February 2024 – ALWIN FELIX's BLOG

Grain

Its important to check the grains, as it will affect the final output of the image and can be picked up in QC

regions especially cleaned up elements has tell tale signs of manipulation while doing QC, so its important to take care of those specific regions and fix grain in them.

Patch Changing Lighting

Wrong method

Patching the tracked with the clone stamp approach will look fine if the image is stationary but we will face lighting problems if the object changes position.

CC By hand

This is also an improper way to work since we hand animate the CC values between frames and this consumes a lot of time on production.
Also if the light changes drastically like a red or green tint suddenly the workload increases tremendously.

Slice Tool

We can use this to monitor the color values on an selected positions of length in the screen.

Frequency seperation patching

We can seperate the image based on Hi and low Frequency which means hi freq is going to be the little details like edges and wrinkles and pores and the low frequency is going to be the general average lighting of the image.
Blur the image to get the low freq and for high freq merge in blur with “from” operation to the original plate.
So what we are doing is we paint the high frequency like clone stamp this will transfer the details to the area and we copy the same to the low freq this will transfer the light data to the high freq clone thus blending it seamlessly.

Paint work divide / multiply

Interactive Light Patch

Light / Glow matching

Sometimes an bright source can create a haze or glow on the lens, this will washout the elements around it, so when comping our elements would look out of place, to tackle this we use curve tool and analyze the light source and apply that data to a grade to change the result.

Roto and Transform

An easy way to remove trackers or cleanup a plate is using track, roto and transform. its basically clone stamp tool but its now tracked.

UV map / ST map

In Nuke, STmap stands for Spatio-Temporal Map. It is a node that allows you to warp and distort an image based on a reference image or sequence of images. This can be used for effects such as morphing between two images, creating fluid simulations, or stabilizing shaky footage. The STmap node is commonly used in visual effects and compositing to create complex image distortions and transformations.

The Expression:

Red Channel:

(x + .5) / width

Green Channel:

(y + .5) / height

Adv. mask removal | Colne and vectors

Denoise the plate using a smart vector node if you have a good GPU and write it to the disk. The output may appear as a black screen, so change the AOV to smart vector.
Adjust the vector detail based on the shot’s needs. Higher detail results in slower processing time. Lower detail can be used for shots with less detail. Write the adjusted detail to disk, as this node can slow down the script.
Pick a specific frame, roto paint the markers, blur to avoid crisp edges, and frame hold.
Use a vector distort node (in wraped src mode) to carry this information to other frames by plugging in the smart vector to the painted frame.
Use a vectorToMotion node to convert the smart vector to motion vector to apply motion blur.
Regrain from the original footage and premult to finish the process.

Adv. mask removal | Colne and vectors | method 2

Another method of the above is to use STMap, we use this because it is very faster.

Adv. mask removal | Colne and vectors | Invert STMap | method 3

Through this method we can stabalize an whole image and apply corrections and reverse it to track through the surface

Adding or removing details from plate

We can add or remove textures in a plate with STmap

keying in Nuke – 13/2

In Nuke, keying is the process of isolating specific elements, typically from a video or image, based on their color or luminance values. This is often done to remove backgrounds, separate objects, or create matte elements for compositing.

HSV

HSV Color Scale: The HSV (which stands for Hue Saturation Value)

scale provides a numerical readout of your image that corresponds to the color

names contained therein. Hue is measured in degrees from 0 to 360

R = HUE: Hue literally means colour

G = Saturation: Saturation pertains the amount of white light mixed with a hue.

B = Luminance: Luminance is a measure to describe the perceived brightness of a colour

Hue corrections

Luminance Key

Can be used to create mask with the luminance value of an image.

This can be used to key various channels like RGB/

Luma key can also be used is various ways like experimenting with different color spaces.
Here is an example with keying red channel in various color spaces.

Hue / Luma separator

We can separate Hue from an image by dividing the luminance with the colour of the image in the merge division mode.

Color Difference Key

We can separate individual colors by negating colors from two channels using merge minus function.

Adding Channels

We can create separate channels for certain needs for example we can create a Roto and add it as a new channel and we can use grade or blur node’s mask as the new channel which was created. These Channels are separate from alpha so it is only visible on its assigned name.

ChromaKeyer

ChromaKeyer is basic keying node useful for quick color keying and it is not accurate around hairs or other tiny moving objects.

Image Based Keyer – IBK

IBK stands for Image Based Keyer It operate with a subtractive or difference methodology. It is one of the best keyers in NUKE for getting detail out of fine hair and severely motion blurred edges.

IBK Color: This node we can use it to make up the clean plate for the background of the subject.

IBK Gizmo: With this node we can make use of the produced clean plate of the IBK Color and aid it to key the subject especially for hairs.

KeyLight

Key light It is a really great keyer over all and does color de-spill.

Primatte

Primatte is what is called a 3D keyer. It uses a special algorithm that puts the colors into 3D color space and creates a 3d geometric shape to select colors from that space.

Ultimatte

The advantage is you can get phenomenal keys that can get really fine detail and pull shadows and transparency from the same image.

Green De-spill

It’s the process of removing green scatter around the subject which occurs mainly due to the green screen.

We first key the green using key-light and after the comes the difference function which will extract the green only from the image and then we desaturate the green and we add back to the image.

we can use the same principle for the blue screen.

De-spill map

We can create a De-spill map with the help of minus function with the color function. Here we use Green – Red thus getting a alpha as a result which can be used for de-spill.

Edge extend

Sometimes we will get black halo/edges in an keyed footage, to eliminate this we use edge extend, It works by extruding the color values around the edge of the alpha and we can use this value to reduce the black halo which will be appearing.

Fixing uneven green screen

Having an irregular green screen will develop problems down the pipeline especially during keying. so to counter that we need to even out the greenscreen in comp.