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# ALE Rendering Chains

The ALE incremental renderer is configured using rendering chains. Rendering chains are constructed hierarchically from: invariants, SSFEs (scaled sampling filters with exclusion), SSFs (scaled sampling filters), and sampling filters. Each of these components is described in a section below. Defaults are outlined in the final section.

## Chains

The chain is the top level of the rendering chain hierarchy, and is composed of a sequence of invariants. Syntactically, a chain is specified as a number of invariants separated by commas:

> `invariant1,invariant2,invariant3,...,invariantn`

Each rendered pixel is assigned a value at the first invariant whose weight exceeds the weight threshold, or otherwise at the last invariant in the sequence.

### Chain Options

    --wt <w>          Set weight threshold for defined pixels    (default is 0.8).
    --dchain <g>      Use chain <g> to render the default output.
    --ochain <g> <o>  Use chain <g> to render output file <o>.
    --achain <g>      Use chain <g> to render the alignment reference image.

### Chain Examples

                 triangle:2                           ALE 0.6.0 merging (roughly)
                 fine:box:1                           Drizzling (roughly)
                 fine:sinc*lanc:8                     High-frequency preservation
                 last:nex:sinc*lanc:8                 Useful for video stabilization
                 fine:box:1,triangle:2                Multi-resolution rendering
                 fine:sinc*lanc:8,sinc*lanc:8         Multi-resolution rendering

## Invariants

Invariants determine whether the rendered pixel value should be taken from a single frame, or from an average of all frames. If from a single frame, it can be the first, last, minimal, or maximal pixel value. By default, the average is used. The syntax is as follows:

> Syntax |
> | Description
> --- | --- | ---
> avg:<SSFE> |  | Average value
> max:<SSFE> |  | Maximum value
> min:<SSFE> |  | Minimum value
> first:<SSFE> |  | First value
> last:<SSFE> |  | Last value
> <SSFE> |  | Same as avg:<SSFE>

## Scaled Sampling Filter with Exclusion (SSFE)

SSFEs qualify SSFs, indicating whether exclusion regions should be honored.

> Syntax |
> | Description
> --- | --- | ---
> ex:<SSF> |  | Honor exclusion regions
> nex:<SSF> |  | Don't honor exclusion regions
> <SSF> |  | Same as ex:<SSF>

## Scaled Sampling Filter (SSF)

SSFs indicate whether filtering should occur at the resolution of the output image, or at the minimum of input and output image resolutions. The former prevents loss of details; the latter prevents aliasing.

> Syntax |
> | Description
> --- | --- | ---
> fine:<SF> |  | Filter at full output image resolution
> coarse:<SF> |  | Filter at minimum resolution
> <SF> |  | Same as coarse:<SF>

### SSF Options

    --afilter <s>     Use SSF <s> to interpolate points in alignment.

## Sampling Filter (SF)

SFs are used to limit bandwidth. Primitive functions and pointwise multiplication of these functions (windowing) are available.

> Syntax |
> | Description
> --- | --- | ---
> sinc |  | Sinc filter: (sin πx) / (πx)
> lanc:<x> |  | Lanczos, diameter x.
> triangle:<x> |  | Triangle, diameter x.
> box:<x> |  | Box, diameter x.
> zero |  | Zero function
> <f>*<f> |  | Pointwise multiplication (windowing)

## Defaults

ALE defaults are as follows:

                         dchain         Varies by default setting
                         achain         Varies by default setting
                         afilter        internal (approximates triangle:2)

Copyright 2003, 2004 David Hilvert

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