Materials can be defined by MaterialX VOPs (for objects and SOPs) or MaterialX shader prims (for LOPs). The entire network must be made of MaterialX nodes, so for example “Mtlx Add” must be used instead of “Add”. Materials can be defined for surfaces (polygon meshes, NURBS, subd surface) and curves (polygon, bezier, NURBS). Curves are shown as ribbons with the line width taken from the 'width' attribute.

Output nodes can be one of:

• Mtlx Standard Surface

• Mtlx Surface

• Mtlx USD Preview Surface

• Mtlx Unlit Surface

• Mtlx Displacement

If none of those are the output node, the first input of the output node is used as the emission color, with no light shading for visualization purposes.

BSDF and EDF nodes only have basic support. BSDFs are layered by combining their parameters, where those apply to the viewport’s shading engine. EDFs simply map to the emission color as a uniform color. These give a basic preview of textures and colors, but not necessarily an accurate preview of how an offline renderer will shade them.

Some MaterialX nodes are not supported in GL:

• Curve Adjust

• Blur

• HeightToNormal

• Ambient Occlusion (a viewport setting instead)

• all Light nodes

• EDF and VDF nodes

• Subsurface and Generalized Schlick BSDF nodes

Particle systems can be rendered as sprites if a Sprite POP is present. Sprites are always drawn as transparent objects. A sprite is a small texture that always faces the camera, centered at the particle. Sprites are drawn after all other transparent objects except for volumes when in Medium Quality or High Quality transparency mode.

Volumes can be rendered at a variety of quality levels which improve the look of the volume at the expense of performance. Volumes do not cast or receive shadows, nor does ambient occlusion affect them. Volumes can be rendered without transparency, producing a solid outline of the volume.

Volumes are limited to using a 128×128×128 8 bit 3D texture by default. For larger datasets, this limit can be adjusted on the Optimization ▸ Textures page of the display options.

Volumes are drawn in the final pass of the beauty render.

This provides a simple depth-based fog shader. By adjusting the fog color and density, many different kinds of atmospheric looks can be modeled (mist, smog, dust). It can also be restricted to a range of depth values, to avoid fog close to the camera or prevent solid fog from forming at extreme distances. There is also a depth clip which prevents objects past a certain distance from receiving any fog, which is useful for avoiding fog on background images and plates.

Uniform fog can be controlled by height as well, to make overcast or ground-hugging fog. Finally, the brightest distant light in the scene can be used as the sun which does simple lighting on the fog. See the Uniform Fog display options in the Fog tab of the display options. Uniform fog is available in all lighting modes.

Volumetric fog is a more advanced atmospheric effect that allows lights to illuminate the fog, creating blooms and rays through the fog. It requires High Quality Lighting and its look greatly benefits from shadows. The Quality of the fog determines size of the volume used for lighting, and the overall performance of the effect (Low is fast, High is slow but much higher quality).

Volumentric fog shares many of the same parameters as Uniform Fog. One key difference is that the Depth Range should be tightly bound around the lights illuminating it to avoid grainy looking fog. It’s useful to use Volumetric lighting in the foreground over a small depth range, and Uniform Fog in the background to fill in the rest.

Lighting can be varied per-light with the OpenGL Fog Light properties. To only light the fog with a small number of lights, set Light Intensity to zero, then put the Light Intensity property on those lights.

Bloom is a simple effect that mimics subtle atmospheric or lens effects around bright spots in the scene. Pixels with intensities above the Bloom Threshold will cause a small halo around them. The size of the halo is based on the pixel intensity and can be further scaled by the Bloom Scale. The brightness of the bloom is also based on the pixel’s intensity, and then be further modified by Bloom Intensity. This effect requires OpenGL 4.4.

Depth of Field creates a blurred effect on objects outside of the focus area when looking through a camera with a non-zero fStop. It can be enabled with Camera Depth of Field in the display options (Scene tab). Additional bokeh sprites can be added if the hardware supports GL4.4, which produces bokeh blooms at very bright spots. The bokeh can be circular or sourced from an image (file or COP). The Bokeh Boost increases the number and brightness of these blooms.

Performance in the viewport is affected by two things: what you are displaying (the model), and how you are displaying it (display options). Turning off Material Shaders on the Effects tab of the 3D Viewer Display Options is a good way to determine if the model is the constraining factor, as this will disable most of the advanced viewport effects.

If the model does appear to be the limiting factor, you can try a variety of things:

1. Turn off the display flags some objects.

2. Display the non-subdivided geometry, if a Subdivide SOP is used. Turn on the render flag on the Subdivide SOP, and the display flag of the SOP before it.

3. Turn on some culling options on the Optimization ▸ Culling tab of the 3D Viewer Display Options, such as Remove Backfaces (backface culling) and Cull by drawing 'x' primitives out of every 'x' (polygon reduction).

If the display options are the limiting factor, you can reduce the quality of the displayed geometry. This is more common on graphics hardware with fewer shaders or earlier generations. All of the following options are found on the Effects tab of the 3D Viewer Display Options.

1. High Quality Light Shading is the most performance-intensive quality enhancement available. Disabling this will provide the highest performance increase, especially if many lights are in the scene, and complex lights like area, geometry, environment or portals are used. It is not enabled by default.

2. If many volumes are shown, reducing the Volume Quality will improve interactivity. Turning off Transparency may help as well, though at a fairly large quality decrease.

3. Transparency causes more passes to be rendered and can result in slower rendering, if transparent objects are present. Volumes, sprites, materials with alpha less than one, and RGBA textured materials will all cause a transparency pass to be rendered. Try lowering the Transparency Quality first, then disable it if needed.

4. Shadows can slow down the system, especially if there are many lights in the scene, or if area lights or portals are used and the Shadow Quality is set to Area or above. Try reducing the shadow quality first. You can also disable a light from rendering in the viewport by disabling the Enable Light in Viewport parameter in the Light object.

5. Try using a non-shaded mode such as Hidden Line Ghost or Wireframe.

=== Viewport Memory Management ==

Graphics hardware has a limited pool of on-board memory (VRAM). Exceeding this limit will result in geometry, textures, and framebuffers swapping in and out of system memory. This can be observed as a frame rate stutter which may gradually correct itself.

There are several ways which VRAM can be used by Houdini:

1. Geometry - models and particle systems.

2. Textures - 2D textures on materials and for shadow maps; cube textures for environment maps on environment and area lights, plus omnidirectional light shadows; 3D textures for volumes.

3. Framebuffers - textures and windows used to display information on the screen.

Geometry can be reduced in several ways, as stated above in Viewport Performance.

Texture usage can be reduced by disabling mipmapping, on the Effects tab, or by changing the settings found on the Optimization ▸ Textures page. Reducing the maximum size of a texture, or its default bit depth, can decrease the overall memory use of material textures. Finally, material textures can be reduced by disabling Multi Texturing or removed entirely by disabling Display Textures n the Effects tab.

Textures used for shadows can be reclaimed by turning shadows off, or by reducing the shadow Quality to Point if many area lights are used on the Effects tab.

Framebuffer usage can be reduced by lowering Scene Antialiasing or disabling HDR Rendering. Scene Antialiasing increases the framebuffer size by the antialiasing factor (2x, 4x, 8x) and can be a major source of VRAM use. HDR Rendering increases the framebuffer size by 50%.

Framebuffer memory increases as the size of the Houdini window increases. A high resolution display, such as the 2560×1536 resolution found on many 30" monitors, will consume much more VRAM than a smaller resolution. Using multiple monitors will double or triple the framebuffer use as well.