Hello chadi!
My preference is to not mess with the original plate and add grain only to the areas that were comped (in your case the CG you added).
Here’s how I usually work:
– Add a Regrain node after the comp work is complete (usually right before the Batch output).
– Attach your finished comp to the Front tab, the original plate to the Back tab, and a matte of the comped areas to the Matte tab.
– In Regrain’s blue pop-up menu, select “Custom” as the grain type.
– Press F4 (I think, not at the machine now!) and sample the grain of the original plate by CTRL+dragging a box over a portion of the image that’s neutral (grey or white). Sometimes I take two samples one for highlights and one for shadows.
If your sample was successful, the X & Y grain-size sliders will move and the grain histogram will show points on the RGB curves for every sample you made. This curves graph is the key to accurately matching grain and is only available in the Custom and Mono grain type selections.
Random thoughts:
Flame’s attempt at matching the grain is never 100% correct. In fact, I’d say that it’s wrong most of the time (like when it gives X & Y grain size of 100 for R, G, & B… which NEVER is right). But it’s a starting point. The XY sliders control the size of the grains. In general, the G channel will have the smallest grain, R will be a bit larger than G, and B will be the largest (and why I find it crazy getting all “100”s).
The curves control the strength of the grain (the graininess) per RGB channel, across the luminance range from black to white and they are essential to matching grain. Once an original plate goes through grading, all kinds of weird things happen to the grain. Often you’ll see in high-contrast grades that there is zero grain at all at black or white as it’s just been clipped off. If the colorist pushes a certain selective (maybe the product is red?) then the graininess of the R channel is going to spike at a certain luminance.
To make things easier on your eyesight, vertically expand the grain curves / histogram by holding down SPACE + Z (zoom) and dragging upwards from the baseline of the histogram. Most points on your graph will end up in the bottom 25% of the curve window so by vertically scaling it you get a lot more working room.
Once you’ve made your starting sample or two, start adjusting grain channel-by-channel. SHIFT+R displays a monochrome version of the red channel (SHIFT+G and SHIFT+B for the other two, SHIFT+Z to go back to full color). Toggle between Back and Result so you can see what Regrain is doing, adjusting grain size and adding or manipulating points on the curves to match the grain distribution. Look at all parts of the image, hilights, mids, and shadows. I usually end up with three points plotted per channel.
When finished, disconnect the Back input as it can create edge issues around the matted region.
Alternatively you can use Regrain for comping your elements over the Back input. For simple fixes I do this as it’s doing the regrain and the A-over-B comes for free.
