The Power of Digital
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| Exposure blended image using Layer Masks and Blend Tool with GIMP. |
Digital photography is really quite astounding. Back in the film days I'd shoot a roll of film send it off to be printed and hope that between my work and the work of the developer I'd get a decent couple of shots in return. But I'd never be quite sure what I'd get back.
Today, I shoot all my images digital and then get to process them myself, making minor or major adjustments with a huge array of tools on hand.
Open Source Is A Real Alternative
Open Source developers take this a step further and give me (and you) access to amazing digital photography tools that are free of cost and non-proprietary. While Adobe's Photoshop definitely sets the gold standard, the GIMP (GNU Image Manipulation Program) is a powerful, free and open source alternative.
Exposing For The Light
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| Images with overexposed and underexposed areas. Click on any image to enlarge it and view as a slideshow. |
Getting up early I want to capture the glowing hues in sky as the sun rises over the horizon. At the same time, I want to be able to see the details of the scenery. Here are a few photos I took at Palmyra Cove, just a drive over the bridge from my house. As you can see, these images have areas that are overexposed and underexposed.
I took these images using a tripod and intentionally bracketed the exposures. That is, I took one image (left) based on my camera's light meter for optimum exposure. While the sky looks quite good, the foreground is completely lost in shadow. I took two more exposures - one at -1 stop (half the amount of light) and then another at +1 stop (twice the amount of light). Many cameras have in-built settings that let you auto-bracket in a burst of 3 images, sometimes more.
The -1 stop image exposes the light for the sky quite well. There is a good range of detail and color, and no area is blown out (just white space) or in deep shadow. The +1 stop image exposes for the foreground but overexposes the sky. We will blend these two images together to make on well-exposed image.
These images are in RAW format, so I used Darktable to export them to .JPG format. If you shoot JPG in your camera you can use those images directly.
STEP 1: Download and Install Gimp (Windows, Mac and Linux)
If you don't have GIMP installed it's just a Google search away (GIMP, download). You should be linked to www.gimp.org/downloads/. Go ahead and click the link for your operating system and follow the installation directions. If you have trouble with this step use the contact form below and I'll try to help you out.
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| Download GIMP for Mac, Windows or Linux. |
STEP 2: Open Gimp
GIMP opens with a usual program splash screen. The one below is for version 2.9. The download link will likely give you version 2.8. Both work the same for this tutorial.
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| GIMP splash screen on opening. |
The GIMP opens as three separate windows. While it may feel intimidating, you'll get used to it.
The left window is the tool palette for selecting and working with various tools.
The center window is the image working space, when you open an image file it will display here. On the top of the window are many menus that access various functions like Open, Save and Export, to name a few.
The right window is the palette for layers, mask and history.
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| The GIMP workspace. |
STEP 3: Open as layers in one project file
Let's go ahead and start our project.
First, select the two images from your compter's file system. In the middle window select the File menu, then click on Open as Layers.
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| Open As Layers dropdown menu. |
Find the two images in your file system, highlight them and click on Open. These two images will be opened as one file. One image will be layered on top of the other. You'll see only the top layer is visible.
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| Select images to use as layers. |
Below you can see the image displayed in the main (middle) window. On the right window the layers palette displays both images loaded.
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| Images opened as layers and visible in the layers palette. |
STEP 4: Choose which areas to expose
For our purposes, the easiest kind of image has a very clear line between the over- and under-exposed areas, like a horizon line. Other images, say with a building or tree that juts into the skyline, make this a little more complicated. We won't deal with that today.
STEP 5: Create a layer mask on top layer
One way to approach this problem would be to simply crop the top of the -1 stop image and the bottom of the +1 image and paste them together. However, there is enough of a uneven line through the horizon that this might look noticeable and odd.
A layer mask gradually gives us control over which parts of an image can show through another image, even the amount (opacity) that it shows through. Using the blend tool the top layer will show through the lower layer graduating from fully visible to fully transparent.
Note: I find layer masks to be an invaluable tool for my everyday digital work flow and programs like GIMP do them very well. A graduated blend is just one way of using a layer mask.
