Manual HDR with GIMP

In previous posts, I’ve looked a different software applications for creating HDR images.

This week, I was at a presentation with Guy Edwards. It was a fantastic presentation, Guy is an amazing photographer and incredibly creative with his use of light. He showed lots of landscape shots taken using long exposures and also using light painting (something that I’m going to have to give a try).

For his landscape images where light conditions were too harsh, Guy said that he used images stacks in Photoshop and then using masks and eraser brushes, merged the stack together bring out the image as he remembered seeing it. This was one of the keys in my opinion. Guy wasn’t creating the “Harry Potter” style HDR’s that people seem to either look or hate. He was just using the digital technique to try and replicate natural looking images.

Having seen Guys technique and having played with recently with different HDR software applications, I thought I would try and recreate the process using GIMP.

Step 1: Creating a new image brush

In order to merge the image stack, Guy was using the largest photoshop brush available. The largest standard brush in GIMP was around a quarter of the size of the one guy used so I decided to create my own as described here.

The brush I created was a single transparent layer, 500px square. I then created a black circle, filling the square and applied a 500px Gaussian blur so that the brush would be graduated towards the edges. The graduation wasn’t quite strong enough so I boosted the brightness and the contrast until I got the effect I was looking for.

The final brush created ended up looking like this:

Step 2: Import your images into GIMP

The next step is to import all the images you want to merge into a new GIMP image as separate layers. I started with the darkest image at the bottom of the stack and worked up to the lightest. The three images I decided to work with for this test were:

Of these three images, the centre exposure wasn’t too far off what I’d actually plan to achieve. However, merging in parts of the other images could give the overall image a better dynamic range.

Step 3: Blending the images

Now you’re all set to do the blending. To start with, I turned off the lightest image so I could basically work from the bottom up. Turning the lightest image left the center image viewable at this stage. Next, you need to select the paint brush tool and select the new brush we created.

In order to start merging, we need to add a layer mask to the centre image (White – full opacity) and select the paint brush tool. The brush we’ll use is the open we created in step 1. The last thing to do before we start painting in the merge is to set the opacity of the brush really low – I used 10%. By setting the opacity so low, it slows down the merge giving you greater control.

Now, with the Layer mask selected, you can start painting over the light areas in the image where you feel the image would benefit from being darker. You should now see the benefit of having the low opacity set. You can keep painting until you are happy you have the detail that you want. The next step is to repeat what you have just done with the next layer up. Enable the image, add a layer mask and start painting away the light areas.

The final step I took was to adjust the opacity slightly of the layers to achieve the result I was after. Here is the final result:

The final result doesn’t appear to be hugely different from the original centre exposure but this is something we always expected because the centre exposure wasn’t far from where we wanted to be. However, the dynamic range has been increased and hopefully you’ll agree that we have a natural looking final result.


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