Post-Processing Workflow in GIMP

Colors

Typical color problems are color casts and incorrect brightness or contrast. For automatic enhancement there are tools found in the Colors -> Auto menu. If those don't give the desired results, manual adjustments can be made with the level tool (Colors -> Levels).

Level tool

Contrast

As a rule of thumb, a the darkest spot in the picture should be completely black and the brightest should be completely white. The distribution of tones can be seen in the histogram (here under "Input Levels"). The histogram above is from this image:

Foggy image

The contrast is obviously lacking — the snow is gray and black objects are not black. To correct this, open Colors -> Levels. Under 'Input Levels', move the black triangle (which indicates the black point) to the left side of the graph and the white triangle (the white point) to the right side of the graph. The result should be an image with more contrast:

More contrast

Usually this is enough to make any scene look good. However, the snow is still too dark. To increase the lightness of the entire scene, move the middle (grey) triangle to the left:

Contrast done

Likewise, if the scene should be darker, move the triangle to the right.

To gain more control, use the 'curves' tool (in the Levels tool, press 'Edit these Settings as Curves').

Color casting

Sometimes the white balance is off due to either bad settings or complex lighting in the scene. This should be solved with white balance in camera settings or by shooting in raw mode. In this case, white balance was left in 'tungsten' mode:

Blue street

In the JPEG format a lot of color information is lost. However, there is hope. The 'Levels tool' (Colors -> Levels tool) provides three eye droppers — black, gray and white. With those you can point out which areas in the image are black, gray or white by selecting the appropriate dropper and clicking in the image:

Corrected cast

In this image, I used the gray eyedropper on the painter's shoes. At the first glance the result looks fine, but the shadows are unnaturally blue. With the colors only slightly off, this method works much better.

If there are no gray or white spots, some manual work with the Color balance tool (Colors -> Color balance) can help.

Graduated neutral density filter

One of the irreplaceable filters is the graduated neutral density filter. Yet not always is one available. In this picture the upper half should be darker:

Mountain

A neutral density filter can be imitated:

  1. add a new [transparent] layer;
  2. with the gradient tool create a black-to-transparent gradient which imitates the filter (in this case, the hard-edge filter):

    Gradient

  3. in layers dialog, change the gradient layer mode to "Soft light":

    Soft light

  4. if the filter is too weak, duplicate the layer that contains the filter. If it is too strong, lower the layer's opacity.

Of course, this won't help with blown-out highlights.

An advantage of the digital graduated neutral density filter is that it can be adapted to a more complex scene by erasing the dark spots where they are not needed.


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