You can also press the + and - shortcut keys to respectively increase or decrease the Level values.
Before you render hair, either in a render region or to file, there are some settings that can make a big difference to the quality of the render. This includes setting the quality of the render, setting the geometry approximation, and setting up a BSP tree for hair.
You can choose to render hair with a low quality for quick rendering, or with a higher quality rendering that takes longer to process. This setting affects only pre-processing time (hair generation). If the hair doesn’t have a lot of kink or frizz, and has few segments (such as short hair or fur), use the Low setting for faster processing; otherwise, use the High setting for better quality.
1. Attach a shader to the hair object as described in Connecting Shaders to Hair.
2. Open the Hair property editor and select either Low or High from the Render Quality list.
Setting the Geometry Approximation
When either viewing or rendering hair, you can set its geometry approximation. Geometry approximation specifies how objects and curves should be tessellated (divided into triangles and line segments, respectively, at rendering time).
When you set the geometry approximation for hair, a general Bézier curve is created to represent the hair. You can reduce the number of segments in the hair curves and still render a very smooth surface.
For general information about geometry approximation, see Geometry Approximation in the Rendering guide.
To set the geometry approximation for hair
1. If the hair does not yet have a local Geometry Approximation property assigned to it, assign one by choosing Get > Property > Geometry Approximation from any toolbar.
2. Click the Selection button on the Select panel to display the selected object’s nodes in a pop–up explorer.
3. Click the Geometry Approximation icon to open its property editor.
4. On the Hair tab, specify the degree of curve approximation for the Subdivision - Curve Type:
- Linear creates a linear curve approximation (straight segments). If you select this option, the OGL Level and Render Level options are unavailable because you cannot approximate straight lines with more lines.
- Quadratic creates a quadratic curve approximation (smooth). This option doesn’t guarantee a continuous curve throughout the entire length of the hair. If the original line segments are at too steep of an angle, there may be a cusp where they meet.
If you select this option, additional points required to construct the curves are generated. This results in one extra control point for each generated hair point, less one.
- Cubic creates a cubic curve approximation (smoothest), guaranteeing continuity between hair segments. If you select this option, additional points required to construct the curves are generated. This results in two new control points.
5. Set the values for the OGL Level and/or Render Level of subdivisions. These parameters are independent of each other.
- OGL Level is the subdivision level for approximating the curves when viewing hair in the OpenGL views in the viewports.
- Render Level is the subdivision level for approximating the curves when rendering hair (in the render region or to file).
Either OGL or Render Level generates 2Level line segments for approximation. For example, if this value is 0, the curves are approximated by 1 line segment; for a value of 1, by 2 segments; for a value of 2, by 4 segments, etc.
The Level sliders display a range of 0 to 4, but you can enter a value up to 7 in the text boxes.
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You can also press the + and - shortcut keys to respectively increase or decrease the Level values. |
You can set up a BSP tree exclusively for rendering hair. These settings work in the same way as do the standard BSP tree settings for the scene, but they are calculated separately. This allows you to deal with any special considerations that hair requires for rendering, apart from the rest of the scene.
You should spend a good amount of time tweaking the hair BSP settings to speed up rendering. Draw small render regions on the hair as you go so that previews are fast.
For information on BSP tree settings in general, see BSP Raytracing Acceleration in the Rendering guide.
For some general tips about rendering hair, see Raytracing and Antialiasing.
To set up the BSP tree for hair
1. Open the Hair property editor and make sure that Instancing is not enabled so that you are rendering the hair geometry, not instanced geometry.
2. Set the values for the Hair Geometry parameters on the same page:
• Maximum Depth is the maximum number of branches down the tree can go until it stops splitting into any further branches (1 to 50).
• Maximum Size is the maximum number of hair segments each leaf on the tree can hold before converting that leaf into a branch to allow for further splitting (1 to 30).
Setting the Hair Chunk Size for Rendering
When you render hair, it is split into separate mental ray objects in approximately 400,000-strand chunks. This results in approximately 40,000 hairs per object given the default 10-segment hair.
Splitting the hair objects into smaller hair objects allows you to speed up the rendering. This means that you can render more hair, which increases the overall realism. As well, making smaller hair chunks can help render hair with motion blur because memory requirements increase when motion blur is used.
To set the hair chunk size for rendering
• Enter SI_HAIR_CHUNK_SIZE as an environment variable in the setenv.bat file (in the XSI Application\bin folder) and enter a value for the maximum hair strands you want per hair object.
For more information about setting environment variables, see Environment Variables.
Currently the default is set to 40,000 strands, so say you have 1000 hairs with 80 segments each, this give you a total of 80,000 strands. when you render this we'll generate two hair objects of 40,000 strands each at render time to optimize it.
Deactivating Hair for Rendering
Hair can often slow down the rendering of a scene, and you may want to render the scene without rendering the hair. Instead of hiding the hair or putting it on a separate scene layer and making that layer unrenderable, you can easily deactivate an option in the Render Manager to achieve faster preview rendering. This applies only to hair that uses the geometry render type, not instanced geometry.
To deactivate geometry hair for rendering
1. Choose Render > Render Manager on the Render toolbar.
2. In the Render Manager, click the mental ray tab on the left side.
3. Click the Rendering tab and deselect the Enabled Features > Hair Geometry option. The hair is still generated, but not rendered.
The same command is also available for batch rendering.
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XSIBatch Command: -geo_hair [on|off] |
• For more information about all of the rendering (and render region) options, see Managing Rendering Options in the Rendering guide.
• For more information about batch rendering options, see Rendering Methods in the Rendering guide.
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