Image: Color & Levels#

Color correction, levels, remapping, keying.

15 nodes.

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Contents#

• COP Chroma Key (PCG) — Keys out a colour (green or blue screen) into a matte, with optional despill.
• COP Clamp (PCG) — Clamps values to a minimum and maximum.
• COP Color Correct (PCG) — Adjusts exposure, brightness, contrast, gamma, saturation and hue in one node.
• COP Despill (PCG) — Removes colour spill (for example green reflected on a subject) without producing an alpha matte.
• COP Equalize (PCG) — Shifts or scales the selected channels so that their measured minimum, maximum or average hits chosen target values.
• COP Fit (PCG) — Remaps the measured minimum and maximum of each layer to a chosen output range and reports statistics.
• COP Function (PCG) — Applies a per-pixel unary function such as abs, pow, exp, log, sin, frac, step, smoothstep, mod or sigmoid.
• COP Histogram (PCG) — Equalizes a layer's histogram or matches it to a reference image.
• COP Invert (PCG) — Inverts layer values as Pivot - x.
• COP Levels (PCG) — Remaps values with input range, gamma and output range.
• COP Mono to RGB (PCG) — Maps a single-channel layer through a color ramp to produce an RGB false-color layer.
• COP Quantize (PCG) — Posterizes values into a fixed number of discrete steps.
• COP Remap (PCG) — Maps values through a gradient lookup table.
• COP Tone Map (PCG) — Compresses HDR values into the 0-1 range with a selectable tone-mapping curve.
• COP Vignette (PCG) — Darkens (or brightens) the image towards the edges with a soft radial falloff.

Nodes#

COP Chroma Key (PCG)#

PCG_COPChromaKey · category PCG/cop · Houdini: Chroma Key (COP)

Keys out a colour (green or blue screen) into a matte, with optional despill.

Three matte methods are available:

• distance (default, specific to this plugin): measures the distance to the key colour in the YCbCr chroma plane, normalised by luma, ignoring brightness, so shadows and highlights on a green screen are keyed out together. Tolerance and Softness control the transition.
• chromakey: Houdini's legacy Chromakey, with a hue, saturation and luminance range and Rolloff values that extend a soft edge beyond each range.
• channel_separation: Houdini's Channel Separation, which subtracts the maximum of the two other channels from the screen's main channel and applies Threshold and Width.

Despill is an extension: the amount by which the key colour's main channel exceeds the average of the other two is reduced. The matte is written to Alpha Layer (1 keeps the pixel, 0 removes it).

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Outputs

Parameters

Key

Rolloff

Despill (PCG)

Visualize

Compared with Houdini

The distance method and Despill do not exist in Houdini. Use COP Despill when only the spill removal is wanted.

Tips

• Raise Softness before Tolerance if the edge looks hard; raise Tolerance if shadows on the screen remain.

COP Clamp (PCG)#

PCG_COPClamp · category PCG/cop · Houdini: Clamp (COP)

Clamps values to a minimum and maximum.

Limits every channel of the selected layers to the range Min to Max. Values are only changed by this node; most other COP nodes deliberately do not clamp, so this is the place to bring HDR values back into range.

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COP Color Correct (PCG)#

PCG_COPColorCorrect · category PCG/cop · Houdini: Bright (COP), Color Correct (COP), Contrast (COP), Gamma (COP), HSV Adjust (COP)

Adjusts exposure, brightness, contrast, gamma, saturation and hue in one node.

Applies, in this order: exposure (multiply by 2^EV), brightness (add), contrast (scale around Pivot; 0.18 is mid grey in linear space), gamma (values above 1 brighten, the same direction as Levels), saturation (interpolates towards the Rec.709 luminance, so luminance is kept) and finally an HSV step (hue rotation, saturation multiplier, value multiplier). With a Mask Layer the result is blended back over the original. Alpha is not changed.

Saturation and the HSV step only apply to layers with at least three channels.

Inputs

Outputs

Parameters

HSV

Compared with Houdini

Combines Color Correct, HSV Adjust, Bright, Contrast and Gamma into a single node.