Let's create our layer mask!
First, right click on the top image in the layers palette (right side window). A lengthy menu will pop up. Click on the option "Add Layer Mask", about halfway down the menu list.
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| Right click on the top layer to view the layer menu. |
You should now see the Add Layer Mask dialogue box. It gives a variety of options. You want the "White (full opacity)" button checked. Click OK. Wherever a mask is white the layer is fully visible and the lower layer hidden.
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| The "Add Layer Mask" dialogue box. |
Your workspace in GIMP should now look like this: (note the white box to the right of the top layer)
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| Opaque layer mask applied to the top layer. |
To make the lower layer visible through the mask we want to apply a gradient to the layer mask. As I said earlier, a white mask keeps the top layer visible and the bottom layer hidden. If we draw, paint or alter the mask with black or gray the layer underneath becomes visible. Fully black is 100% visible and shades of gray increase or decrease opacity, for example middle gray makes the lower layer 50% visible.
Remember: keep the layer mask selected!
The first trick is to remember to keep the layer mask selected. You'll know it's selected when it has a white outline around the mask in the layer palette. If you select one of the layers you will be altering that layer and not the mask. It's easy to forget this and wonder why things aren't working they way you expected.
Just for kicks, select the bucket fill tool and fill the layer mask with black. You'll see the lower layer show through. Now click Ctrl-Z to undo.
STEP 6: Blend Tool
Making sure that the layer mask is selected, we're going to switch our attention to the tool palette on the left. Click on the blend tool (if your tools look different to mine simply move the mouse cursor over each tool until the tool tip text says Blend Tool.
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| Select the Blend Tool. |
Using the default settings the blend tool will create a gradient from full black (transparent) to full white (opaque). In my setup, I want the sky from the lower level to show through, so I want the black to be at the top of the image and the white at the bottom.
Left click somewhere in the top 1/3rd of the image. While holding down the mouse button, drag down to somewhere in the bottom 1/3rd of the image. Depending on the size of your image and the power of your computer, you will see the gradient start to work. In this first example I drew the gradient at an angle so you can see how the top left of the image shows the darker sky and leaves the lighter image visible on the right hand side.
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| Click and drag to activate the blend. |
To get a verticle line hold down the Control key will dragging with the mouse. The line should snap to a verticle (the gradient is applied in the direction of the line).
For this image, the gradient transition can happen over a small distance in the middle of the trees. You can adjust the start and end of the blend by dragging the ends of the gradient line higher or lower. Play around with it to see what you can achieve with a shorter or longer gradient. Which do you prefer?
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| Click the Control button to achieve a verticle blend. |
If you look to the right palette you will notice that the layer mask is now half black and half white. By using the layer mask you can clear the mask at any time and start again, leaving both original images unchanged. You could even use a paintbrush to go over an area you want to add or remove from the mask. To let the bottom layer show through use black paint. To hide the bottom layer use White paint.
Now you're done! Well, almost. You've finished the exposure blend. You may decide to do further work on the image. I still find this image too dark, so I might increase the brightness using one of the many options to do so. Once that is all done ...
STEP 7: Export Image (Save As)
You're ready to export your final image. In GIMP saving the file will create a .xcf file which leaves all the layers. It's not a format you can use to print or share with friends on Instagram. So we want to Export. Using the menues on the main window select File, then Export As ... .
Select where you want to export the file to. In most cases you'll want to export as a JPG file, so finish the file name with ".jpg" and GIMP will do that automatically. A second dialogue box (not shown) will ask you for some further options. I generally just move the compression slider to 95 for a high resolution image for printing, or 50 for a low resolution image for sharing on the web.
Final Blended Image: Too Dark
Using a similar repeat of the above procedure I created a duplicate of the final image and brightened the lower layer to better expose for the reeds and lilies. I then used the layer mask to let that show through. The end result is far more pleasing.
You could get the same result by raising the brightness of the bottom layer in Step 3. To save me redoing all the screen shots I did at at the end. This also shows that layer masking, once mastered, is a highly useful tool.
My Final Image
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