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The standard known as Rec.601 or CCIR-601 (Studio Encoding Parameters of Digital Television...) defines standard quantization levels: for luminance, black is 16 and peak white is 235. This is the normal luminance range. For chrominance, the range is 16 to 240, or 128±112. "The signal may occasionally excurse beyond" these ranges according to Rec.601.BT.601-7 (Annex 1, Table 3)

Luminance or chrominance values outside their ranges are known to cause problems with some TV sets, so Limiter was created to clip them (or clamp them - both terms are used) to the ranges [min_luma, max_luma] and [min_chroma, max_chroma], which you may specify, but by default are the Rec.601 values listed above.

Limiter can also colorize out-of-range pixels (with show) to warn of a potential problem. See the example images below. Naturally, this option should not be enabled for final output.

AVS+ By default all parameters are non-autoscaling – they must be scaled to the target bit depth.
By using parameter paramscale the range parameters are treated to be of 8 bits range and will be scaled to the current bit depth automatically


[edit] Syntax and Parameters

Limiter(clip clip [, int min_luma, int max_luma, int min_chroma, int max_chroma, string show ] )
AVS+ Limiter(clip clip [, float min_luma, float max_luma, float min_chroma, float max_chroma, string show, bool paramscale ] )

clip  clip = (required)
Source clip. YUV only.
AVS+ also supports YUVA.
int  min_luma = 16d
Y (luma) lower bound.
int  max_luma = 235d
Y (luma) upper bound.
int  min_chroma = 16
U & V (chroma) lower bound.
int  max_chroma = 240d
U & V (chroma) upper bound.
string  show = (empty)
Colorizes the pixels outside the specified [min_X, max_X] range.
If set, show must take one of the following values:
show Effect
"luma" colorizes out of bound luma
"luma_grey" colorizes out of bound luma, and makes the remaining pixels grey
"chroma" colorizes out of bound chroma
"chroma_grey" colorizes out of bound chroma, and makes the remaining pixels grey
The colorizing is done as follows:
  • YUY2: j,k=1,2 (chroma shared between two horizontal pixels p1 and p2: Y1UY2V)
  • YV24: j,k=1 (no chroma shared)
  • YV12: j,k=11,12,21,22 (chroma shared between 2x2 pixels Y11UY12V: Y21UY22V)
Pixel Condition Colorization
 show="luma"  show="luma_grey"
Yj < min_luma   red (pj)   red (pj)
Yj > max_luma   green (pj)   green (pj)
Yj < min_luma and Yk > max_luma   yellow (pj and pk)   puke (pj)  olive (pk)
 show="chroma"  show="chroma_grey"
U < min_chroma   yellow   yellow
U > max_chroma   blue
V < min_chroma   cyan
V > max_chroma   red
U < min_chroma and V < min_chroma   green
U > max_chroma and V < min_chroma   teal
U < min_chroma and V > max_chroma   orange
U > max_chroma and V > max_chroma   magenta
AVS+ bool  paramscale = false
If set, minimum/maximum luma/chroma values are treated as they were in 8 bit range (but non-integer values are allowed), limiter will autoscale them to the current bit depth of the clip.
Conversion uses the bit-shift method for integer bit depth conversion (from 8 to 8-16 bits). For 32 bit float clips the conversion is value/255.0 for Y (luma) channel and (value-128)/255.0 for U/V chroma channels. Do not use paramscale when you want to limit 32 bit float chroma exactly to +/-0.5, give the parameters directly instead.
Default: does not scale at all, parameters are used as-is. Parameters now are of float type to handle 32 bit float values..

[edit] Examples

# 1. Original image
Trim(4685, 4685)
# 2. Exaggerated luma and chroma range
Levels(16, 1.0, 255-16, 0, 255, coring=false)
Tweak(sat=2.0, coring=false)
# (insert Limiter here)
Limit Ex02.jpg
# 3. Limiter("luma")
Limiter(16, 235, 16, 240, "luma")
Limit Ex03.jpg
# 4. Limiter("luma_grey")
Limiter(16, 235, 16, 240, "luma_grey")
Limit Ex04.jpg
# 5. Limiter("chroma")
Limiter(16, 235, 16, 240, "chroma")
Limit Ex05.jpg
# 6. Limiter("chroma_grey")
Limiter(16, 235, 16, 240, "chroma_grey")
Limit Ex06.jpg
# 7. Putting it all together
\    Last
\  , StackVertical(
\       Histogram("levels", 0.5).Crop(Width, 0, 256, 256).AddBorders(0, 0, 0, Height/2-256)
\     , Histogram("color2"     ).Crop(Width, 0, 256, 256).AddBorders(0, 0, 0, Height/2-256)
\    )
\  , StackVertical(
\       BicubicResize(Width/2, Height/2).Limiter(16, 235, 16, 240, "luma_grey")
\     , BicubicResize(Width/2, Height/2).Limiter(16, 235, 16, 240, "chroma_grey")
\    )
\ )
Limit Ex07a.png

[edit] Changes

AVS+ r2724 Added support for float.

Added paramscale

AVS+ Added support for 8-16bit color spaces.
v2.60 Added support for YV24.
v2.56 Added show to show out of bound luma/chroma.
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