How to create a fake long exposure from a bunch of frames

Question

I made several frame screenshots at one angle and I want to join them into a long exposure shot similar to those pictures of night traffic. Like this example.

But I can't get my head around, how to combine the images I have into one blurry line. My guess is I need to edit it in Photoshop or other app, slice the changing part on every image, then join in layers, apply blur. But this is a very time-consuming process. Is there any other way to achieve the desired result?

Answer 1

Hard to give a full answer without seeing what you start with. But in Photoshop and Gimp there is a "Lighten only" (and possibly some variants thereof) blend mode that applies only the parts of the layer that are lighter than the image composition below, so it only transfers lights.

So in your case you would stack all the pictures and set them all to "Lighten only" mode (except the one at the bottom).

Answer 2

You can also use the frame averaging technique. This takes a set of images and calculates the average (or median) value for each pixel. This technique can be used to reduce noise, remove moving people from your photo but also to simulate long exposures.

Some advanced camera's can actually do this in-camera, but you can achieve the same effect in Photoshop or other software such as GIMP and ImageMagick.

In Photoshop you would:

1. Load all images into separate layers
2. Select all layers
3. Align all layers (if they are not already aligned) via Edit > Auto-Align Layers
4. Choose Layer > Smart Objects > Convert to Smart Object
5. Choose Layer > Smart Objects > Stack Mode and select a stack mode from the submenu
   • You can try out different modes to see what gives you the desired effect, I would start with mean and median.

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Answer 3

How to combine the images I have into one blurry line

With "normal" techniques using Ps you can not. Depending on how different each image is from the previous one, what you will get is a stroboscopic effect. Which can be awesome btw.

But depending on the trajectory of the lights you could add them by combining a layer with only the lights and some motion blur (Either directional or zoom) and some opacity blending mode. But this is now in the realm of special effects rather than photography.

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I suppose there are some new AI algorithms that analyze the difference between each frame and generate the proper blur. In video that is a common technique, so probably you can apply it to your original image sequence to get either more frames and more blur, so you minimize the stroboscopy effect.

I'll try to complement this answer later.

Answer 4

Depending upon the tools you have, there are various approaches. Here is a way I did it some time back, so it may be dated, using only freely available software. This was to produce a long exposure effect on a waterfall, alter the modes for your needs.

1. Export your images as 16-bit TIF

2. Align Images with Hugin Hugin download

# align_image_stack -a aligned_  -v -m -g 10 -C ../*.tif

or using a GPU

# align_image_stack -a aligned_  -v -m -g 10 -C --gpu ../*.tif

3. Stack Images (be sure to switch to directory of aligned images. 3 example stack approaches)

(a). Imagemagick
     # convert *.tif -evaluate-sequence median -alpha off median.tif

(b). Hugin Enfuse
     # enfuse -o enfuse.tif -d 16 --exposure-weight=1 --saturation -weight=0.2 --contrast-weight=0 --soft-mask aligned_*

(c). Hugin Stacker
     # hugin_stacker --output=median2.tif --mode=median --bigtiff ../*.tif

Regardless of the tools you use, the general approach is:

1. compatible image type
2. Align
3. stack

Depending upon your images, alignment may be readily automated or need manual alignment guides created.