Nuke Colour Correction

Here are just notes I had taken down or copied from the book. I don’t claim this as my own information, I merely decided to copy it down so that I can absorb the information better.

  • Nuke is a 32-bit float linear colour compositing application.
  • Most compositing and image-manipulation programs are 8-bit.
  • 8 bit is good for displaying colour, but not good enough for some calculations or operations
  • It may result in banding – inaccurate display of gradients, change in colour happens abruptly rather than smoothly.
  • Nuke bumps 8 bit and 16 bit up to 32 bit
  • Float – 8 bit = 256 colour 1 = black, 256 = white, 128 = grey.
  • 32 bit allows for what is brighter than white (super white) or darker than black (sub-blacks), 8 bit colours would get clipped. E.g. Paper and light bulb in a room. Different shades of white = float. if they were the same shade =  Non Float.
  • Linear means linear colour space.
  • Computer monitor doesn’t show an image as the image appears in reality <- I learnt this in the lighting masterclass.
  • Has a mathematic curve called gamma that is use to display images. Most often they have a gamma curve called SRGB.
  • Because it’s not showing the image as it appears in reality the image needs to be corrected.
  • Process represents basic colour management.
  • If a gamma curve has been applied to make a middle grey in an image, it will react different to colour correction.
  • Correct way to manipulate imagery – in whatever way – is BEFORE the gamma curve has been applied to an image.
  • Correct way: Linear Image > Colour correct it > Composite it > Transform it > then apply a reverse gamma correction as well as negating the correction you just applied. Nuke works like this by default.
  • In Nuke 0 is still black but white is now 1 to make it simpler.
  • Dynamic range changes from image  – Image usually has black, white and everything in between.
  • Dynamic ranges are split according to their brightness values:
  • Shadows or low lights – darkest colours
  • Midtones – neither dark or bright
  • Highlights – Brightest colours
  • In Nuke there is also:
  • Super whites
  • Sub blacks
  • Colour > Maths > Add – Add operation adds colour to everything uniformly. The whole image changes together.
  • Colour > Maths > Multiply > Strongly boosts the highlights quickly.
  • Colour > Maths > Gamma
  • Gamma deals with midtones
  • Colour > Roll off Contrast > Contrast Value low lights get pushed down and highlights are pushed up.

Centre Value

  • Colours above the Centre Value will be considered bright and pushed up.
  • Colours below the Centre Value will be considered dark and pushed down.
  • Similar to Multiply Node – Virtually the same
  • Lift is virtually the same as Roll off contrast
  • Lift – Value looked at 1, the farther the values are away from 1, the bigger the effect.
  • Multiply operation multiplies the dynamic range by a value. Multiply operation is virtually the same as changing the exposure in a camera or increasing light.
  • I/O Graph – Input vs Output graph
  • x axis – colour coming in
  • y axis – colour going out.
  • Perfectly diagonal line represents no colour correction.

Expression 1 Script

  • Reformat – defines resolution of image. this case it’s 256 x 256 > Ramp (from draw tool box) generates ramp, have a black to white horizontal ramp from edge to edge > Backdrop – Highlights areas in the tree (from Other toolbox). Tells you where to add your colour correction nodes in the next step > Expression (in Colour > Math toolbox) lets the user write an expression with which to draw an image.
  • Can do with an expression node:
  • Simple colour operations (Adding, multiplying but wasteful)
  • Complex warps or redrawing of different kinds of images all together.
  • In this it draws values of a horizontal black to white ramp on – screen a white pixels in the corresponding weight in image.
  • Gamma change midtones without changing black or white.
  • Multiply – Affects highlights more than low lights.
  • Add – Changes everything all together.
  • Contrast pushes two parts of the dynamic range away from one another.
  • Centre value at 1 exact same as multiply curve
  • Centre moves the low lights and highlights point.
  • Centre value at 1 – White point locked while everything else change – opposite of multiply.
  • Soft clip smooths out the edges of the curve so the colours don’t suddenly turn to black or white.
  • Images need to be unpremultiplied then premultiplied again.

  • Curve tool Max Luma Pixel – finds the brightest and darkest pixels in the image.
  • Image – Constant – Temporarily replaced black background with middle grey background.
  • Colour Correct Node  controls for:
  • Saturation
  • Contrast
  • Gamma
  • Gain (Multiply)
  • Offset (Add)
  • Range tab shows graph
  • Highlights = White
  • Midtones = Grey
  • Shadows = Black
  • Green/magenta mix of two ranges.

This is from the files the book provides as exercises

This was before the colour correction, you can clearly see that the car doesn’t match the background plate.

This is how I managed to achieve this:

  • Car’s matching blacks and whites
  • Ctrl and drag Curve tool 1 Minimum Luminance Pixel Animation > Grade 1’s Black point animation.
  • Ctrl + drag Maximum LP Animation > Grade 1’s White point animation.
  • Background ctrl + drag Curve tool 2 Minimum Luminance Pixel Animation > grade 1’s Lift Animation
  • Ctrl + drag Maximum Luminance Pixel Animation > Gain
  • This has matched the foreground’s shadows and highlights to those of the background.
  • Viewer – Y – Luminance = Greys
  • Edit gamma with the help of multiply.
  • Still need to change colour – look into red channel.
  • Have to change multiply + offset values for perfect results – distance from car from camera means black point will be higher and white point will be lower.

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