You can create skies and make them pop with a few easy tweaks in GIMP so long as you take your photographs thoughtfully the renowned open-source image editor can help . These tweaks are also a great introduction to some of GIMP’s more advanced features such as layer modes, layers and layer masks.
When taking your photographs, you can do an easier job of creating skies by employing a couple of steps. If your camera supports changing speeds, shoot at a slow ISO speed. This should always be the slowest speed you have for point and shoot cameras. This is less important if you have a digital SLR, nevertheless you should still use slower speeds.
When taking your photos, use a graduated neutral density filter. Since skies are typically much brighter than whatever is below them, a graduated neutral density filter blocks out light in part of your photo which allows you to get a more consistent exposure across your photo. You should consider getting one. But, if you have no way of fitting filters to your camera or lens or you don’t have one, just follow the next step.
If you’re not using a neutral density filter, underexpose. Anything that ensures that your sky does not go pure white and this can be by as much as a stop or two. The lost details in dark areas can usually be recovered later. An overexposed sky can never have blown highlights. For film cameras even massive overexposure can never get to blown highlights, but shadow detail is usually impossible to recover. This is why you should shoot at a slow ISO as shadow detail recovery brings out noise. Also this technique brings out any noise that is in the sky, too. If your photo is already noisy, then the end result will be ugly.
Using Overlay or Hard Light instead of Soft Light can give better results for your sky layer. If you are using a digital SLR, employ center weighted metering. Aim your camera at a bright part of the sky and click your auto-exposure lock button. Recompose and shoot. This causes the sky to be appropriately exposed.
You are now ready to edit your photo using GIMP. Follow these easy steps.
Open your image (File -> Open).
Open the Layers dialog with Ctrl+L and create a duplicate layer of your photo. Right click the Background layer then go to Duplicate or use the button.
Right click on the “Background copy” layer and go to Add Layer Mask…. Hit “Add” to add a layer mask to your duplicated layer
Bring up the gradient tool from your tool box or press L to select the gradient tool. Go to the main GIMP window to check if the background is set to white and the foreground color is set to black. Set the gradient to black-to-white by hitting FG to BG.
Put on the “Show Layer Mask” to view it. On your layer mask, draw a gradient while holding down your Control key to ensure a straight line. Left Click the horizon line and hold down your left mouse button. Drag your mouse pointer upwards to some arbitrary point; you need to experiment with this to see what works well for your photo. Release your left mouse button. In the Layers dialog, right click the “Background copy” layer and hit Show Layer Mask to view if you’ve done it correctly. Hit the same tab to turn off the display of your layer mask. You can start to see the difference when you set the layer mode to “Soft light” from the drop-down “Mode” list. You will notice that the color and contrast of your sky has improved to the “soft light layer”.
Duplicate your “soft light layer” until you are satisfied with the overall look. Go to View -> Zoom -> 1:1 to zoom your photo to 100%. In the layers dialog, right click your “soft light layer” then go to Duplicate Layer. Repeat doing this until you’re happy or until you start seeing large blocks of color. Right click on any layer in the Layers dialog and go to Merge Visible Layers and hit “OK” to merge your layers.
Kick up the saturation in the sky a little bit more. Use the freeform select tool by hitting F to loosely select the sky. Go to Select -> Feather; use a Radius of about 1%-2% of the height of your photo in pixels. Go to Colors -> Hue/Saturation and slide the Saturation slider over to the right to taste. Zoom to 100% to ensure that you are not bringing out too much noise in the process. Hit “OK” when you’re satisfied with the results.
Use the “dodge” or “screen “layer mode to recover shadow detail from below the horizon. You can skip this step if you used a neutral density filter on the sky. Add a layer mask after duplicating the bottom layer. Repeat steps 2 to 7; except this time, do it backwards by drawing the gradient starting just above the horizon line to just below the horizon line. The end result will be that the ground stays in the white area of the layer mask and the sky goes in the black area. Experiment on this again until you’re satisfied with the end results. Set the layer mode to Dodge or Screen. Duplicate until you’re happy and then merge them.
Experiment on any other processing that you feel fit to do. You can try de-noising your sky a little with a selective Gaussian blur by going to Filters -> Blur -> Selective Gaussian Blur; then sharpen the area below the horizon by going to Filters -> Enhance -> Unsharp Mask, and use a”tone mapping” script to finish it off. Admire the end result. If you’re not happy, do it all over again. Practice makes perfect so they say.