COP Despill (PCG)#

PCG_COPDespill · category PCG/cop · Houdini: Despill (COP)

Removes colour spill (for example green reflected on a subject) without producing an alpha matte.

The channel in which Spill Color is largest is limited to a ceiling built by mixing the other two channels with Bias; the part removed is the spill. Luminance can add the removed brightness back so the result is as bright as the input. The removed colour is written to a separate Spill Layer. Use COP Chroma Key when a matte is needed.

On pure colours (where the other two channels are equal) Bias has no visible effect.

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Parameters

Compared with Houdini

Matches Despill: it only removes spill and does not output alpha.

COP Equalize (PCG)#

PCG_COPEqualize · category PCG/cop · Houdini: Equalize (COP)

Shifts or scales the selected channels so that their measured minimum, maximum or average hits chosen target values.

Measures the minimum, maximum and average of a luminance (chosen by luminance_type) and then moves all selected channels together to the target: stretch to Black/White, move the minimum to Black, move the maximum to White, move the average to Target Average, or scale so the maximum length hits White.

This is not histogram equalization; for that use COP Histogram.

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Outputs

Parameters

COP Fit (PCG)#

PCG_COPFit · category PCG/cop

Remaps the measured minimum and maximum of each layer to a chosen output range and reports statistics.

Rescales values so that the actual minimum and maximum found in the layer land on out_min and out_max. COP does not clamp values, so the real range of an image (noise in plus/minus 1, an SDF in hundreds of pixels, depth in metres) is often unknown; Fit is the quickest way to inspect it without knowing the range in advance. The second output is a one-line statistics text for the chosen layer.

By default all channels share one range, which preserves hue. With per_channel on, each channel is stretched on its own, which changes color relationships. A constant layer (min equal to max) is written as out_min without dividing by zero.

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Parameters

COP Function (PCG)#

PCG_COPFunction · category PCG/cop · Houdini: Function (COP)

Applies a per-pixel unary function such as abs, pow, exp, log, sin, frac, step, smoothstep, mod or sigmoid.

Equivalent of Function. The function is applied to every channel of the selected layers.

The node never produces NaN: log and sqrt of negative numbers give 0, and the reciprocal of 0 gives 0. A NaN would be smeared by a downstream blur and turn the whole image black with no visible source.

pow keeps the sign (x^a applied to the magnitude). step is 1 where x >= a. smoothstep runs from a to b. mod is x mod a (result has the sign of a; 0 when a is 0). sigmoid is the logistic function.

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Parameters

COP Histogram (PCG)#

PCG_COPHistogram · category PCG/cop

Equalizes a layer's histogram or matches it to a reference image.

Works by rank rather than by binning, so the result is exact and has no bin steps. The mapping preserves order, and ties get their average rank, so flat color regions are not stretched into gradients (the classic histogram-equalization pitfall).

• Equalize: the values of each channel are spread evenly over that channel's own original [min, max].
• Match Reference: each channel takes the value found at the same quantile in the reference image. Useful for pulling a batch of AI-generated textures with inconsistent colors onto one reference. Matching an image to itself is the identity.

Channels are processed independently, so matching RGB will also pull hue toward the reference. To match only brightness, convert to luminance first. The node needs the CPU (sorting), so GPU layers are handled through numpy.

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COP Invert (PCG)#

PCG_COPInvert · category PCG/cop · Houdini: Invert (COP)

Inverts layer values as Pivot - x.

Every value x becomes Pivot - x. The default pivot of 1 gives the usual photographic negative. The alpha channel (the fourth channel) is left alone unless Invert Alpha is enabled.

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COP Levels (PCG)#

PCG_COPLevels · category PCG/cop

Remaps values with input range, gamma and output range.

The classic Levels operation:

t   = (x - In Black) / (In White - In Black)
t   = t ^ (1 / Gamma)
out = Out Black + t * (Out White - Out Black)

The default settings are a bit-exact identity. Gamma uses the reciprocal exponent so that raising Gamma brightens the image, the Photoshop / Nuke / Houdini convention (the opposite of pow(x, gamma)).

