The Preview viewport uses the MODO render engine in a progressive refinement mode that allows users to see all changes in a scene with a fully rendered image. This viewport is thread aware so that when it is not the current viewport it will not interrupt interactivity from other views. This allows the user to work on their project without performance degradation yet anytime there are "free cycles" the Preview viewport will begin a progressive render starting at low quality and filling it in to higher quality as time permits. In the Preview viewport users can view changes to the geometry, materials, textures, cameras, lights and even render settings such as ray tracing and global illumination.
In traditional systems the OpenGL viewport gives a proxy view of the project in real-time and a separate offline render engine (if available) is summoned periodically to generate a full quality image. This offline rendering is a modal effect which requires the artist to completely turn over control of the system to the render engine until the image is complete. This "start and stop" process is a complete workflow killer. Even waiting a several seconds to review a change takes the artist out of their creative flow. The Preview window enables the artist to simply pause to glance at the progress of their project and then continue the process of refining their vision resulting in a much more fluid and art conducive process.
Usage
Navigating the camera around in the Preview window works just like any other 3D GL viewport set to 'camera', with the additional ability to define a rectangular area by pressing Ctrl+Alt+RMB to define a box area for zooming. Note though that navigation only works when the "Full Resolution' option is disabled, when it is enabled, users will only be able to pan around the rendered image itself. Users may also wish to zoom in for a closer look without affecting the cameras position, it is only possible with a middle scroll wheel mouse, by first pressing and hold the 'Shift' key on the keyboard, then placing the cursor over the center point of the zoom target and then roll the mouse wheel in and out to zoom. To reset the view, use the Render Done/Pause button at the top of the viewport.
The buttons at the top of the viewport also offer additional useful functionality. Users can LMB+click any of them to open popup menus or enable/disable specific functions. The 'Options' button opens a menu with controls to customize the Preview rendering engine for additional optimization (covered below). The 'Effects' category button allows users the ability to render different Render outputs or select from a wide variety of 'Effect' types for easily previewing specific shading input, useful in troubleshooting the Shader Tree. The next button is a status update of sorts that gives users useful feedback of Previews current state, users can LMB click this button to pause Preview rendering temporarily. When pressed again Preview will refresh the image, starting a new render.
The LUT button allows users to choose the Colorspace of the monitor, unless there is a specific need, users should just leave this setting on the the default 'sRGB' setting. For more information on Color Management, please reference that page of the documentation. The Rendering Camera popup allows users to select any alternate camera from the scene. The IC button can be a big time saver; if nothing is changing in a scene except surfacing values, then pressing the IC button will 'Hold' the current values, instead of reevaluating them each time a refresh is triggered. The button is a toggle that pressing again will trigger a refresh evaluation of the IC values.
Options
The Preview options provide extensive opportunities to optimize the performance of the viewport by limiting or focusing the rendered data. These controls are found under the 'Options' menu. Users can LMB+click it like a button to open the Options menu, users can enable or disable certain functions with a mouse click. The check mark will mean the feature is enabled, no check mark- disabled.
Preview Type: Users can select from different display types for the actual preview window. The 'Scene Preview' option will render the active scene based on the settings of the Shader Tree. When used as such it will also work as an interactive view of any changes made to the scene, updating as the changes are made. The 'Sphere', 'Plane' and Preset Sphere' options are useful to adjust settings individual materials in the Shader Tree, providing a way to see adjustments made interactively away from the surface the material might be applied to. The top-most Shader Tree layer always takes precedence in this mode. The 3 primitive options use the current scene environment and lighting.
Quality: The quality control allows users to set the level of fidelity for the preview window, in essence controlling the speed at which the preview works. There are three separate options-
Draft Quality- Cuts most ray evaluations values by half, providing a high quality, yet very fast rough preview of the scene.
Final Render Quality- Uses the full number of ray evaluations as defined by the Shader Tree producing a image that is nearly the equivalent of an actual rendered image.
Extended Refinement Passes- Uses an extended amount of ray evaluations, beyond those defined in the Shader Tree (up to the maximum amount defined in the Preferences). This will act like many dedicated progressive render engines that will continue to refine the image as long as it is left alone to bake. The longer it is left, the higher the quality. When the maximum number of rays has been reached the 'Rendering' button will say 'Done'.
Full Resolution: When the 'Full Resolution' option is enabled, the Preview will render at the resolution defined by the Render Item 'Frame' settings, regardless of the window size. When disabled, the render resolution is defined by the actual window itself.
Render All Outputs: When the 'Render All Outputs' option is enabled, Preview will render all the visible 'Render Output' items from the Shader Tree simultaneously, allowing users to switch between the various layers (using the 'Effect' menu) while the scene is rendering. Selecting between the different outputs will not trigger a Preview refresh, though picking any of the other alternate 'Effect' types will.
Shadows: The 'Shadows' option will toggle the evaluation of direct light source Shadows, these are lights such as 'Point' and Spot Lights'. When disabled shadows will not be evaluated in the Preview, providing a speed boost dependent on the number of lights in the scene and their contribution. When enabled all active direct light shadows will be evaluated as defined by each lights Shadow settings.
Individual Evaluation Toggles: This section of the menu contains a number of individual functions that can be toggled (enabled/disabled) controlling whether or not they are evaluated. Most should be apparent from the function name; draft options, when enabled, will automatically halve the associated value allowing providing a faster Preview evaluation.
Automatic Geometry Updates: Preview can be useful to see a fully rendered view of geometry as it is created and tweaked. Each time the mouse is released on a modeling action, such as a bevel, the geometry cache will be updated to reflect the change, this will cause a new full evaluation of the scene by Preview. By disabling the 'Automatic Geometry Updates, users can go into a manual mode where Preview will reset the geometry cache only when the 'Reset Render' option is invoked or the Preview itself is paused and then reactivated.
