How To Use Gimp’s Exposure Blending Technique
Amber asked this question:
Hi, I’m signed up for your mailing list and I’m a new gimp user. I’ve tried sooo many different techniques and I can’t figure out anything that works to change the background in this picture I attached. The background is too yellow and I want to lighten it to a soft white or pink and blend it into the image but I can’t figure out how to do it. I was wondering if you could link me to a technique you think would work? I did these newborn pics for a friend of mine but I can’t get them to look good because of the background!
Now before we get into all of this, I recommend that you watch these Gimp tutorials:
Once you watch those, the rest of this lesson will make a lot more sense.
How To Use Gimp 2.8 To Change the Background Color
To change the background color of this image, I ended up using a method that is growing on me more and more as I use it. That method is called Exposure Blending. If you aren’t familiar with this, it is essentially where you take multiple duplicates of the same image, change their colors, and then mask off what parts of the image you do not want to have that exposure. Sound confusing? Let me show you an example with a few images.
Here is what I came up with for your image:
To make this image, I combined two different exposures of your image. I made each exposure by duplicating your image twice, and then using a few of the tools in the colors menu on each duplicated layer. Here is what each of my resulting images that I combined to make the image above look like:
If you take those two images, and keep only the background of the bottom-most image and keep the rest of the top-most image, you get the result I showed above. On top of that, I also used my skin retouching technique to work on the skin on the baby’s face.
Now that you understand what I did, let’s talk about how to use gimp 2.8 to do this.
Part 1 – Create Your Layer Mask
We are going to brighten the entire image, but the background (the dark wall) and the foreground (the relatively light baby and blanket) both need different amounts of brightness to make this photo expose properly. To do this, we are going to break the images into two exposures, just like we talked about above. The thing is, we need to be able to tell Gimp what part of each exposure should be visible, and what part should not be visible. To do this, we’re going to use a powerful tool called Layer Masks. We’re going to use my background isolation using color masking method to do this. Don’t worry, it’s not as hard as it sounds. Essentially we’re going to use our image to create our layer mask for us (mostly.)
Duplicate your background layer
click colors>;>;>;use GEGL
Tip: GEGL is a more-powerful option that is not-yet left on by default in Gimp. If you’re ever adjusting the exposure of an image, I highly recommend turning this on before you do anything because it gives you a bit more accuracy in your color modifications.
click colors>;>;>;levels>;>;>;adjust the levels until the edge of your blanket is white, and the top is mostly black, like this:
Repeat this step, but this time only select the now smaller white area, like so:
And finally, repeat this process with the right-side, like so:
click select>;>;>;none to deslect your previous selection
Now that we have most of our layer mask created, it’s time to transfer this over to a layer as a layer mask.
right-click on the black and white layer that you’ve been working on and click “add layer mask.” Initialize the mask to greyscale copy of layer.
Right-click on the layer mask, and click “mask to selection.”
right-click on your original image and click “add layer mask.” Initialize the mask to “selection.”
hide your black and white layer.
Your image should look something like this:
Touching Up the Layer Mask
Using this isolation method, we quickly masked off most of the tricky edge on the blanket. Now all we need to do is touch up the mask to bring back some of the details lost in the baby’s face, as well as the top of it’s head. To do this, we’re going to make use of the paths tool.
Right-click on your layer mask, and click “disable layer mask.” This will temporarily stop the mask from being used so that we can clearly see what we need to select with the paths tool to correct the missing areas.
use the paths tool to trace the baby’s arm and head, like so:
Right-click on the path, and click “path to selection,” or press SHIFT+V to do the same thing.
re-activate the layer mask by right-clicking on the mask and un-checking the “disable layer mask” option
Your image will now look something like this:
Right-click on your layer mask, and click “show layer mask.” This will help you find the areas that are not completely white in the bottom-half of the image.
Fill all of the areas in the bottom-half of the layer mask with the color white, like so:
Right-click on your layer mask, and un-check the “show layer mask” option. Now that we have a complete layer mask, it’s time to actually use it to lighten up our background.
Lighten Up the Background Using Gimp 2.8
First off, our layer mask is backwards, so click on your layer mask and click colors>;>;>;invert. This will fix that.
right-click on your image and click “duplicate layer.” You now have two copies of the same layer, with the same layer mask. We only need this mask on the topmost layer.
delete the layer mask from the bottom-most image by right-clicking on the bottom-most layer mask and clicking “delete layer mask.”
click on your topmost layer and click colors>;>;>;hue/saturation>;>;>;adjust the hue and saturation until you end up with a dark pink color. We will increase the brightness in the next step.
click colors>;>;>;curves>;>;>;adjust the curve until your wall looks good.
Lighten Up the Foreground Using Gimp 2.8
To lighten the foreground, click on your bottom-most layer and click colors>;>;>;curves, and adjust the curves until you find something that you’re happy with. Once you do that, you’re done! If you want to learn a bit about how I touched up the skin of the baby using Gimp, check out my skin retouching tutorial.