This is a test to composite for images that is hard to render at a time, for example a scene including many hairs.
Usually, such compositing needs overlay of alpha transparent images.
But in this post, I tested a way to overlay images with volume scatter environment, because it seemed to be easier than transparent images. And it is suit for scenes of deep sea as well.
Compositing for distant view:
First, I added volume scatter for volume in world environment. It produces many particulates in the atmosphere to scatter light, making it looks deep sea or night.
And made some parts of seagrass field image separately like this.
The images are combined by nodes for color > mix (change to add).
The volume scatter work well to overlaying images without post processing.
Compositing for near view:
Importing the resulting backdrop image as a plane, I placed it behind the seagrass.
This setting makes the object more clear. It is good for near view.
And volume scatter also work fine for depth between objects and backdrop.
The resulting swath of seagrasses trying to soak up sunlight.
The advantage of this setting is the way to update backdrop. If you want add more seagrass, you can import the latest image, then use as a backdrop recursively.