Correct The Exposure: Multiply & Screen Blending Modes

Step 3: Darken The Sky Using Layer Mask

The screen mode increases the highlight of an image, multiply the shadow. The biggest problem we have now is that the highlights layer (screen mode) makes the sky too much brighter, unreal, loosing almost all the details. In order to fix the sky I am going to use a layer mask on the highlights layer.

Layer masks are very useful in lots of photo retouching scenarios, so please consult the GIMP manual for more details on this subject. Basically, you may consider a mask as a fully transparent foil placed over all the surface of a layer. After placing the foil, you start to paint over it in order to hide some parts of that layer. To be completely transparent, this foil is composed only of pixels with no value (0, which is equivalent to black). Pixels with maximum value (255, which is white) will obstruct completely the visibility to that layer. Anything between 0 and 255 will me partially transparent, the lower the value, the higher the transparency. Finally, when you are happy with the painting you have made to that foil (mask) you will stick it to the layer, using a fully transparent glue (which means apply layer mask).

If you got the point with this masks I suppose you already know what I am going to do in order to this the sky: use a layer mask on the highlights layer, fully transparent (black) over the sky and fully white over the rest of the picture. In order to improve the transition between black and white I will use a gradient.

In order to create the gradient I will use the blend tool. This tool fills the selected area with a gradient blend of the foreground and background colors by default, but there are many options. To make a blend, drag the cursor in the direction you want the gradient to go, and release the mouse button when you feel you have the right position and size of your blend. The softness of the blend depends on how far you drag the cursor. The shorter the drag distance, the sharper it will be. For more details please consult the GIMP manual.

For this photo I want a sharper gradient, just above the line where the sea begins. To do this, I fist create a layer mask (no meter what option you select when creating this mask because the gradient will replace it anyway). To create a layer mask, select the layer you want to affect, than either right click and select Add Layer Mask, or go to Layers / Mask / Add Layer Mask. Click Add in the dialog that opens.

Notice that white rectangle next to the highlights layer? This means we have a fully opaque layer mask on this layer. Now, select this mask by clicking inside the white rectangle (in the picture above is already selected). In the layers dialog you can see where the current selection is by the color of the border surrounding each thumbnail. All, except the selected one are surrounded by a black border. The selected is surrounded by white, but since our mask is white we cannot observe that white border around it (the absence of a white border on other thumbnail should be the clue). While the layer mask is selected, pick the blend tool, leave all options to default and make sure your foreground color is black and the background color is white.

If you look carefully at the photo you will notice that I already have a gradient at the sea level (lighter close to the beach, darker at the horizon line). This is where I will place my gradient also. Usually you will do it just above the horizon line, but since I have also the sea I will do it below the horizon and above the beach. Draw a vertical line starting from the line where the sky meets the sea and stop just above the beach line. Make sure the line is perfectly vertical and release the mouse.

Now watch the result below, the sky is not so over burned anymore (to sharp gradient, indeed but it will be fixed later as a side effect). The highlight layer is not applied anymore above the sky part of the picture: