As I continue to use Nuke in ways in which it was never intended to be used (read: motion graphics), I keep finding small bits of friction that I just can't help but remove with app customizations.

My latest annoyance stems from an animated project that involved more traditional motion-graphics-style animation than the typical interface design and animation I usually create. I built all the graphic assets I would need for the video ahead of time, then assembled and animated them into a sequence, entirely in Nuke.

Again and again, I would merge a new graphic asset onto my shot, and I would have to do some math to figure out how to transform it into the center of the frame. Since the origin (0,0) of a Nuke frame is the bottom left corner, by default, images show up in the lower left of the frame rather than the center. Which is not what I want.

So, I'd add a Transform to the asset and move it to the center of the 1920 x 1080 frame. Since I care about precision, I didn't just eyeball the transform. I want it to be exact.

As long as I add a Transform to a graphic element with the upstream node selected, the Transform will detect the width and height of the asset and place the transform jack in the center of the object. As a Nuke user, you already knew that.

Then, I place my cursor in the x translate parameter box and type 1920/2 - whatever value was in the x center position, as determined by the upstream node. I repeat this process for the y translate parameter, using 1080/2 to match the frame's height.

And lo, we have discovered another simple, math-based operation, prone to human error, ripe for automation. The formula is simple:

The x translate parameter should be defined as half the frame width minus half the asset width.
The y translate parameter should be defined as half the frame height minus half the asset height.
If we have added the Translate node directly to the asset — which is to say we have not added it to our script unconnected — the x center and y center parameters will be automatically filled with the half-width and half-height values of our asset.

In Nuke Python, this formula would be expressed as:

n = nuke.thisNode()

# Get the x and y values of the Transform's center point
xVal = n['center'].value(0)
yVal = n['center'].value(1)

# Get the width and height of the frame format
rVal = nuke.Root()
xfVal = rVal.width()
yfVal = rVal.height()

# Define the variables to set the translate values
txVal = n['translate'].value(0)
tyVal = n['translate'].value(1)

# Find difference between center of frame and center of transform
cxVal = xfVal/2-xVal
cyVal = yfVal/2-yVal

# Translate to center of frame format
n['translate'].setValue(cxVal, 0)
n['translate'].setValue(cyVal, 1)

Next, we take that nicely formatted Python script and shove it into an addOnUserCreate function within our Menu.py file thusly:

def OnTransformCreate():
  nTR = nuke.thisNode()
  if nTR != None:
    script="n = nuke.thisNode(); xVal = n['center'].value(0); yVal = n['center'].value(1); rVal = nuke.Root(); xfVal = rVal.width(); yfVal = rVal.height(); txVal = n['translate'].value(0); tyVal = n['translate'].value(1); cxVal = xfVal/2-xVal; cyVal = yfVal/2-yVal; n['translate'].setValue(cxVal, 0); n['translate'].setValue(cyVal, 1);"
    k = nuke.PyScript_Knob('center_trans', 'Center Transform')
    k.setCommand(script)
    nTR.addKnob(k)

nuke.addOnUserCreate(OnTransformCreate, nodeClass="Transform")

Now, every Transform node created will have a nice big "Center Transform" button added to it automatically.

So, when I bring in a 584 x 1024 graphic asset like, say, this:

And I merge it over a 1920 x 1080 background...

...add a Transform node — which will find the center point to be (292,512)

All I have to do to center my graphic asset is click this button...

...and boom. Automated.

Update – 2020-04-20

Back in January, reader Birger sent me an email explaining his method for centering non-root-sized images over a background.

He writes:

For me, the easiest way would be to put a reformat (1920x1080) underneath the asset and set resize type to none. Would that work for you too?

As I replied to Birger, this will definitely accomplish the same thing once you tick the "black outside" checkbox. Additionally, the Reformat node concatenates just like the Transform node, so if you need to stack transforms, you wont lose any quality due to filtering.

The only arguments I can make for using my version over a Reformat node are:

I like to see a Transform node on the node graph when I'm repositioning things because it helps me understand what's happening at a glance.
When I'm working, I often put down a Transform node before I know I need something centered, so it's easier for me to just click the "Center" button.
In the event that I want to start with a centered image and then move it slightly off-center, I can use the same node, center the object, then move from there. But I probably wouldn't do that since I can add an additional Transform after the Center operation and the nodes would concatenate into a single Transform operation, so this one isn't really valid.

Anyway, thanks Birger for the additional solution!.