I have rendered an image and am doing composition on it. Is it possible to apply the composition to the already render image without rerendering it completely? It would save me a lot of time.
Remove the Render Layers and the Composite node. Then use the Image node under Add > Input > Image and import the image of your render. Then do your compositing, and use the File Output node under Add > Output > File Output.
To output the file, use the Render Image button again. It should not re-render your scene.