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27/06/2009

Adding noise in The Gimp

The noise (or the “grain”) in an image is often an unwanted effect when shooting at high ISO level, specially on older SLRs like Nikon’s D70.

In some cases, though, we want to add new noise to give the shot an oldish look, as if it were shot a long time ago.
Getting such an effect in The Gimp is quite simple. We’ll take a look at the options we have.

We take the initial shot, in NEF, and we import it into The Gimp with UFRaw.

Once in The Gimp, we convert it to black and white. In this case, I’ve used the Channel Mixer (take a look at my first tutorial on converting an image to B&W), 100% Red, 50% Green and 0% Blue, without selecting “Preserve Luminosity”. This gives the bonus of a slight over-exposition of the image.

Now we have to add the noise. The Gimp gives us several options, some really simple, some really complex.
All the filters are on “Filters > Noise”.

I usually use “HSV Noise”, though others will work, also.

In the case of “HSV Noise”, I usually play with “Holdness” and “Hue” in order to get the desired amount of noise.

The problem with these filters is that they provide “pixel level noise”, that is the noise they apply to the image has the size of a pixel. As resolutions go up, the size of a single pixel decreases in relation to the total size of the image.

So, what I do is:

  1. Create a new layer, about 33% the size of the image.
  2. I fill it with solid black colour.
  3. I pick “HSV Noise” with these settings.

    This creates the noise layer. We can play with the “Holdness” slider to get more or less noise. The other values give the color of the noise (white, in our case).
  4. Now I resize this layer to 100% of the image.
  5. Finally, one can play with noise layer’s opacity and mixing mode to get the desired effect. “Overlay” or “Soft Light” give good results.

This is the result. Since resizing the image “kills” the noise, this is just a thumbnail. Click on it to see the image full-size:

pic-3

There are more ways to add noise, for example the script “Add Film Grain” you’ll find in “Filters > Noise”. This is a Script-Fu script, which has a lot of options and creates a multi-layer image.

As usual, just experiment. The Gimp is free as in beer and as in speech.


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