High contrast and HDR images in photogrammetry

[Pauldanc24]

Does anyone on the forum have any experience in using HDR images in photogrammetry. It sounds like it should help in the alignment process. Alternatively does anyone know if increasing the contrast in an image set also improves the alignment. If so, could I not use the increased contrast images to align the photos and then build the texture with the unadjusted ones?

[Dave Martin]

Paul,

In general, I would say avoid any such modification especially HDR. Contrast stretching might be OK if you can do it identically for all images so the same transformation is applied to all. In photogrammetry alignment the software is trying to find 'identical' key points in multiple images to tie the image geometry together. With HDR, or contrast expansion on a per-frame basis, frames will be modified differently, so reducing the potential number of tie points. And, should you try and texture using HDR or images whose contrast was individually adjusted, then you may well get increased boundary effects in the texture.

So, if you really must, you could apply an identical WB shift to all images; or if absolutely necessary a contrast stretch so long as you can apply exactly the same changes to all images.

Otherwise, general rule is use the rawest images you can.

Dave

[leszekp]

For some difficult datasets, I've had success increasing the contrast on raw images and sharpening the images a bit in Lightroom or Camera Raw. I then export them as 16-bit TIffs or JPGs for import into Photoscan. This works best if all of the photos had the same exposure settings.

[cdschroer]

I agree generally with the advice listed here. We stay away from contrast curves.  You can apply a white balance and an exposure compensation when needed, and this can be done for individual images or sets of images.  The main thing when doing an exposure compensation is that you are trying to get a matching histogram across the images (not exactly, and will potentially vary for different parts of a subject)  The point is that you can make some adjustments that will give you better photos, without applying contrast curves or sharpening.  If you have heavily shadowed areas within a photo and some areas are bright, you can also bring up the shadowed area.  This is essentially an exposure compensation applied only to the dark parts of an image.  Don't go crazy with these modification though.

As for the second part of your question, you absolutely can make new jpegs or tiffs from your RAW or DNG files with exposure changes.  As long as they have the same name and are in the same location on your hard drive, (or you reset the path to point to them) Agisoft software will use those images for any operations after you replace them.  We have done that to correct for lighting differences across an image set and to remove what Dave referred to as "boundary effects"  or light and dark stripes or patches across a surface.  This can occur for a variety of reasons including shooting something in the round outdoors where one side is in shadow and one side is in full sun. Also if the sun is going in and out of clouds you can have these issues. Ideally you would correct for that when you are shooting by adjusting shutter speed. 

Carla