Use Multiple Threads: The 'Use Multiple Threads' option will intelligently use available CPU cores, interactively adjusting the number based on the current needs of the system. This helps to keep the system interactive while still keeping preview fast.
Use All Threads: The 'Use All Threads' options, when enabled, will use all CPU cores, same as a full render would. This option maximizes Previews speed but it can also result in sluggish performance when working outside of Preview. This is best to enable when the extended refinement passes option is selected and the system won't be used for other purposes for a while.
Show Only Selected Items: When 'Show only Selected Items' is enabled, only items that are selected in the Items List will be evaluated in the Preview viewport. When disabled, all items set to Render will be evaluated. The Render toggle is defined in the Items properties panel.
Show Only Visible Items: When the 'Show Only Visible Items' option is enabled, an items contribution to the Preview will be determined by the visibility column of the Items List, where toggling the eye icon on or off will change the target items Preview visibility.
Pause Render: When enabled the 'Pause Render' option will stop the Preview as its current state with no further evaluations. When disabled the Preview render will reset itself and begin evaluating a new image.
Reset Render: The Reset Render will do exactly as it states, resetting the Preview, refreshing the geometry cache and the GI cache if necessary, starting a new image.
Progressive Render: The Progressing Render options work best when the 'Extended Refinement Passes' option is selected. Using these Progressive Render controls, users can 'Save' the current state of the progressive render to disc (warning- files may be quite large depending on the scene) for further evaluation at a later time. The 'Load' option will load any previously saved progressive render states and allow the continuation of the render when loaded. However, when reloading a progressive render state, the same scene must be loaded and active and be in an identical state as to when the progressive render was saved. Even with these limitation it can be quite useful when a users needs to stop a render to use the computer for another purpose but wants to continue the rendering once the task is complete.
Save Image: The 'Save Image' command will save to disc the current state of the Preview image to a variety of image formats.
Save Layered Image: When the 'Render All Outputs' option is enabled, users can simultaneously save all of the Outputs in their current state into a single layered file in either the PSD or EXR formats. Each layer will be named after the Render Output it represents.
Update Under Mouse: The 'Update Under Mouse' will concentrate Preview evaluations to the area under the mouse when it is moving, allowing users to control, in a way, what areas update first in any given Preview. When the mouse is not moving or not over the Preview viewport, the standard random evaluation pattern will be used, evenly spreading the CPU calculations across the image.
Trackball Rotation: When the 'Full Resolution' option is disabled, users can navigate a preview window like any other standard 3D viewport. Toggling the 'Trackball Rotation' option will enable/disabled the function when navigating. Keep in mind that any navigation of the Preview viewport will actually move the Camera itself in 3D space.
Tool Activation: The 'Tool Activation' option allows users to toggle tool visibility in the 'Preview' viewport. When the option is enabled users will be able to see Tool handles directly in the viewport, which can be used to transform objects interactively. When disabled tool handles will be invisible and users will not be able to modify items interactively.
Render Animation: The 'Render Animation' option will open a special animation saver called the 'Preview Animation Saver' that will allow users to leverage the Preview rendering system to render out animations. Users should note that it cannot be used to render Passes, only the current Pass will be rendered. Otherwise, all the current settings of the Preview window are respected when rendering animations. Once started a progressive render can only be aborted by pressing the 'Abort' button in the progress bar, or by pressing 'Esc'. The dialog has a few options for the user-
Render Time: The 'Render Time' feature is the greatest feature ever invented. Well, maybe thats an overstatement, BUT it is super useful for deadline driven workflows. It allows users to control how long it takes to render an entire sequence and it will evenly slice up the time between all frames, and will progressively refine each frame until time is up (or the frame finishes, which can definitely happen if you're in Final or Draft mode). Instead of fiddling with settings to get a frame rendered within a specific time limit, one simply specify in 'hours:minutes:seconds' how long for MODO to spend rendering the entire sequence. So if you have a 120 frame animation and you tell MODO you need a preview in 4 hours to show the director, then MODO will spend 2 minutes on each frame doing the best it can in that amount of time.
First/Last Frame: Determines the frame range that will be rendered.
Save As: Users can specify if the Preview will be saved as a 'Movie' file or as individual frames in an 'Image Sequence'.
Below the Preview 'Options' button is a Camera pop-up selector allowing the user to easily select the current camera for rendering, without the need to specify it specifically in the Render Item. Directly next to the options is an 'Effect Button' that allows the user to define a specific rendering channel for previewing, allowing for a number of diagnostic possibilities, such as previewing the amount of Fresnel reflectivity on a surface. Additionally, users can Drag and Drop any Render Output from the Shader Tree onto the Preview window automatically switching it to that view that Effect. the "Current Layer Overlay" option allows users to toggle the visibility of a selected layers contribution to a scene, visualized as an semi-transparent overlay, by enabling that option. Once enabled it displays the current texture as an overlay on top of the render, visualizing the raw value unaffected by things like 'Blending Mode', 'Opacity' or shading. So if you have an image map on the Subsurface Color channel, it will display the raw color of the map. For 'Groups' it displays the group mask opacity. This is very handy to resolve texturing issues and see what each layer is contributing to the final render.
Visually Selecting Shader Tree Layers--
To make it easier to find a particular surface, users may RMB-click over any area of the preview image itself, opening a popover menu displaying all shader tree layers that contributed to the pixel directly under the mouse pointer when the RMB was pressed. Clicking on any of the layers in the dialog will select them in the shader tree.
