Autoscale parameter

AVS+
This feature is specific to AviSynthPlus. It is not supported in other AviSynth versions.

Autoscale is the term being used to describe filter and function arguments that adjust themselves to the current bit depth – the BitsPerComponent of the main source clip. A related concept is 'd' notation: 16_d-235_d refers to normal TV-range levels at any bit depth.

Examples

To see a filter with non-autoscaling arguments, consider Levels:

Levels(clip C, 
\     int input_low, float gamma, int input_high,
\     int output_low, int output_high [, 
\     bool coring, bool dither])

All the arguments in blue expect user inputs in the normal 0-255 range – assuming 8-bit video. For 10-bit, the input range is 0-1023, and for 16-bit, it's 0-65535. Scripts that specify 8-bit Levels arguments need to be altered for use with other bit depths. For those used to "thinking in 8-bit," writing scripts for multiple bit depths can be cumbersome.

That's not to say Levels is wrong or buggy – what it does makes sense: it is the humans that need to adjust. There are utility functions to help with this, which will be discussed below.

Another example is RGBAdjust with its bias arguments: rb=16 always lifts the Red channel by 16 units, whether on the 0-255 or the 0-65535 scale. It does what you say, not necessarily what you mean.

To see a filter with autoscaling arguments, consider ColorYUV:

ColorYUV(clip [,
\    float gain_y, float off_y, float gamma_y, float cont_y,
\    float gain_u, float off_u, float gamma_u, float cont_u,
\    float gain_v, float off_v, float gamma_v, float cont_v,
\    string levels, string opt, bool showyuv, bool analyze,
\    bool autowhite, bool autogain, bool conditional ] ))

The off_x arguments (in blue) add or subtract a given amount from all pixels in the given color channel, and the amount is always on the 0-255 scale – that is, off_y=255 will always turn black to white, at any bit depth.

In 8-bit video, setting off_y=16 will raise all luminance values by 16 units on the 0-255 scale – but in 10-bit video, off_y=16 will raise all luminance values by 64 units on the 0-1023 scale.

The argument value has been adjusted by the filter to give the same visual effect. That's autoscaling.

User-Defined Autoscaling functions

For filters and function that don't autoscale, there are some helper functions you can call upon, which are included in raffriff42's Utils-r41.avsi script. These three are the ones most important for autoscaling:

sc8f(clip T, float f, [bool cx])

Scale an 8-bit value for target clip T

sc8x(clip T, float f)

Scale an 8-bit value for target clip T; clamp output

sc8s(clip T, float f, [int decimals])

Scale an 8-bit value for target clip T; string result

You can use them to make Levels autoscale, so that the old 8-bit arguments give the same visual result in any bit depth:

function LevelsPlus(clip C, 
\               int input_low, float gamma, int input_high,
\               int output_low, int output_high,
\               bool "coring", bool "dither")
{
    return C.Levels(
    \           C.sc8x(input_low), 
    \           gamma, 
    \           C.sc8x(input_high),
    \           C.sc8x(output_low), 
    \           C.sc8x(output_high), 
    \           coring, dither)
}

LevelsPlus is included in the script previously mentioned.

You don't have to write wrapper scripts like the one above; you can insert them only where needed:

RGBAdjust(rb=sc8f(8), gb=sc8f(-8), bb=sc8f(12)) 

Limiter(min_chroma=sc8x(128-64), max_chroma=sc8x(128+64))

These statements will have the same visual effect at all bit depths.