there is more you can do. Currently, the gauge just floats on a black background, rather than existing as part of a steam room. Adding a Background with Render Layers Load the file "CompositeStage6.blend" and render to fill the passes. You will notice that the two layers with additional objects, Layers 2 and 11, have been re-enabled.

The node network to composite the layers with different settings. With the other layers showing, the gauge now rests against a simple plane, textured with an altered copy of the brass material, and given a custom paint job to show oil staining with the Image painting tools. Also, a bunch of duplicates of the gauge appear in the background, attached to a wall. The main gauge's wall is on Layer 11. The gauges and wall in the distance are all set to Layer 2. The node setup in this file uses two separate Render Layers: one for the foreground gauge you have been dealing with up until now and its wall, and a separate one for the distant background elements. Having the background appear on a separate Render Layer will let you perform a completely different set of composite effects to those objects.

Render Layer settings for the foreground.

Render Layer settings for the background. Notice that the bottom set of Layer buttons for the "1 Render Layer" layer only includes objects from scene Layers 1 and 11. The Layer buttons for the "Background" layer include objects from scene Layer 2. Looking at the node network, a new Render Layer node has been created with Add->Input->Render Layers, and set to use the "Background" render layer at the bottom of the panel. As you will only be darkening and blurring this layer, you can stick with the default "Combined" pass.

The "Background" Render Layer node. Immediately after the "Background" Render Layer, we have added an RGB Curves node to darken and reduce the contrast of the render. Contrast can be reduced by performing the opposite of the "S Curve" — darkening the light areas and brightening the shadows. Before putting both layers together, though, you can use an old trick to help bring out the foreground objects. Quick and Dirty Depth of Field A simple blur applied to the background makes it look as though the camera lens is focused on the gauge.

The Blur node for the background. A Gaussian blur has been applied with X and Y settings of 5. We have used the Gamma button to emphasize the bright parts of the image, ensuring that the out-of-focus dials remain visible. Also, as we're pretending that the background is blurred due to camera focus, it might be worth it to use the Bokeh option.

The blurred, darkened background layer. The combination of the steam gauge with the background can be accomplished, once again, with the Mix node. This time, however, you will use the default Mix mode. How can you get the node to not blend the entire area of the images together, though? As you've already learned, adjusting the Factor affects how much of the image from the lower input socket is composited over the other. In addition to just being a number, though, the Factor setting can also use an image as its input. By connecting the Alpha pass from the original Render Layers node, portions of the image that were completely opaque (the gauge itself) receive a Factor setting of 1.0, while the non-rendered areas receive a Factor of 0.0. The result is that the Alpha pass is used as a mask for the Mix node.

The Alpha Channel from Render Layer 1.

Mixing the background with the rendered element.

The rendered, composited image with background. Before you finish, you'll look at one more excellent use of the Compositor, one that's suited to animation but that can also enhance single-frame renders. Vector-Based Motion Blur Load the file "CompositeStage7.blend" and render to fill the passes.

The node network for compositing the spinning pointer. With this file, you will produce the animation. However, as the only thing that moves is the pointer on the gauge's face, it would be a waste of time to render the entire image once for each frame. The animation for this piece is 250 frames long, and each frame takes, on the computer used for this discussion, almost a minute to render. That is almost four hours of render time. If you use a single minute-long render to produce a background, then render only the pointer as it spins, you can reduce the per-frame render time to around two seconds, saving nearly three hours and fifty minutes of render time! In this new file, you will see that only three objects exist: the pointer and the main body and face of the gauge. You will only use the render of the pointer when you make the final composite, but the shape of the gauge itself will be useful too. When producing an effect like this, you will need to have already rendered the rest of the image, without the animated portions, to use as a background. We have already done that in the example file, bringing the image into the Compositor with an Image node found in Add->Input->Image. Also, the only 3D objects left in the file are the pointer itself, the main gauge body and face, and the lamps. If you had wanted, you could have simply moved the extra objects to a disabled layer. Blender has two methods of producing motion Blur. The older method, available with the "MBLUR" toggle in the Render buttons, relied on rendering an entire scene several times on fractional frame numbers, then combining the results. Of course, this came at the cost of having to render your whole scene up to sixteen times per frame. Vector-based Motion Blur, on the other hand, uses the Compositor to examine how the objects in a scene are moving, then builds a new image with moving objects smeared along their trajectories and blended into the scene.

The Vector Blur node. Vector Blur is found under Add->Filter->Vector Blur. To make it work, you will need to have some sort of image to blur (either a Combined pass or a composited image), and the Z and Vec passes enabled in the Render Layers tab of the Render buttons. In this example, all three input sockets connect directly to their output counterparts on the "1 Render Layer" node.

The pointer blurred on Frame 189, before compositing. Obviously, objects will need to be in motion on the rendered frame for Vector Blur to have any effect. Note: As you need a good Alpha channel with which to composite, remember to switch the Renderer from Sky to Key mode on the Render tab.

Key mode on the Render tab. Completing the Shot To finish this example, the dial needs to be mixed back over the background image.

The Alpha Over node. When compositing an image with built-in Alpha (a render of a lone object like the pointer), the AlphaOver node does the job. AlphaOver is found in Add->Color->Alpha Over. It follows the same socket stacking rules as the other nodes, with the base image in the upper socket and the image with Alpha in the lower socket. In the example, the saved image of the gauge is used as a backdrop in the top Image input, while the vector-blurred pointer with built-in Alpha fills the bottom Image input. You can see from the final composite, though, that something is wrong.

Composite with the pointer sticking out. One last trick, then, to properly mask the pointer. This is why you still have the gauge body and face hanging around. In this file, both the gauge body and face have been placed on Layer 2, and a separate Render Layer created for them called "Gauge Body." With the body itself selected, it has been assigned an Object Index by using the "PassIndex" spinner on the Objects and Links panel of the Object buttons (F7).

The PassIndex of the body set to 1. In the Render Layer settings for the "Gauge Body" layer, you can see that all passes have been disabled with the exception of the IndexOb pass. You don't need to care about colors, materials or shading here: you want a pass that will generate a mask of this object to use on the pointer. The PassIndex value of all objects defaults to 0 unless changed by you. By assigning a PassIndex of 1 to the gauge body in the Object buttons, you will be able to single it out in the Compositor.

The node network to build a mask from an Object Index pass. The IndexOb pass from the Gauge Body Render Layer (note how with no Combined pass sent, there is no image at all in the preview) is connected to an ID Mask node, from Add->Converter->ID Mask. The ID value in the ID Mask node is set to 1, to correspond with the value you set on the 3D object. After that, an RGB Curves node is used to invert the resulting mask. That image fills in the Factor input socket on the AlphaOver node, correctly masking the spinning pointer and completing your shot.

The final shot, correctly composited with the animated blurred spinner. Getting the Shot Out of Your Department So, you’ve finally finished the job. The managers who have been planning the production had allocated five hours to your department on this shot for rendering and sweetening. Because you’re a pro with the Compositor, you were able to set up the nodes in only a half hour (perfectly reasonable once you’re experienced), and rendered the finished animation frames before the rest of the first hour was up. Have a sandwich. Grab some coffee. You’ve earned it. Well, the Compositor’s earned it, but you can take the credit.