i posted a matching grain article a while ago and this is a follow on.
Below is one method of match grain when replacing part of an image or background.
Look at this image, it is part of a man’s shoulder and the background could do with evening out or replacing altogether.
Here I have just selected and deleted the back ground to white. It does not look at all right.
Above a blur might clean up the background. Whilst evening out the background it still does not match very well.
Here I have added some grain (noise) but it still does not match. If I apply a blur to this then we can achieve a better result.
With a slight blur its much better and using the correct selection technique for the original background selection it looks fairly convincing. Using this matching grain technique and varying the amounts of grain and blur ratios and perhaps even repeating the process a few times along with varying the type of noise, we can achieve different patterns of grain to suit nearly every situation.