Clamp Input is on by default; turn it off for SDFs, normals and HDR data. With clamping off, t can be negative, and a plain power of a negative number would give NaN, so the node uses sign(t) * |t|^g instead, which is continuous at 0, monotonic and odd-symmetric (for an SDF it compresses or stretches inside and outside symmetrically).

Setting Out Black to 1 and Out White to 0 inverts the values. For SDFs, Out Black 0 and Out White -1 negates them (swaps inside and outside).

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Parameters

COP Mono to RGB (PCG)#

PCG_COPMonoToRGB · category PCG/cop · Houdini: Mono to RGB (COP)

Maps a single-channel layer through a color ramp to produce an RGB false-color layer.

The value is normalized as t = (v - Input Min) / (Input Max - Input Min) (or the layer's own min and max with Compute Range), the ramp is evaluated at t, and the color is scaled into the Output Min..Output Max intensity range. With Scale by Value, the color is additionally multiplied by the unclamped t, so values beyond the range keep getting brighter. If Input Max equals Input Min, t is 0.

Outside Range decides what happens to t outside 0-1: Clamp at End holds the end colors; Repeat takes the fractional part (a value exactly at the top of the range still gets the last ramp color).

The ramp is written as position:r,g,b entries, for example 0:0,0,0 1:1,1,1. If a Ramp Layer is given, colors are instead read along the horizontal middle row of that layer, with t = 0 at the leftmost and t = 1 at the rightmost pixel. To simply copy one channel into three, use COP Channel Convert.

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Parameters

COP Quantize (PCG)#

PCG_COPQuantize · category PCG/cop · Houdini: Quantize (COP)

Posterizes values into a fixed number of discrete steps.

Computes round(x*(N-1))/(N-1), so the end points are preserved: Steps 2 gives {0, 1} and Steps 4 gives {0, 1/3, 2/3, 1}. A floor(x*N)/N formulation would never reach 1.0 and drop the brightest level.

Values outside the Range are not clamped; they continue to be quantized on the same grid. Quantizing an SDF therefore produces evenly spaced concentric bands, useful for contour lines. Useful as a step before cel shading or banded dithering.

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COP Remap (PCG)#

PCG_COPRemap · category PCG/cop · Houdini: Remap (COP)

Maps values through a gradient lookup table.

The ramp uses the same syntax as COP Gradient and Attribute Fade, for example 0:0 0.5:1 1:0, and the right of the colon may be r,g,b. The behaviour is chosen automatically from the number of ramp components.

A 1-component ramp is applied per channel, keeping the channel count; use it for grey curves, contrast and level compression. A multi-component ramp first reduces the image to a scalar t (first channel or Rec.709 luminance, see Mono Source) and then looks up that many channels. This is the greyscale to false-colour route, e.g. colouring SDFs, depth or noise, or turning a distance field into contour bands.

The lookup runs in NumPy, so on a GPU image there is a round trip; avoid it in per-frame hot spots.

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COP Tone Map (PCG)#

PCG_COPToneMap · category PCG/cop · Houdini: Tone Map (COP)

Compresses HDR values into the 0-1 range with a selectable tone-mapping curve.

Applies exposure (value x 2^Exposure, negative values clamped to 0) and then one of the curves: Reinhard, Reinhard Extended (where White maps to 1), ACES (Narkowicz fit), Hable (Uncharted 2, with White as the white point) or Exposure Only, which just clamps. Only the RGB channels are changed; alpha is left alone. The result is clamped to 0-1.

Enable Encode sRGB only on images you are going to look at directly; leave it off for images that still go into compositing, since compositing expects linear values.

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COP Vignette (PCG)#

PCG_COPVignette · category PCG/cop · Houdini: Vignette (COP)

Darkens (or brightens) the image towards the edges with a soft radial falloff.

Multiplies the RGB channels by 1 - Amount x smoothstep(Radius - Softness, Radius, d), where d is the distance from Center. Distance is measured so that 1 is reached at the corners of a square image. With Roundness 1 the falloff is a circle based on the short side; with 0 it is an ellipse that follows the frame. Alpha is not changed.

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