High dynamic range image - Exposure blending question

Software GIMP
1

Hello everyone,

I learned how to use the bracketing option on my EOS T7, to take 3 identical picture with 3 different exposures, the image bellow is my test result.

Left (A): Normal exposure
Middle (B): Underexposed (This is where i like the clouds)
Right (C): Overexposed with a somewhat better terrain (Which is not very interesting actually but still better then A or B)

I followed this interesting tutorial to learn how to blend images to modify exposures using GIMP.

But it’s not doing what I want exactly.

In the tutorial the teacher is working with 2 pictures, in my case B and C, adding a gray mask to B. This will create a new image, more balanced with a better exposition for the ground. The problem is, it’s giving way too much exposure to my sky, i want to keep the sky exactly as in the middle picture.

Even if I work the sky with the burn tool, it’s jut not doing what I want.

QUESTION: In your opinion, is it possible with Gimp, to blend B and C without touching the upper part of B? (The sky)

https://i.imgur.com/Aqt4BID.png

2

You can drag and drop your image on the editing window here, you don’t need to use imgur.

3

Isn’t this a classic usecase for Graduated-Neutral-Density-Filter-Using-Masks technique? Both images as layers, doesn’t matter which is which, then add a layer mask to the top layer and paint on the mask to allow through the part you want visible. Or, now that the Brush Tool has an ‘erase’ setting, just remove on that layer what you don’t want so the part in the layer below shows through. Probably have more control with the mask technique.

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4

I agree with @gstalnaker about using a simple mask to blend the best exposure for the sky with the best exposure for the foreground would suffice for the landscape image @Pic-N3p is trying to work on. Simply open both images as layers, create a white mask on the top layer and then use the gradient painting tool to create a gradient mask to give a natural looking blend.

image
This shows how the top layer is retained as a gradient that will allow the bottom layer to show through as an inverse gradient. This is an effective and quick method of combining exposures in many landscapes.
image

2 Likes
5

Yes, you use a layer mask on the top image, and initialize the mask using a grayscale copy of the layer. Assuming B is on Top, the terrain in B will produce a very dark mask that makes B transparent there (and so make the terrain in C prevalent), while the sky will be much less transparent, so it will come mostly from B. You can adjust how/where this transition happens by applying Color tools (Curves, Levels, Brightness-Contrast) to the layer mask.

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Whoa!!! I’m having to think through this logic. I’ve only ever used fully opaque/transparent masks and painted where I wanted the layers to show/not show.

So …

Copy the layer.
Paste into Mask as gray
The lighter the original pixel, the darker the mask pixel, so it becomes opaque.

That is too cool.

Assuming I’m grokking this correctly.

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7

And … I had no idea I could use Color Tools on the mask itself!!

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The lighter the original pixel, the darker the mask pixel, so it becomes opaque.
That is too cool.
Assuming I’m grokking this correctly.

Almost.

  • The lighter the original pixel, the lighter the mask pixel, so it stays opaque
  • The darker the original pixel, the darker the mask pixel, so it becomes transparent

Of course, you can invert the mask on creation or using Color > Invert, in which case the top layer provides its darker parts and the lighter parts come from the bottom one.

9

@Terry yes that is something like that I had in mind, like using a gradient in darktable, but I never used one in GIMP yet.

I will try to use the gradient mask on GIMP first. Thanks!

@gstalnaker Neutral density filter, :grinning: i want one (; My lens is very basic, EF-S 18-55mm f3.5-5.6 IS II, would a ND filter work well with that? There is a thread at the end of the lens to screw something.

And how much should I pay for this type of filter?

Thanks

10

A plain neutral density filter just makes everything darker - primarily useful to enable longer exposures or wider apertures in a given lighting conditions. They will indeed screw on to your lens with no trouble.

A graduated neutral density filter has half of it clear and half darker - these generally are square bits of glass that fit into a filter holder that itself fits to your lens, maybe with an adaptor ring. They’re square so you can slide it in the holder to position the ‘slope’ between dark and light where your image needs it.

I’ve considered getting one or two but have always just put up with shooting multiple images if need be, like your are. :slight_smile:

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11

Yes, I should think that a Neutral Density (ND) or Graduated Neutral Density (GND) filter will work with your camera/lens. The difference between these two is that the GND is HALF filter and half clear. The YouTube link below explains why this is useful.

You need to determine your use cases for using one. Here is a short YouTube video describing why one would use an ND/GND filter: How to use ND Grads: A step by step guide. There are other YT vids as well, and you may benefit from watching them.

There are many types of ND/GND filters based on the number of stops of exposure the filter provides (e.g., a 1-stop ND/GND will transmit 1-stop LESS of light through it, a 2-stop ND/GND will transmit 2-stops less, etc.). You decide which to buy based on your expected kind of use.

Also, there are several kinds of filtering systems. One is a ring, with the filter inside the ring, that screws onto the front of your lens. You’ll need to know the lens diameter (e.g., 55mm) to buy the correct sized type of this filter. Another is a filter-holder that attaches to the lens, then the filter is slid into a slot on the filter-holder.

There are advantages and dis-advantages to both. You get to decide which makes sense for the kinds of photography you do. There will be cheaper and more expensive brands based on the decisions you make above.

B&HPhoto is a good source for looking at options:

https://www.bhphotovideo.com/c/search?q=neutral%20density%20filter&sts=ma

All of this said, you can do exactly what you asked about in this forum post - use the sky from one bracketed image, and the foreground from another, and the much more sophisticated masking techniques your photo editor provides (as Ofnuts explains above) to “blend” them into a desired image that better captures what your eye could see in person.

Note that there ARE reasons to use ND/GND filters that bracketing cannot help with (waterfalls!!) as in ND Filters for PHOTOGRAPHERS!. This is why I have ND filters.

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12

Quick and Dirty:

Made mask from the light version using Color>Threshold and inverted it:

mask

Added as a mask to the dark image then edited the ground and stuff to taste.

Exported:

masked

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13

Thresholds can be an excellent way to make a mask in GIMP with the right subject.
Here I make a threshold adjustment to turn the sky white and the foreground black. I then use the select by color tool to select the sky which is white.

image

I then add a layer mask based on selection. Sometimes you have to invert the mask if it comes out the opposite to what you intended. That depends on if you select the subject to keep or the background to replace. You might also want to google luminosity masks in GIMP. I have played with them in the past, but find a good RAW file replaces the need for many HDR composites.

image

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14

I am still here, this is not over, I need to catch up a bit with your last messages. :grinning:

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@123sg @gstalnaker Thanks for the information about the filter, obviously what I would need is the gradient version of it. I keep the filters in the wish-list, I just bought a new monitor a couple of days ago!

@xpatUSA I learned how to do this already! using the parametric mask to select certain pixels exposition or hue, I learned various ways to use masks in Daktable, this is pretty cool.

I will try to optimize my images RAWS individually first then I’ll try to create a HDR image, merged in Darktable, the automatic version.

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