Creating lightening flash with curve tool

For this (Below) shot from Valiant I was asked to add some lightning flashes in the clouds. Thought about it for a second and realized I am not the best animator so i turned to the curve tool.  

First thing I did was go to youtube and find reference that I liked. I picked this video.

I brought my footage into nuke from youtube hooked up the curvetool and picked an area I liked.

I added the time offset just to get the footage to frame 1.

Picked the area I liked for the curve tool to calculate

Then I hit go in the CurveTool tab and let it calculate.

Here is the image we will add the curve to.

Added a keyer to my clouds background to create an alpha of the area I wanted to have effected.

I copied and pasted the CurveTool Intensity Data tab into an exposure node in all three channels RGB.

This was my result!

You can also change your keyer to get different results or add an expression to the exposure tool to make it brighter or darker.

simple expression multiplying it by 2

Nuke Curve Tool

The curve tool is great for a few things. I am going to show you how to use the autocrop feature of the curve tool.

The curve tool is under the very first tab in the nodes panel in the image section.   You can always hit Tab and type curves and it will come up.

What the autocrop feature does is it looks for an alpha then crops your bounding box to fit the alpha channel. This can save some time in render and just over all workflow. 

 

Below is the same image but its alpha channel and bounding box.  

Alpha Channel 

I hook up the curve tool to the node I want it to analyze and under the curve type I change it to auto crop then click go!

Once it is done analyze you will get a data set under the AutoCropData tab.  

Now all you have to do is add a crop node and hold command/ctl and click the animation menu from the curves animation menu to the crop animation menu.

Here is our new bounding box! Now instead of reading a 4k or 2k plate for the whole image it will only update what is in the bounding box! 

Here is what my end result nodes look like. 

This trick can save you a lot of time when using large images. Sometime it can also take a while to analyze so you have to figure out if the analyze wait is worth the end result.