What is it called when multiple photos are adjusted for further blending?

Question

I have taken a series of noisy photos without a tripod.

I want one less noisy version, so I need average them down to one photo.

But details of the individual images do not exactly match, so that naive attempt will lead to a blurred/ghosted image, so I need to move/rotate[/scale/whatever] all images to match the first and then average them?

What is this technique called?

1. Adjusting
2. Stabilisation
3. Synchronising
4. Converging
5. Deshaking
6. some other term?

@related http://superuser.com/questions/256787/how-to-adjust-and-combine-multiple-lower-quality-photos-into-one-better-using-fos

Asked by Vi0

Answer

The answer you are looking for is Image Aligning.

When you merge images for HDR on Photoshop, there is an optional 'Automatically Align Images'. The same option exist in several cameras with built-in HDR capabilities.

It is also sometimes called Image Registration but that term is overloaded as it has other meanings, for example relative to copyright.

Stacking, HDR Merging, Exposure Blending are what you do with images that are aligned already.

Answered by Itai

Check more discussion of this question.

How can I overlay two images exactly by scaling one of them in gimp?

Question

I have two different pictures of the same object, one of which is taken from roughy the same angle but has a different scale and rotation. I want both images to overlap so that the upper one matches the lower one as exactly as possible. Is there any option in gimp where I can define a number of key points and gimp aligns the images according to these points? For example I say that corner X in picture 1 has to match corner Y in picture 2. It should be sufficient to define 3 such points in each image to get a good result already.

If it is not possible, what would be the best way to reach this goal aside from just scaling and rotating until it looks ok?

Answer

I don't believe there is anything in Gimp to auto align images.

You can do it manually by putting the images in layers, setting the top opacity so you can see the underlying image and scaling/rotating one of them.

Or you can use a tool like Enfuse

Check more discussion of this question.

How can I salvage an HDR image from non-optimal source images?

Question

During a recent wedding, I took a number of shots with my Nikon D90 at different exposures, hoping to merge them into HDR pictures. However, it was done in a hurry, i.e. with no auto-bracketing or tripod, etc. The images are of people standing, with bright sky as background.

When I use Hugin's align_image_stack command to align the images, and enfuse to do an exposure merge, the resulting image has a noticeable border around where the background (sky) merged with my subject like this:

Is this a problem with the alignment? It is possible that both my subject and my hands moved a little bit between the shots.

In any case, is there a way to salvage these photos so that I can still merge them into a correctly exposed shot?

I have access to Hugin, GIMP, and pfstools right now.

Answer

I would align the pictures by hand in GIMP before attempting the HDR processing. Just load them all in layers, pick one as a reference then transform the others to match. You may need to translate, rotate and/or scale to achieve the best alignment. If a perfect alignment is not possible, then favor the high-contrast border areas, like where the sky meets the horizon, trees, etc.

Then do the HDR processing with these manually aligned pictures, skipping the align_image_stack step. Hopefully you'll get a better result that way.

If the people moved between shots (more than likely they did) and that resulted in blurring, then bring into GIMP that blurry tonemapped shot along with one of your source images, the one where the people is best exposed. Do manual surgery in GIMP to insert the people from the good LDR shot into the tonemapped picture.

Good luck.

Check more discussion of this question.

Why is the Alignment step necessary in HDR?

Question

A basic step in HDR (I guess) is aligning the bracketed images together, but what I don't understand is why we need this step. The images are all the same, since they were taken from the same location (maybe on a tripod) with time lag of only a fraction of a second. So, it looks like all the images have the same settings except the exposure. So, what is alignment all about?

Answer

With high resolution images, even the tiny-est vibrations will be visible in the image when looked at the pixel level. The same holds for slight movement of the subjects in the image (from wind, motion, etc.). Misaligned images degrade the quality of the result of the HDR processor.

Note that if you shoot RAW, you can use a single image to generate a few bracketed jpegs of different exposure levels. These jpegs are perfectly aligned, so if the dynamic range you want to encompass in your HDR image is small enough, you can use these aligned images as input to the processor.

Check more discussion of this question.