Remove Color Cast: Levels & Curves Tools

Step 2: Select The Black Point

We will use this layer to help us in selecting the black (shadow) point:

1. Select the Background copy layer;
2. Open the Threshod tool;
3. Make sure the Preview is checked;
4. Move both sliders to 0 (to the left, or enter 0 in both text boxes);
5. Move the white slider to the right until a useful amount of white pixels are displayed in the image;
6. Click Ok to apply and close the threshold tool.

Now, in the image window, click on the above ruler and drag down until crosses over a white zone. Now click on the left ruler and drag right until intersects the previous line over the white zone (CLICK on image below).

After that, in case we plan to use the curves tool, we will insert a sample point on the intersection. Keep Ctrl pressed while clicking on any of the rules area (top or left) and drag over the intersection of the guide lines. If you do not have open the sample points dialog, you can activate it from Windows / Dockable Dialogs / Sample Points. Make sure the sample point indicates 255 for red, green and blue, which indicates that you really selected a white spot in the image.

At this point you can delete the copy layer on which we worked until now. As the background layer becomes selected the sample points indicates the corresponding values from this layer. I use sample points because I consider that are really helpful when making corrections using the curves tool. For levels adjustments, this points are not needed (actually better don't have them).