From K-3D

DRAFT

Shading & Sampling

Shading Rate

This is you number one image quality knob. The Shading Rate tells the renderer the frequency at which the shaders should be run on the geometry in your scene. Confusingly, it is expressed as an area in pixels, not as a frequency per pixel, as one might be trapped to guess from its name. Tip: The lower the Shading Rate, the more time it will take to render your image and the crisper it will look.

Most RenderMan renderers available today are more or less based on an algorithm called REYES, which stands for
"Renders Everything You Ever Saw".

REYES renderers dice geometric primitives into tiny micropolygons. The granularity of the resulting micorpolygon grid is proportional to Shading Rate in these renderers. For this reason Shading Rate also directly affects silhouette quality. The following description applies to REYES-based renderers:

A Shading Rate of 1 means that no micropolygon's size will exceed the area of one pixel. Imagine how even the simplest objects will generate thousands of micropolygons and render beautifully and without any nasty polygonal silhouette edges. Tip: A good setting for Shading Rate is 1 if you do high-res work. If you render for screen or web, consider a value of 0.5 or 0.25. Lower values will also increase the image's sharpness. For previews, a value of 10–20 should be ok.

Always keep in mind that Shading Rate determines area respective size of a micropolygon, not its edge length! At a Shading Rate
of 9, micropolygons should end up with a maximum edge length of around 3 pixels on screen, namely the square root of 9. 
Warning: Halving Shading Rate usually doubles the number of micropolygons in your image!

Pixel Samples

This is your number two quality knob. It tells the renderer how often to sample the scene at each pixel resp. how strong the geometric and temporal antialiasing will be.

Under sudden circumstances, if you have a lot of high-frequency content in your image (e. g. many tiny objects, which have a size close to Shading Rate), Pixel Samples may even become more important than Shading Rate. Warning: Pixel Samples have a major impact on rendering time! For previews, set them to 1×1 samples; this will essentially turn antialiasing off. To capture very high frequencies in geometry or shaders/textures and/or get noise-free images with motion blur or depth of field, you will need to increase them considerably.

Filtering

Pixel Filter

This is the function used to filter the Pixel Samples into the final color. It will have a subtle yet visible impact on the look of the image. Tip: Think of the Pixel Filter as a filter you would normally apply in your 2D imaging application; like e.g. 'Sharpen' or 'Gaussian Blur'. Only this one is applied to the samples before quantizing to the display's color depth and before even pixels exist. The result is of much higher quality than it would be had you applied a similar filter in your 2D imaging application after the image has been rendered.

It works like this: the samples are averaged together to calculate the final pixel color. Each sample is weighted before
the average is calculated. The weight of the sample is a function of its distance to the center of the belonging pixel.
This function, which essentially determines how much of the respective sample will go into the pixel, is called the
Pixel Filter. In general, the farther away a sample is, the less impact its color will have on the final color of
the pixel being processed. Several pixel filters exist. Some renderers offer additional ones to those found in every
RenderMan renderer.

Box

The simplest filter. Averages samples together without weighting them. This means that the sample's distance to the filter's center is not used to determine its influence on the resulting color. Used for (shadow) depth-map renderings and other special stuff. Avoid this filter if you want to create nice looking images. For previews it's pretty ok though.

Triangle

Samples are weighted linearly. This filter is a compromise between quality and speed. It looks better that [#Box Box], but is far from a the quality of a [#Catmull-Rom Catmull-Rom] or [#Sinc Sinc] filter.

Catmull-Rom

Samples are weighted by a hermite curve that has a negative lobe near its border. This filter will increase contrast at edges in the image and therefore works a bit like those sharpening filters you probably know from your 2D imaging application of choice.

Gaussian

Samples are weighted by a Gaussian bell-curve. Images will look more blurry than with [#Catmull-Rom Catmull-Rom]. By using a bigger [#Filter_Width_.26_Height Filter Size] with this filter, you will get very smooth looking images. This is the preferred filter if you work for output to film.

Sinc

Samples are weighted by a filter that looks similar to [#Catmull-Rom Catmull-Rom] and which, too, has a negative lobe near its border. This filter will increase contrast at the edges in the image and give very sharp images, in particular with many [#Pixel_Samples Pixel Samples] and a smaller [#Filter_Width_.26_Height Filter Size], e.g. 2×2.

Blackman-Harris

A separable 4 term (-92 dB) Blackman-Harris filter.

Mitchell

The recommended filter from Don Mitchell and Arun Netravali's 1988 Siggraph paper on reconstruction filters - the separable version of the (1/3, 1/3) filter.

Separable Catmull-Rom

A separable version of the catmull-rom filter. Must be 4x4 wide.

Lanczos

Bessel

Disk

Filter Width & Height

This controls the width or radius of the used [#Pixel_Filter Pixel Filter]; i. e. how many pixels will contribute to the filtered pixel's color. Imagine applying a filter in your digital imaging application, as described in the [#Pixel_Filter Pixel Filter] section. Filter Width is similar to the 'radius' parameter of such a digital imaging filter. Tip: Using a narrower Filter Width & Height will yield sharper images, regardless of the [#Pixel_Filter Pixel Filter] being used.

Here is a list of suggested Widths & Heights for some of the filters. It is sorted ascending by expense as the rendering time increases with larger filter support:

Box: 1.0
Triangle: 2.0
Blackman-Harris: 2.0
Gaussian: 2.5
Catmull-Rom: 4.0
Separable Catmull-Rom: 4.0
Mitchell: 4.0
Lanczos: 6.0
Bessel: 6.49
Sinc: 8.0

Reyes

These are options that apply to renderers using an algorithm called REYES.

Bucket Width & Height

The width and height of a bucket. The image gets divided into cells called 'buckets'. These get processed by the render one by one, ensuring that the memory requirements stay as low as possible.

Imagine that when the R.E.Y.E.S. algorithm was designed, machines had less than 1MB of RAM but people wanted to render 2k frames with complex geometry diced into millions of µ-polygons and motion blur without going into swap.

Grid Size

The maximum number of micropolygons in a grid. This should usually be the product of [#Bucket_Width_.26_Height Bucket Width] and [#Bucket_Width_.26_Height Bucket Height] divided by [#Shading_Rate Shading Rate]. Read:

GridSize = ( BucketWidth × BucketHeight ) ÷ ShadingRate

For example, if your [#Bucket_Width_.26_Height Bucket Width & Height] are set to 32 respectively and your [#Shading_Rate Shading Rate] is 2.0, your Grid Size should ideally be 512.

Texture Memory

Sets the amount of memory, in kilobytes, the renderer uses for texture mapping. Increasing this value may improve texture map, shadow map and environment map access performance and thus shorten render time at the cost of a higher memory usage during rendering.

The memory required to hold all the textures for some scene may exceed the amount of physical memory available. To render scenes like this without swapping to disk, most renderers use a memory caching system that keeps the texture memory footprint below a threshold which is defined through the value of [#Texture_Memory Texture Memory].

Eye Splits

Opacity Threshold

This indicates the opacity level beyond which semi-opaque surfaces should be culled.

Can significantly speed-up your renders with tightly packed semi-transparent primitives, like fur.

Image

Bit Depth

0: float
8: 8-bits
10: cineon style.
16, etc...

Gain

This alters the contrast of your image. A value of 1.0 is neutral, values above 1.0 will increase the contrast, values below 1.0 will decrease it.

Gamma

1.0 is neutral and is the default.

Crop Window

View Crop Window

Check this to make the crop window appear in your camera view. Use the sliders to adjust it.

Left

defaults to 0.

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