Levels

Adjusts brightness, contrast, and gamma. This is done using the following transfer function:

output = ( (input - input_low) / (input_high - input_low) )^{(1 / gamma)} * (output_high - output_low) + output_low

• input_low and input_high determine what input pixel values are treated as pure black and pure white.
• output_low and output_high determine what output values are treated as pure black and pure white.
• gamma controls the degree of non-linearity in the conversion.

This is one of those filters for which it would really be nice to have a GUI. Since we can't offer a GUI (though AvsP does), we at least make this filter compatible with VirtualDub's when the clip is RGB. In that case you should be able to take the numbers from VirtualDub's Levels dialog and pass them as parameters to the Levels filter and get the same results. Unlike VirtualDub's filter however, the input and output parameters can be larger than 255.

When processing data in YUV mode, Levels only gamma-corrects the luma information, not the chroma. Gamma correction is really an RGB concept, and is only approximated here in YUV. If gamma=1.0 (unity), the filter should have the same effect in both RGB and YUV modes. For adjusting brightness or contrast in YUV mode, it may be better (depending on the effect you are looking for) to use Tweak or ColorYUV, because Levels changes the chroma of the clip.

Syntax and Parameters

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

clip  clip = (required)

Source video.

int  input_low = (required)

Higher input_low darkens the output; input values below input_low are clipped, that is, rounded up to input_low.

float  gamma = (required)

Gamma adjustment. See examples.

If 1.0, there is no change to the output gamma.

int  input_high = (required)

Lower input_high brightens the output; input values higher than input_high are clipped, that is, rounded down to input_high.

int  output_low = (required)

Dark values brighten to gray as output_low becomes larger.

int  output_high = (required)

Light values darken to gray as output_high becomes smaller.

bool  coring = false

When true (the default), input luma is clamped to [16,235] and the chroma to [16,240]; this clamped input is scaled from [16,235] to [0,255], the conversion takes place according to the transfer function above, and then output is scaled back from [0,255] to [16,235]. When false, the conversion takes place according to the transfer function, without any scaling.

coring was created for VirtualDub compatibility, and it remains true by default for compatibility with older scripts. In the opinion of some, you should always use coring=false if you are working directly with luma values (whether or not your input is [16,235]).

bool  dither = false

When true, ordered dithering is applied to combat banding. Default is false.

Examples

# does nothing (unity transfer function):
Levels(0, 1, 255, 0, 255)

# the input is scaled from [16,235] to [0,255], the conversion [0,255]->[16,235] takes place 
# (accordingly to the formula), and the output is scaled back from [0,255] to [16,235]
# (for example: the luma values in [0,16] are all converted to 30)
Levels(0, 1, 255, 16, 235)

# gamma-correct image for display in a brighter environment:
# example: luma of 16 stays 16, 59 is converted to 79, etc.
Levels(0, 1.3, 255, 0, 255)

# invert the image (make a photo-negative):
# example: luma of 16 is converted to 235
Levels(0, 1, 255, 255, 0)

# does nothing on a [0,255] clip; does nothing on a [16,235]:
Levels(0, 1, 255, 0, 255, coring=false)

# scales a [0,255] clip to [16,235]
Levels(0, 1, 255, 16, 235, coring=false)  
# note both luma and chroma components are scaled by the same amount,
# so it's not exactly the same as ColorYUV(levels="PC->TV")

# scales a [16,235] clip to [0,255]:
Levels(16, 1, 235, 0, 255, coring=false)  
# note both luma and chroma components are scaled by the same amount,
# so it's not exactly the same as ColorYUV(levels="TV->PC")

# makes a clip 100% black
Levels(0, 1.0, 255, 0, 0)

# apply fading on gamma corrected source (same holds for resizing and smoothing else well)
clip = ... 
gamma = 2.2
clip.Levels(0, gamma, 255, 0, 255) # undo gamma (also called gamma correction)
FadeOut(n)
Levels(0, 1.0/gamma, 255, 0, 255) # redo gamma

Changes

v2.60	added dither
v2.53	added coring (true by default, which reflects the behaviour in older versions)